been put forward (e.g., Davis and Stubbs, 1977), which are
based on intra-abdominal pressures generated in the act of
lifting.

Repetitive Liftine

In a series of studies on healthy young males and females
who were trained to lift boxes, Lind and coworkers observed
the following:

A. Aerobic capacity. The V0 max for men who lifted by the
cantilever (bent back) method from the floor to a bench 60
em high (24 inches) was,

1) substantially lower when lifting boxes than on the
bicycle ergometer (15% lower when lifting a box
weighing 36.4 kg, or 80 lb).

2) directly related to the weight of box, from 1 kg to
36.4 kg.

3) determined, by subjective impression, by the rate at
which the trunk and head could be moved for boxes
weighing up to 6.8 kg (15 1b) but by fatigue in the
forearm muscles when the box weighted 36.4 kg (80 1b).
(Lind et al., 1979; Petrofsky and Lind, 1978).

The VO. max of women lifting with the bent-legs method
from the floor to a bench 60 cm high (24 inches) showed,

1) no differences in VO. max for boxes weighing 15.9 kg
(35 lb) and 22.7 kg 450 lb) and were not statistically
different from that measured on the bicycle ergometer.
The difference in pattern of response with that of the
male subjects is attributable to the different method
of lifting.

2) the Vo2 max increased with the weight of box, from 2 kg
(4.4 1b) to 22.7 kg (50 1b) (Lind et al., 1979, Williams
et al.. 1980).

B. Level of aerobic capacity associated with fatigue. When
men lifted boxes from the floor to a bench 60 cm high (24
inches) for one hour,

1) the evidence from oxygen uptake, heart rate and arteri-
al lactate support the view that fatigue was generated
when the work rate exceeded 50% of the VO2 max for the
given weight of box (i.e., at levels as low as 35% of
the Yo2 max established on the bicycle ergometer for
these subjects).

46


vision of the cqntainer, the workplace around the container and
the surface on which the operator stands. Following the IES Code
for Interior Lighting, an tlluminance of 150 lux in each of these
areas is a recommended minimum.

Type of lighting can have a large effect on visual performance,
particularly in the areas of depth perception and surface texture
perception. The aim in both of these areas is to provide suffic-
ient visual contrast for safe operation. Contrast, which depends
on the difference in light reflected from two surfaces, can be
controlled either by differential illumination or by differing
reflectance. Differential illumination should apply to stairs
or changes in walking surface such as shoe/surface friction changes.
Low, angled illumination is recommended for enhancing surface
texture to warn operators of changes in shoe/surface friction.
Depth perception errors are.usually controlled by changing the
reflectance of the container and task to provide additional con-
trast. Color contrast can also be used in critical areas such

as edges of steps, loading docks and ramps where the consequences
of misperception are severe.

The American National Standard 253.1, ]967, Safety Color Code recom-
mends the Yellow be used for marking hazards that may result in acci-
dents from slipping, falling, striking, etc. Yellow indicates
flammable liquid storage cabinets, materials handling equipment,
radiation hazard areas, etc. Often black stripes or a checkerboard
pattern can be used with the yellow color.

In situations where the operator may be unfamiliar with a particu-
lar container, or where a variety of containers are handled, appro-
priate labels and markings should be put on the container. These
should be designed to ensure that the best handling position is
adopted and that the appropriate force is applied. All lettering
should be of a uniform style (e.g., MIL STD 33558).

Labelling should indicate handhold positions, cautions against
single person lifting if the load is heavy, or a note of caution
if its center of gravity is not near the center of the container,
or if its center of gravity is likely to shift. The center of
gravity and total weight warnings are particularly important with
modern packaging methods, where the size of the container may bear
very little relationship to its weight and dynamic characteristics.

Labels should be printed on all sides of a container, facing in at
least two directions so that they are visible to the operator at
all times. Labelling should aim for brevity and simplicity if it
is to be used and understood on the maximum number of occasions.
THE THERMAL ENVIRONMENT

Early studies on mine workers showed a relationship between environ-
mental (air) temperature and accident rates (Vernon and Rusher, 1920).

110


A truly accurate identification of the site and origin of back
pain is seldom easy. The tissues. concerned are usually deep from
the skin surface. The quantity of pain felt and the sites from
which it may arise are not closely related. And because of the
absence of a nerve supply, the disc and the joint-facets may be
injured without causing pain. Often the pain after back injury
only begins after the secondary effects of the injury and the
ensuing irritative state spread to neighboring structures.

The radiological appearance is seldom of great help in identifying
the site and origin of pain. Any observed abnormality such as
disc degeneration, a congenital defect or any other structural
derangement is likely to be present both before the onset of pain
and after its relief. Such changes have frequently been seen in
people who have never had back pain.

Although it is widely believed that disorders of the lumbar inter-
vertebral disc may be the most common single factor to which back
and sciatic pain may (directly or indirectly) be ascribed, there
are a growing number who consider that disorders of the apophyseal
joints are of equal importance. It is misleading to select a
single factor for the etiology of back pain. Many possible con-
tributory factors must be borne in mind, for example:

1. fatigue
2. postural stress
3. trauma
4, socio-economic and emotional
stresses
5. personality
6. degenerative changes
7. congenital defects
8. reduction in the size and shape
of the spinal canal and inter-
vertebral foramina
9. genetic factors
10. stretching, angulation, compression
or adhesion of nerve roots
11. neurological dysfunction
12. the duration of symptoms
13. physical fitness
14. body-awareness

This list is far from complete. Further, it does not fully con-
vey the complexity of the problem which is compounded by the way
in which these contributory factors interact. Another problem
in applying epidemiological data obtained from medical clinics
is the knowledge that the patients who are seen in practice only

represent a small fraction of the affected population (Westrin,
1973).

With regard to the "conditioning" factors which predispose the
back to injury, mention has been made of environmental and

12


ADMINISTRATIVE CONTROLS

Given the preceding detailed analysis of the physical stresses, a
lifting job (or tasks within a job) can be classified into one of
3 zones:

abo Acceptable (below the AL),
Ao Unacceptable for most individuals (above the MPL), or

So Unacceptable for some individuals (between the AL and
MPL).

This section deals with selecting and training of workers for jobs
of the latter type (3). A more detailed discussion is presented
in Chapter 6. Alternative engineering controls are then summar-
ized for conditions 2 and 3. A more detailed discussion appears
in Chapter 7.

Selection

In order to safely match a worker to the physical demands of a
given task, several selection procedures can and have been used.
For the many manufacturing, distribution, and service industries
which have no medical staff, self-selection has been the principal
method. Other methods commonly used are:

1. Questionnaires on health and medical history

Bo Tests of visual, auditory and pulmonary function, blood
pressure, mobility and a chest X-ray

3. Clinical examination.

The problem with these is that they are unreliable predictors of
medical risk of injury. Also, the worker is usually not followed
medically during the first few weeks of training when many pos-
tural stress related problems could be prevented,

In view of these problems, routine lumbar spinal radiography has
often been prescribed by occupational medical advisors as a prere-
quisite for employment in heavy manual work. Using X-rays for
this purpose, however, is very controversial because:

ako There have not been any statistically significant dif-
ferences shown in the incidence of radiological abnor-
malities between workers with known history of back
pain and those without.

Bo There is a problem of X-ray interpretation. No reliable
data exists on the predictive rating for observed ab~
normalities. It is also impossible to quote a value for
the increased probability of back trouble for different
abnormalities.

145


50.cm

Figure 8.12: Example 4, Depalletizing Operation.

(v) and vertical travel (D) vary from one lift to the next. Also
note that this task requires both lifting and lowering (for those
high cartons) as well as possibly some twisting and carrying.

These later aspects are outside this Guide and their effects should
be minimized by a slowed pace for lifting and delivering pallets

as close to the conveyor as possible.

Figure 8.13 presents an analysis which approximates the stresses
of this activity assuming the carton dimensions are 40 cm x 40 cm
x 40 cm, unloaded 12 per minute, and the person is free to climb
over the pallet to get close to each carton. Basically the job

is divided into 5 tasks representing the 5 tiers of the loaded
pallet. The base frequency (12/min) is divided between each tier
(2.4 lifts/min/tier). The horizontal location is estimated as

H = (15 + 40/2) = 35 cm. The vertical locations at the origin
represent the position of the hands under the cartons (ignoring
the pallet height for purposes of this example).

Two separate analyses are warranted in this case. Each task should
be analyzed separately and then collectively. The most stressful
task (or tier) is task 5. For this task the V factor = (1 -
-004(160-75)) = .66. The V factors for each of the other tasks will

140


WORK PRACTICES GUIDE FOR MANUAL LIFTING

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Centers for Disease Control
National Institute for Occupational Safety and Health
Division of Biomedical and Behavioral Science
Cincinnati, Ohio 45226

March, 1981

For sale by the Superintendent of Documents, U.S. Government
Printing Office, Washington, D.C, 20402


2. A power grip in which the object is clamped between the
partly flexed fingers and palm with the thumb opposing
the grip and lying along the plane of the palm, and

So A precision grip in which the object is pinched between
the flexor aspects of the fingers and opposing thumb.
Most handles, hand-holds or gripping aids on containers force the
worker to use a hook grip (the least effective) or a power grip.
This latter gives a good gripping force and allows a large surface
area of hand to be used but it is inefficient if accurate control

of the container is needed. Frequently, however, the weight of a
container will not allow a precision grip to be used.

The'other main consideration in handle design is the nature of the
forces that are to be transmitted through the handle to the con-
tainer. Figure 7.1 shows the forces and torques associated with
handle use. Most studies of handle design have assumed that the
best size, shape and texture of the handle can generalize from

one use to another: their results do not show close enough agree-
ment to support this contention.

——

Push/ Pull Force

Utnar/Rediat
Deviation Torque

Pronation/Supination
Torque

Thrust Force

Figure 7.1: Hand Forces and Torques on Handles

106


shows the major tasks and personal factors known to affect
metabolic energy expenditure rates. A brief description
of the effects of each factor follows.

Table 4.2: Major factors affecting metabolic energy
expenditure rate.

Worker Variables Task Variables
1. Gender 4. Load
2. Body Weight 5. Frequency of

loading of body
3. Lifting Techniques

6. Vertical travel
distance

7. Vertical origin
of lift

8. Temperature and
humidity

The effects of age and body weight have been discussed in
the section on dynamic exercise (p. 41).

Other factors affecting the metabolic rate include:
1. Body Posture (or Technique). The body posture used to

lift a load may affect the metabolic cost of the job
considerably. The following are some of the reasons:

1. Different body postures affect the loading of
different muscle groups and different muscle
groups have different metabolic efficiencies.

2. Body posture affects the muscle moment arm and
length of the muscle. Metabolic efficiency is
related to muscle tension which is a function

of length of the muscle and speed of shortening
(Hill, 1938).

55


PHYSICAL STRESS JOB ANALYSIS SHEET

DEPARTMENT_ASSEI UW pate _3///80
JOB TITLE. A24deER_ ss ANALYST'S NAME __CKA
TASK OBJECT Ce OC aIONs TASK

DESCRIPTION | WEIGHT | | v yy | FREQ] ALI MPL|} REMARKS

Ave Max | om cml cm cm

PRoouer A 10 | 40 |4o| 0 | sola

/ 7.8 |234|| Reduce product

Variabilitg
PRoover B 15 |} 30 |30| o| 30,15 | 2 Ue 1348
PRoovcr C 5 | 70 |s}75|20]00| = |I26.8 |Go4
Tervar |8l 22|47 8 7-7 |23.1

Figure 8.14: Example 5, Highly Variable Job.

TASK 2:

TASK 3:

AL = 40 (15/30) (1-.004(75)) (.7+7.5/25)(1-2/12)
= 40 (.50) (.70) (1.0)(.83) = 11.6 kg
MPL = 34.8
AL = 40 (15/15) (1~.004(0)) (.7+7.5/25) (1-5/15)

40 (1.0) (1.0) (1.0) (.67) = 26.8 kg
MPL = 80.4 kg

Two technical aspects of the above calculations are noteworthy:

a.

Travel distances less than 25 cm should be considered
as 25 cm. In task 2, this cutoff was used. (Factors
must all be less than or equal to 1.0).

Since this job is performed continuously, Fa, 9 factors
(from Table 8.2) of 12 and 15 were used for ftEts from
less than and greater than 75 cm.

143




Despite such guidelines, gross occupational injury and illness
statistics continue to dramatize the problem in the United States
and elsewhere. In 1973, the State of California reported 27% of
all compensable injury and illnesses were due to overexertion
(National Safety Council, 1975). Worker's compensation reports
for Arkansas in 1976 show the same percentage (BLS, 1978). The
later study also revealed, 1) an average of almost $3,000.00 per
incident was spent in lost wage compensation and medical payments,
and 2) 68% of the overexertion incidents involved lifting and 20%
involved pushing and pulling objects. The State of Wisconsin re-
ported the number of overexertion injury claims doubled between
1971 and 1977, as indicated in Table 1.2. The State of California
reported over 68,000 overexertion injuries in 1973 alone (National
Safety Council, 1975). Though the total for the United States in
1980 is not known, it will probably exceed 500,000 injuries.

Table 1.2: Wisconsin occupational injury and illness
compensation records (Workmen's Compensation
Division, 1978)

Year Total Number of Overexertion Claims
1971 7,160
1973 9,875
1975 10,795
1977 14,411

Some industries have a greater overexertion injury rate than others.
Table 1.3 depicts several high injury industry groups for Arkansas
(BLS, 1978). A study of back strain and sprain injuries in Wis-
consin in 1973 showed similar patterns between industry groups
(Taugher, 1973). This latter study also reported that 61% of the
back pain cases listed overexertion as the cause of the problem,
and that about 7% of the back pain cases became permanently dis-
abling to the individual. In this regard, a recent in-depth
follow-up of 549 persons classified as partially but permanently
disabled due to on-the-job injuries disclosed that less than
one-third of the severe cases (particularly back problems) were
able to return to work at any job, and that the worker's compen-
sation payments replaced less than one-third of the wage earning
capabilities of the workers (Grinnold, 1976).

If low-back pain cases alone are used as an indication of the eco-
nomics involved, a recent report by the National Safety Council
estimated over $1 billion dollars was spent on worker's compensa-
tion and medical payments in the U.S. in 1974 (Hirsch, 1977).
Average awards in the railroad industry, which is not covered by
the worker's compensation system are 10 times those awarded under
the Worker's Compensation Act in some states (Hirsch, 1977).

2


Conveyors
These gravity or power devices are commonly used for moving uniform
loads continuously from point to point over fixed paths where the
primary function is movement.

abo Common examples include:

a. roller conveyors,
b. belt conveyors,

So screw conveyors,
d. chutes,
e. monorails, and

Efe) trolley conveyors.

2. Conveyors are generally useful when:
a. loads are uniform,
b. materials move continuously,
@o routes do not vary,
da. loads are constant,
e. movement rate is relatively fixed,

$35 conveyors can bypass cross traffic,
g. the path to be followed is fixed, and
h. movement is from one point to another.

Cranes and Hoists

These overhead devices are usually utilized to move varying loads

intermittently between points within an area fixed by the support-

ing and guiding rails, where the primary function is transferring.
tho Common examples include:

a. overhead traveling cranes,
b. gantry cranes,

c. jib cranes,
da. hoists, and
e. stacker cranes.

ao Cranes and hoists are most commonly used when:

a. movement is within a fixed area,

b. moves are intermittent,

Cc. loads vary in size and weight,

d. cross traffic will interfere with conveyors, and
e. loads handled are not uniform.

115


Table 8.1: Emphasis on task variables by alternative

approaches.
no
D un 3)
toa) i)
3 ol D o
dan Gn nn aa
ON gm on Bayt
ol at d ‘a,
5 . Oe Oe Oe
G oc ad aa
yon) EO av aS)
a 6c a4 Sans
Os “ a i)
i) i) a a
Object Weight (L) x x xX x
Horizontal Location (H) x x x x
Vertical Location (V) x x x x
Travel Distance (D) x x
Frequency of Lift (F) x x x
Duration or Period (P) x

importance of object weight (for example) is highly dependent on
where the weight is located (horizontally and vertically), how far
it must be moved, and how frequently. Thus, none of these variables
should be evaluated independently.

Secondly, the four approaches taken separately may lead to differ-
ent conclusions. For example, metabolic criteria (Chapter 4) can
lead one to believe lifting heavy loads infrequently is preferred
to frequent lifting of lesser loads (due to the cost of

moving the body). From a biomechanical or strength point of view,
(Chapters 3, 5) object weight should be minimized regardless of
frequency.

Another example has to do with object location. A person is gen-

erally strongest when lifting with the legs and back. Were
strength the only criterion, (Chapter 5) one would favor leaving

123


Table A.1 (cont.)

ISOMETRIC
% CAPABLE

*The currently available models suggest that reach posture
(P) tather than strength is most limiting in these cases.

180


Perey, O. "Fracture of the Vertebral End-Plate in the Lumbar
Spine: An Experimental Biomechanical Investigation." Acta
Orthop. Scand. Suppl. 25, 1957.

Petrofsky, J.S. and Lind, A.R. “Aging, Isometric Strength, Endur-
ance and Cardiovascular Response to Static Effort." J. Appl.
Physiol. 38:91-95, 1975.

Petrofsky, J.S. and Lind, A.R. "Metabolic, Cardiovascular and
Respiratory Factors in the Development of Fatigue in Lifting
Tasks." J. Appl. Physiol.: Respirat. Environ. Exercise Physiol.
45:64-68, 1978.

Petrofsky, J.S. and Lind, A.R. "Comparison of Metabolic and Ventil-
atory Responses of Men to Various Lifting Tasks and Bicycle
Ergometry." J. Appl. Physiol.: Respirat. Environ. Exercise
Physiol. 45:60-63, 1978a.

Pfauth, M.J. and Miller, J.M. "Work Surface Friction Coefficients:
A Survey of Relevant Factors and Measurement Methodology."
Jd. Safety Res. 8:77-90, 1976.

Pheasant, S. and O'Neill D. "Performance in Gripping and Turning:
A Study in Hand/Handle Effectiveness." Appl. Ergonomics.
6:205-208, 1975.

Pollock, M.L.; Miller, H.S. Jr.; Janeway, R.; Linnerud, A.C.;
Robertson, B. and Valentino, R. "Effects of Walking on Body
Composition and Cardiovascular Function in Middle-Age Men."
J. Appl. Physiol. 30:126-130, 1971.

Porter, R.W.; Hibbert, C. and Wellman, P. "Backache and the Lumbar
Spine Canal." Spine. 5:99-105, 1980.

Poulson, E. and Jorgenson, K. "Back Muscle Strength, Lifting and
Stoop Working Postures." Appl. Ergonomics. 1:133~-137, 1971.

Powell, P.J.; Hale, J.; Martin, J and Simon, M. "2,000 Accidents;
A Shop Floor Study of Their Causes Based on 42 Months' Contin-
uous Observation. App.17." Ergonomics of Good Handling, Nat-
ional Institute of Industrial Psychology, London, 1971.
pp 106-108.

Rais, 0. "Heparin Treatment of Peritenomyosis (peritendinitis)
Crepitans Acuta: A Clinical and Experimental Study Including
the Morphological Chenges in Peritenon. and Muscles." Acta
Chir. Scand. Suppl. 268, 1961.

Redfield, J.T. "The Low Back X-Ray as a Pre-employment Screening

Tool in the Forest Products Industry." J. Occup. Med. 13:219-
226, 1971.

169


EB, = &E

iT + F (AE)

P

where F is the frequency of lifting. The net metabolic cost
of each lift (AE) is composed of two parts, the energy
necessary to move the body and the energy necessary to move
the load. Depending on the posture and vertical location:

75

LIFT RANGE
0 to50 cm

LIFT RANGE
50 to100.

C=ENERGY EXPENDITURE
(los keal/ kg-m)

20
WEIGHT (kg)

25 30

Figure 4.7 Energy Efficiency with Weight and Range of
Lifting (Frederik, 1959).

61


the erect back, squat lift posture predicate their views on a
simplistic mechanical logic; namely that this posture allows the
load to be held close to the torso and, therefore, the spinal
bending moment and compression forces on the back will be small.
In addition, the stresses on the vertebrae will be better distrib-
uted, e.g., Floyd (1958), Davis (1959), Muchinger (1962), Himbury
(1967), Anderson (1970) and Nachemson (1971). In such analyses,
little concern is shown for the dynamic loadings on both the back
and the knees during the lifting sequence, not withstanding the
practical fact that many heavy objects are too large to be lifted
between the knees, as is required by the squat lift method.

Research by Clark and Russek (1958), Brown (1973), Jorgensen and
Poulsen (1974), and Garg (1976) disclose that leg lifting from a
squat position is metabolically more demanding, thus possibly
leading to more fatigue related injuries (e.g., slips and falls,
or dropping of object). Chaffin (1969) found that the location
of the load relative to the back is more important than the lift-
ing posture in generating high compressive forces in the spine.

Confagunding the issue of which posture is the safest for lifting is
the realization that low-back pain can occur due to sudden slips
and the resulting postural corrections necessary to regain balance
(Hult, 1954; Brown, 1958). Therefore, to protect the back as well
as other body segments, one must maintain a posture which assures

a maximum stability over the period of the activity. Toward this
end, the National Safety Council (1971) and the International

Labor Office (1967) have chosen to emphasize the kinetic method of
lifting, which is based upon the squat lift approach and the ef-
ficient use of body weight. However, Anderson (1970), the original
developer of the kinetic method, believes that

"it is safer to allow workers to use their own common
sense and muscle sense than to teach them new drills

in performing certain jobs in which a series of pre-

determined positions must be consciously assumed."

Observations of workers experienced in the handling of heavy loads
show that the squat lifting posture is rarely used (Shephard,
1974). Davis, et al., (1965) suggested this is because the method
is impractical. The leverage exerted by the quadriceps muscle in
this posture is ineffective and the average worker cannot develop
sufficient force to raise heavy loads. For loads away from the
body, Park and Chaffin (1974) have shown that forces on the erec-
tor spinae muscles and the lumbosacral disc can be as much as

50% higher when using the recommended squat posture compared to
the stooped posture. However, for compact objects close to the
body, they recommend the squat method based upon a shorter moment
arm for the body weight and the load acting at the lumbosacral
Ls5/S, disc.

19


and Coulter, 1921; Becker, 1961; Moreton, et al., 1958; McGill,
1968; Rowe, 1971) and it is generally agreed that the primary

aim is to identify those who have had previous episodes of back
or sciatic pain, This is based on the finding that the probabil-
ity of episodes of. back pain increases by a factor of 3 or 4
after the first reported attack (Dillane, et al., 1966). However,
other than the scars of surgery, there are few reliable and ob-
jective signs of previous back problems, and the medical history
is often of skeptical value for this purpose (Rowe, 1971).

After some type of evaluation, assuming no gross abnormalities
have come to light, new employees are certified as fit for gen-
eral work, still subject to training. It is comparatively rare,
unfortunately, that the orientation and training period is under
medical supervision. It is proposed that with such supervision
during the first few days on the job, many postural stress related
problems could be prevented. Clearly, for any physical work which
is unfamiliar, a period of adaptation and conditioning is needed.
Tolerance for postural stress, and for kinetic stress arising

from rapid trunk movements, is likely to increase over a period

of days or weeks. Similarly, the magnitude and frequency of the
loads which can be handled without discomfort may increase with
physiological adaptation and the acquisition of skill. However,
the processes of adaptation to postural induced kinetic stresses
may lag (scientific evidence is limited in this regard).

It is recognized that selection must be concerned with both the
initial screening and placement of employees and their acclimit-
ization to the physical stresses of the job. Further, very few
companies are now capable of such aggressive Management. For-
tunately, some are developing and evaluating formal selection/
placement/conditioning and training programs. It is clear that
these efforts must be encouraged.

Criteria for Physical Assessments
There are many different methods by which a concerned physician
may evaluate a person's capability to handle heavy loads safely
in a future job. Some of these methods may have merit. Others
are of questionable value. In providing any such assessment it
is important that certain medical, social, economic, and legal
criteria must be met. In choosing between alternative methods,
it is suggested that the following criteria be applied:

abo Is it safe to administer?

2. Does it give reliable, quantitative values?

Jo Is it related to specific job requirements?

4. Is it practical?

Elo Does it predict risk of future injury or illness?

90


Another biomechanical fact illustrated in Figure 3.2 is a large
shoulder torque (and therefore shoulder muscle flexor force)
produced due to the load acting through a large moment arm. The
average male arm length is about 63 cm to the center of grip.

This means that each shoulder torque is about 630 kg-cm (i.e.,

63 cm multiplied by 10 kg). If one includes the weight and dis-
tribution of the masses of the arm, this value becomes closer to
734 kg-cm. Empirical investigations have found this torque exceeds
the voluntary strength capabilities of 90% of the female industrial
population and about 40% of the male industrial population (Chaf-
fin and Baker, 1970; Martin and Chaffin, 1972). The major point,
then, is that the shoulder joint is not well suited to withstand
high forces when flexed or, as discussed by Tichauer (1978) when
abducted.

One might suspect that such lifting requirements do not exist in
industry today. One example of exactly this situation is shown
in Figure 3.3. The layout of many machines, materials handling
equipment and storage devices often compels the operator to
assume biomechanically awkward and potentially injurious postures.
Because the required posture has caused the worker to be strain-
ing himself close to his expected arm and shoulder strengths, any
sudden slip of the object either could cause an overstrain injury
or the object could fall onto the worker's foot. (This also il-
lustrates the mutual concern for both health and safety in most
operational situations.)

Figure 3.3: Illustration of lifting task requiring high arm and
shoulder strengths

26


REFERENCES

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Handling." Ergonomics Research Society Annual Conf., 1961.

Aberg, U.; Elgstrand, K.; Margnus, P. and Lindholm, A. “Analysis
of Components and Prediction of Energy Expenditure in Manual
Tasks." Int. J. Prod. Res. 6:189-196, 1968.

ACGIH. Threshold Limit Values for Physical Agents. Adopted by
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Ad Hoc Committee on Low Back X-Rays. "Low Back X-Rays: Criteria
for Their Use in Placement Examinations in Industry." J. Occup.
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Alston, W.; Carolson, K.E.; Feldman, D.J. Grimm, Z and Gerontinos,
E. "A Quantitative Study of Muscle Factors in the Chronic
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American Heart Association. Exercise Testing and Training of
Apparently Healthy Individuals. A Handbook for Physicians,
1972.

American Industrial Hygiene Association. "Ergonomics Guide to
the Assessment of Metabolic and Cardiac Cost of Physical
Work." Am. Ind. Hyg. Assoc. J. 32:560-564, 1971.

Anderson, T.N. “Human Kinetics in Strain Prevention." J. Occup.
Safety. 8:248, 1970.

Apple, J.M. "Materials Handling." In: Industrial Engineering
Handbook. H.B. Maynard, Ed., McGraw Hill, New York, 1971.
Chapt. 11.5.

Aquilano, N.J. "A Physiological Evaluation of Time Standards for
Strenuous Work as Set by Stopwatch Time Study and Two Pre-
deternimed Motion Time Data Systems." J. Indust. Eng. 19:
425-432, 1968.

Armstrong, J.R. Lumbar Disc Lesions. Williams and Wilkins Co.,
Baltimore, 1965.

Asmussen, E. and Heeboll-Nielsen, K. "Isometric Muscle Strength
of Adult Men and Women." Communications from Testing and
Observation Institute of the Danish National Association
for Infantile Paralysis. 11:1-44, 1961.

Asmussen, E. and Heeboll-Nielsen, K. "Isometric Muscle Strength
in Relation to Age in Men and Women." Ergonomics, 5:167-169,
1962.

155


- the individual's skill level--training and
experience on the job

- the individual's activities outside of work--
second job, housework, etc."

In industrial work there is the additional problem of static
muscular effort (posture maintenance and holding of worklaods)
which reduces blood flow. This would indicate that aerobic
capacities for this type of work would in general be lower
compared to the dynamic capacities assumed in the above
studies (Lind, et al., 1979; Williams, et al., 1980).

For the purposes of this Guide, the lower (33 percent of
aerobic capacity) will be assumed for 8 hour work duration.

Working Time Prediction

With reference to a normal, healthy, 35-year-old working man,
three limitations in physical work capacity as a function of
working time were proposed by Bink (1962) and Bonjer (1962):

1) An upper energy work limit of 16 Kcal/minute for
four minutes (i.e., aerobic capacity)

2) an eight-hour continuous work limit of 5.2 Kcal/
minute, which is 33 percent of (1).

3) a 24-hour performance limit of 2.85 Kcal/minute
(based on dietary considerations, (V.V.Vv. 1958).

The resulting logarithmic relationship of working time to
work capacity is illustrated in Figure 4.2. The use of such

a logarithmic relationship of the working time and the average
work capacity has also been discussed by Bonjer (1971) and
Moores (1970).

An example application of the working time prediction concept

is illustrated with a coal mining job consisting of the
following productive activities (Garry, 1952).

Average Metabolic Weighted Metabolic

ISRENESS/ % of Time Rate (Kcal/min.) Rate (Kcal/min.)
Loading 42% 6.3 2.64
Standing 7% 1.8 0.12
Walking 23% 6.7 1.54
Hewing 4% 6.7 0.27
Timbering 24% 5.7 1.37

Predicted Average Metabolic Rate = 5.94 Kcal/min.

51


Figure 8.11: Example 3, Horizontal Obstruction.

139


The study also examined worker selection and training practices
and concluded

"No significant reduction in low back injuries was
found in employers who used medical histories,

medical examinations, or low back x-rays in select-
ing the worker for the job. Similarly, no signifi-
cant reduction in low back injuries was found in
employers who trained their workers to lift properly."

The topic of muscle strength and capacity is thus an important
one. Chapter 5 is devoted to a detailed discussion of worker
strength capability as a basis for job design.

SUMMARY

A wealth of literature can be identified which relates manual
materials handling to musculoskeletal injuries in the workplace.
With particular reference to low-back pain, this chapter presents
an overview of but a few studies. More extensive bibliographies
can be found in Brown (1972), Herrin, et al., (1974), and Drury
(1978).

Due to the problems of measurement and interpretation of the "low
back pain syndrome" it is recognized that longitudinal studies
provide the most reliable estimates of hazard and risk. It is
concluded from recent longitudinal studies that heavy load lifting
contributes to increased frequency and severity rates for low-back
pain. This is true regardless of the repetitive or dynamic nature
of the lifting. If, however, such lifting is performed repetitive-
ly, the medical hazard extends beyond low-back problems to other
musculoskeletal strain/sprain injury risk, particularly for

weaker workers.

In this latter regard, gender, age, and anthropometry are known to
modify these risks for populations of workers. The inherent
variability between workers and within any worker over time pre-
clude the use of such factors to assign risk to any particular
individual. Strength testing, however, is supported as one means
for identifying high risk workers who need to perform manual
materials handling. Studies to carefully document the effectiveness
of this form of selection procedure, however, are still needed.

22


Table 4.1: Average energy cost while performing selected
activities. Values apply for 70-kg (154-
pound) man. For most activities adjustment
for cost is proportional to body weight.
(Durnin and Passmore, 1967)

Total
Energy
Cost Keal/min
Body Position and Activity Typical Range
Heavy activity at fast to maximum pace 10.0-20.0
Jogging, level, 4.5 mph 7.5
Lifting, 20 kg, 10 cycles per min.
floor to waist 8.2
floor to shoulder 10.8
Reclining, at rest aboe)
Running, level, 7.5 mph 12.7
Shovelling, 18 1b. load 1 yd. with l-yd.
lift, 10 times per min. 8.0

Sitting, at ease
light hand work (writing, typing) 1.7 1.6-1.8
moderate hand and arm work (drafting,
light drill press, light assembly,

tailoring)
light arm and leg work (driving car
on open road, machine sewing) 2.8 2.5-3.2
heavy hand and arm work (nailing,
shaping stones, filing) oS 3.0-4.0
moderate arm and leg work (local
driving of truck or bus) 3.6 3.0-4.0
Standing, at ease 1.9
moderate arm and trunk work
(nailing, filing, ironing) Sot 3.0-4.0
heavy arm and trunk work (hand
sewing, chiselling) 6.0 4.0-8.0
Walking, casual (foreman, lecturing) 3.0 2.5-3.5
moderate arm work (sweeping,
stockroom work) 4.5 4,0-5.0

carrying heavy loads or with heavy
arm movements (carrying suitcases,

scything, hand-mowing lawn) 7.0 6.0-8.0
transferring 35 1b. sheet materials 2
yds. at trunk level, 3 times per min. 3.7
pushing wheelbarrow on level with
220 lb. load 5.5 5.0-6.0
level, 2 mph Bods
3 mph 4.0
4 mph 5.9.

54


Table A.2: Maximum Permissible Limit Sensitivity Analysis

ISOMETRIC
_% CAPABLE

181


Is the test safe to administer? Perhaps one of the most widely
used procedures in the past for evaluating a person's physical
capability, the low-back X-ray, provides an example of

these criteria. As will be discussed later in this section, the
evidence which supports the use of such X-rays for routine pre-
employment and placement screening is usually insufficient to
warrant the procedure, especially when the radiation hazard is
considered.

Tests of a person's physical strength and endurance have also be-
come popular in many industries, primarily because they can meet
some of the other criteria. It is imperative that such tests be
carefully evaluated to assure that they are safe. For instance,

a test which sets a specific goal for the worker, (i.e., the

person must lift an object of specified weight to obtain a job),

is a situation which may produce overexertion injury in an overly
motivated subject. On the other hand, if there is no specific

goal stated or direct feedback given as to how great the exertion
is during the test, as recommended by the AIHA Ergonomics Committee
(Caldwell, et al., 1974), isometric strength testing has proven

to be safe in several studies (Chaffin, 1974; Chaffin, et al.,
1978; Laubach, 1976). It is recommended, however, that any such
tests be included as part of a medical examination only after

the person's medical history has been screened for any past history
of musculoskeletal or coronary problems, and a general physical
examination has been performed.

Does the test give reliable, quantitative values? Though clinical
impression is important in assessing a person's physical capability,
specific tests with quantitative scoring should be performed.
Stereotypes based on age, gender, and body weight have all been
shown to be only weakly correlated with physical capability to
exert high forces (Chaffin, et al., 1978; Bernauer, et al., 1975;
Laubach, 1976, and Kamon, et al., 1978).

A test of physical capability, like any other diagnostic test,
should produce repeatable results. A common measure of such is
the coefficient-of-variation of the repeated tests, which is the
standard deviation of the repeated values divided by their mean
value. This is expressed as a percentage, and thus, represents
the percent error in the test due to measurement procedures. It
should be possible in physical capability tests to achieve a co-
efficient-of-variation of less than 15% (Chaffin, et al., 1977),
with well-controlled laboratory tests often achieving 5%.

Is the test related to specific job requirements? It has been
disclosed by several studies that one physical attribute of a
person does not correlate well with another. For instance, grip
strength is a poor predictor of other strengths (Laubach, 1976).
Considering anthropometric and specific strength values together,
one can achieve a better prediction of the strength scores achieved
by people performing physical acts common to industry but a high

91


Brown, J.R. Lifting as an Industrial Hazard. Labour Safety
Council of Ontario, Ontario Dept. Labor, Ontario, Canada,
1971.

Brown, J.R. "The Contributory Factors to the Development of Low
Back Pain in Industry." Paper presented at the American Indus-
trial Hygiene Association Meeting, 1974.

Brown, J.R. "Lifting as an Industrial Hazard." Am. Ind. Hyg.
Assoc. J. 34:292-297, 1973.

Brown, J.R. "Manual Lifting and Related Fields: An Annotated
Bibliography." Labour Safety Council of Ontario, Ontario
Ministry of Labour, Ontario, Canada, 1972.

Brown, J.R. "Factors Involved in the Causation of Weight Lifting
Accidents." Ergonomics. 2:117-118, 1958.

Bureau of Labor Statistics. Arkansas Workers Compensation-Supp.
Data Sys. 1976.

Bureau of Labor Statistics. 1976 SDS Report for Arkansas, 1978.

Caldwell, L.S. "Body Position and the Strength and Endurance of
Manual Pull." Human Factors. 6:479-483, 1964.

Caldwell, L.S.; Chaffin, D.B.; Dukes-Dobos, F.N.; Kroemer, K.H.E.;
Laubach, L.L.; Snook, S.H. and Wasserman, D.E. "A Proposed
Standard Procedure for Static Muscle Strength Testing." Am.
Ind. Hyg. Assoc. J. 35:201-205, 1974. ~~

Caldwell, L.S. and Grossman, E.E. "Effort Scaling of Isometric
Muscle Contractions." J. Motor Behav. 5:9-16, 1973.

Caldwell, L.S. and Smith, R.P. "Subjective Estimation of Effort,
Reserve and Ischemic Pain." U.S..Army Medical Research Lab.
Report No. 730, Ft. Knox, Ky. 1967.

Chaffin, D.B. "A computerized Biomechanical Model: Development
of and Use in Studying Gross Body Actions." J. Biomech. 2:
429-441, 1969.

Chaffin, D.B. "Some Effects of Physical Exertion." Dept. of
Industrial Engineering, University of Michigan. 1972.

Chaffin, D.B. "Human Strength Capability and Low Back Pain."
J. Occup. Med. 16:248-254, 1974.

Chaffin, D.B. "Manual Materials Handling and Low Back Pain."
In: Occupational Medicine, Principles and Practical
Applications. C. Zenz, Ed. Year Book Medical Publishers, Inc.
Chicago, 1975. Chapt. 19.

158


1. Transportation of Raw Materials. Each delivery of mate-
rials to the plant whether by road, air, rail or water
requires material moving equipment and people. Often
such transportation of raw material is performed by out-
side carriers over which the plant management has little
control. Still, their interaction with the manufactur-
ing process must be recognized as important.

30 Receiving. Unloading the material from the transporting
vehicle and moving the materials to their assigned places
constitutes materials handling. Furthermore, inspection
for weight, quality, quantity, etc., generally invdélves
handling operations. Often, the handling itself takes
more time than the inspection or storing action.

So In-process Handling. The handling of material within

the plant and particularly between work stations con-
stitutes major materials handling problems with respect
to effort, cost, and danger of accidents. Pieces may

be moved many times in manufacturing plants either as
single elements or combined with others in subsystems

or units. Storage of in-process parts between operations
is a very critical area of materials handling.

4. Handling at the Work Place. This is one of the most
overlooked but costly operations in materials handling.
It usually involves manual movement of material among
fixtures and jigs, into and from machines, and the dis-—
posal of by-products and wastes. This is a phase in
materials handling which contributes significantly to
injuries and accidents.

Bo Handling of Wastes and By-products. Rejects, wastes .and
by-products produced in the manufacturing process, or
used packaging material, must be moved and disposed.
This may involve sorting of materials that can be used
again or recycled, and of those materials that are dis-
carded, This is also an often overlooked aspect of
materials handling.

6. Warehousing. The storing, stocking, order picking, as-
sembly of orders, packing and movement out of the ware-
house are material handling operations.

Uo Distribution. The transportation of the finished or
Processed goods and materials is the last part of mate-
rials handling in the manufacturing cycle. This usually
involves, like the transportation of raw materials,
road, rail, air or water vehicles, and is often done by
outside carriers.

Materials handling systems can be classified in Many ways by their
function and purpose. The International Material Management Society

113


often are insufficiently developed to allow the person to "lift
with the legs" when instructed to do so. Thus, the rule about
"lifting with the legs while keeping the back vertical" must be
qualified to include the physiologic fact that many people will

not be able to perform such lifting without first increasing their
leg strengths. In addition, with some individuals, muscle-stretch-
ing exercises will be needed to provide the necessary range of
motion in the knees, hips and ankles. It should also be recognized
that lifting from a squat position will require lifting of the
torso, thus requiring extra energy expenditure, as discussed in

the next chapter.

A second, more complex qualification on the classic "squat lift"
rule must also be realized when lifting large objects that cannot
pass between the knees. Studies by Park (1973) and Park and
Chaffin (1974) disclose that when a large object is lifted around
the front of the knees, as required in the squatting type of leg
lift just described, it necessarily causes the moment arm of the
load about the low back to be large. This causes the moment at
the low back to be large, and hence high spinal compressive
forces and muscle forces result. In contrast, the more often
used stoop-back method of lifting allows the person to "move over"
the weight to be lifted and thus reduce the load moment arm about
the lower back. Figure 3.8 illustrates this concept. For the
calculation of the forces in the example, a 15.5 kg (35 lb.) load
is being lifted from a position that is 38 cm (15 inches) in
front of the ankles and 38 cm (15 inches) above the floor. Nowii-
nal anthropometry and abdominal assistance values are assumed,

as described by Chaffin (1975). It can be seen that the "stooped-
over" position results in about one-third less compressive stress
on the low back than the squatting type of lift. It should also
be evident that the stooped-over position allows the person to
reduce the load moment arm of 35 cm (14 inches) about the low
back even further by moving in over the load more than is shown,
whereas the load moment arm of 50.9 cm (20 inches) for the squat
lift is as small as possible due to interference of the upper

legs and the load. A further limitation on lifting large objects
with a squat lift arises from the fact that the arms must be ex-
tended farther in the forward horizontal direction than with the
stooped-over posture. As discussed earlier, such a position of
the arms means that a high torque will be produced at the shoulders,
which may not have the strength to move the load upward. There-
fore, the person normally will lean forward more to lessen the
load moment arm about the shoulders, and in so doing will cause
greater stresses on the low back both by the effects of gravity
acting on the torso mass and by hyperflexing the lumbar column.
Such hyperflexion places a greater stress on the posterior posi-
tions of the annulus of the disc, thus distributing the compres-
sive loads unevenly within the disc. As Davis and Troup (1965)
have described moderate flexion of the torso does provide effec-
tive abdominal pressure assistance during lifting, thus reducing
the low-back stresses. Therefore, some torso flexion appears to be

32


Shoulder
Horizontal

Trunk Lateralr
Bending!
Angle!

Figure 5.3: Body Angles Used to Depict Posture (Garg and
Chaffin, 1975).

and hence the user can easily determine when balance is critical
to task performance.

The final constraint in the model is based on evidence that lumb-
ar compression forces may limit a person's volitional capability
as discussed in Chapter 3. Thus, an assessment similar to the
Morris, Lucas and Bresler Model of low-back compression has been
included in the strength model, with acceptable compression limits
being selected by the user.

Figure 5.4 illustrates the strength capabilities of a strong 2.5
percentile male (97.5% of males have a lower strength capability)
predicted with a two-dimensional form of the model (Chaffin, 1974).
Similar force contours could be defined for other population per-
centiles and anthropometric characteristics (i.e., age, gender,
body weight, stature, etc.). These are summarized at the end of
this chapter.

In summary, this model produces three key pieces of information.

First, it allows the rank ordéring of the gross strength require-
ments of the various tasks involved in a job. Second, it identifies

74


Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

Table

1.1:

1.2:

JoeB

1.4:

4.1:

5.1:

5.2:

5.33

5.4:

Bos

LIST OF TABLES

ILO suggested limits for occasional weight
lifting (kilograms) (ILO, 1962)........seeees

Wisconsin occupational injury and illness
compensation records (Workmen's Compensa-
tLONEDAVI STON wel O16) ieteledeleletetslsleleteletetisteletetetotete

Arkansas 1976, Supplemental Data Systems
REPOL tw (BLS/ Mel 97/6) lelelelelels}slelelola}elolotelolellsieteteisle}el

Estimated work force performing manual
materials handling activities in Wisconsin
(Wisconsin Bureau of Research and
Statistics, 1974) ...c.cscesccrcccccsccsevece

Average energy cost while performing
selected activities. Values apply for
70-kg (154-pound) man. For most activities
adjustment for cost is proportional to

body weight (Durnin and Passmor@ 1967)......

Major factors affecting metabolic energy
expenditure rate. cccccccccccccccssecccenace

Maximal voluntary isometric strength
(kilograms) .. cece cece cece ccc cece cceeacece

Prediction equations using anthropometry
(Keyserling, et al., 1978) ...ccececceccceee

Maximum acceptable weight of lift for
Males (kg) (Snook, 1978) ...cceecccencccecces

Maximum acceptable weight of lift for
females (kg) (Snook, 1978) .....ceseececceees

Mean and standard deviation of maximum
weights (kg) of lift acceptable to male

and female industrial workers (corrected
for one lLift/mMin.) ..ccce cere cece cece cecccace

Summary of dynamic strength regression

MOdE1S. ccecesnccsccecccccccccncceccscesscces

xi

54

55

70

71

78

79

80

81


Contents (cont.)
Chapter 5: BASIS FOR GUIDE: PSYCHOPHYSICAL APPROACH......

PSY.CHOPN YS 1 CSiteleleleloleleleleteieleleleteleteleleleleleiclelelsReleleteleteteleicteteteiers
Static Strength......
Static Strength Models.....
Dynamic Strength. ...sseceee
Dynamic Strength Models......
Psychophysical Design Criteria.....ssssscseececeseeeee

Chapter 6: ADMINISTRATIVE CONTROLS......eeeeccccccccccaces

Selectionyof Workers!2\\-)-<)<lolslololel clolelslol=lelolelelele)s)elol=)sele}=1<) =
Training OL WOLKELS!\lllelelelelslelolelelsleleislelslolelelelelelsyelelels}elslel=)>)

Chapter 7: ENGINEERING CONTROLS......ceccccccccccccccccens

The Mechanical Environment.......ceccccecccsccsccceces
The Visual Environment..........0.
The Thermal Environment. eee ceeceee
Materials Handling System Alternatives....
Potential Safety and Ergonomics Problems...........006

Chapter 8: RECOMMENDATIONS.....cceccsccccecccccccececceces

Definition of a Lifting Task. ....ceeeseccccccccccccece
Lifting Task Variables.....
Criteria for Guideline..
Guideline Limits...............
Job Physical Stress Evaluation.
Examples...ccccccscccccccscccece
Administrative Controls........
Ergonomic/Engineering Controls.
CONCLUSION. . se secerececccvcccccccrccccces

References.

APPENdix. coeecccececsccccccccccccccccccccscusescsescccccces

iv

67

67
68
71
77
78
83

89

89
99

103

103
109
110
112
116

121

121
122
124
126
129
135
145
148
p53)

155
175


PHYSICAL STRESS JOB ANALYSIS SHEET

DEPARTMENT__AKE Hose pate __ 2/8/80
JOB TITLE ZEFALLET/ZIVG__—ANALYST'S NAME __@24

HAND LOCATI
DESCRIPTION welgut iia peste aa AL] MPL|| REMARKS
Ave Max) em cm| cm cm
Uoao Pauers (2 fos

TieR 1 5 |S [3s|ol|3s}s 24

TIER Z 5 | 5 13s| 40] 3s) 50} 24

TIER 3 S |S |35/8|3s| 50] 24

TieR 4. S |S |35|n0/|35] 50] 2.4

TIERS 5 |S I3s 400|35 | 50 | 2.4 8 |-2¢

Terau = 35 | 412 47 14

Figure 8.13: Example 4, Analysis Sheet.

be larger since this vertical origin is most distant from 75 cm.
Note that with more complicated tasks such a simplification will
not necessarily be possible. It is only possible with this job
since all other variables remain constant.

For this most stressful task, then

AL 40 (15/35) (.66) (.7+7.5/110) (1-2.4/18) = 8 kg

MPL = 3 (8) = 24 kg

It is concluded that individually the elements of the job are
quite reasonable (5 kg is below the AL).

Now consider the tasks collectively. The following approximation
is not exact but should provide a reasonable composite estimate,
Derive a weighted average for each variable in the job analysis
according to frequency. In this case frequency is constant across
tasks and vertical origin and travel distance are the only factors
which vary.

141


bearing cartilage end-plate's) ability to withstand such stresses.
Figure 3.6 illustrates these data by age group. In general, the
data from male cadavers under 40 years of age disclose a mean of
about 675 kg before the cartilage end-plates begin to exhibit
microfractures. The fracture levels range, however, from as low
as 250 kg (over 60 years of age) to more than 950 kg (under 40
years of age). Sonoda (1962) estimates that the female's spinal
compression tolerance is about 17% less than the male's. This
would be consistent with the smaller force-bearing area of the
vertebral bodies in a woman's spine.

To

1000

800,

2)

fo}

Oo
T

AT Lg/S, LEVEL
>
fo}
fo}
q

200-

COMPRESSIVE FORCES RESULTING
IN DISC-VERTEBRAE FAILURES

=
on <40- 40-50 50-60 >60

AGE

Figure 3.6: Mean and Range of Disc Compression Failures by Age
(Adapted from Evans, 1959, and Sonoda, 1962).

29


Other types of mechanical aids are:

a. Conveyors are used primarily when loads are uniform,
movement is from one point to another, and materials
move constantly.

b. Cranes and Hoists are most commonly used when loads
vary in size and weight, moves are intermittent, and
cross traffic will interfere with conveyors.

c. Industrial trucks are most often used when materials
are moved intermittently, over varying routes, when
cross traffic would prohibit conveyors, and loads are
uniform or mixed in size and weight.

Potential Safety and Ergonomic Problems

Reducing the physical hazards or manual lifting to the musculo-
skeletal system can often result in other types of safety problems.
Some of these are:

a. Conveyors: Aside from mechanical entrapments, the
major problem with conveyors is that the work is extern-
ally paded. This can cause time-stress induced errors.
Conveyor designs which take into account the cycle-to-
cycle variability of the operator can reduce such stress.
Buffer stocks between stations achieve the same effect.

b. Cranes and Hoists: Because of the dangers involved when
a load is suspended, cranes are among the most hazardous
of material handling equipment. The crane cab and all
controls should be designed for good visibility and
safe operation. The loading capacity of the crane should
be clearly marked. Standard signals should be used
between the crane operator and the signalman on the
ground.

c. Hand Trucks: Safety hazards include falling of the load,
collision between the truck and other objects, and the
operator getting pinned between the truck and another
object. These can be reduced by such design features
as: covering all exposed handles with knuckle guards,
guarding or recessing all wheels, brakes that apply
automatically when the handle is either fully raised or
fully lowered, or released.

152


5. Vertical Location. Not only is the net vertical distance
of the lift important but also the absolute vertical
heights of the beginning and end point of the lift.
Lifting the same net vertical distance from two different
vertical heights, may result in assuming different body
postures and varying amounts of movement of the center of
gravity of the body. For example, consider the following
two tasks (from Garg, 1976):

Task A - Lifting 4.5 kg from a vertical height of 91 cm
to a vertical height of 168 om at a pace of 25
lifts per minute.

Task B - Lifting 4.5 kg from the floor to a vertical
height of 91 cm at a pace of 21 lifts per minute.

In these two cases, the total mechanical work is practi-
cally the same (i.e., 86.6 and 86 kg-m). The average male
energy requirements for the two tasks were 3.56 and 6.77
Keal/minute respectively (Aquilano, 1968), even though the
Task A was performed at a higher pace as shown in Figure
4.6 (Tasks 5 and 1 respectively). Here again, vertical
body travel is a critical factor in estimating metabolic
rate. Frederick (1959) suggested that the best area for
manual lifting is between 100 and 150 cm from floor level
for a standing man of average height. The physiological
efficiencies (expressed in Keal/unit of work) for four
different vertical ranges of lift as given by Frederik
(1959) are illustrated in Figure 4.7.

6. Temperature and Humidity. As previously mentioned, meta-
bolic energy expenditure encompasses two forms of energy.
One is mechanical energy, which is the basis of the motor
performance capability of man, and the other is heat. The
proportion of total energy which is converted to mechanical
energy varies from zero in static work to about 30 percent
for walking (Grandjean, 1969). This Guide assumes an
ambient environment of 70-80°F and 40-50% humidity.
Possible controls for heat stress are discussed in Chapter
Yo

MODELING TASK AND PERSONAL VARIABLES

Each of the task and personal variables described in the
previous section were combined into one predictive model for
lifting in a study of Garg, et al., (1978). According to this
model the metabolic rate for any task (Ep) consists of two
parts--the metabolic rate necessary to maintain posture Ep

and the energy required to:lift (AE) in the following form:

60


Although repeated strength tests on a particular muscle group are
highly reliable, the strengths of different muscles, even within
subjects, are only weakly correlated (Thordsen, et al., 1972;
Laubach, et al., 1972). In these studies, 51 subjects were asked
to perform maximal isometric exertions in 44 different tests. Of
the 768 simple correlations among the 44 measures taken, only 13
had correlation coefficients of .71 or higher (i.e., could explain
at least 50 percent of the variance). This finding indicates that
when it is necessary to determine a person's ability to perform a
particular job element, it is often more accurate to simulate the
job's activity in a strength test, rather than trying to predict
job strength from standardized tests.

A number of studies by Asmussen and Heeboll-Nielsen (1961), Back-
lund and Nordgren (1968), Chaffin (1974), Kroemer (1969), Laubach
and McConville (1969), Snook, Irvine and Bass (1970), Snook and
Ciriello (1974), Troup and Chapman (1969), Nordgren (1972) report
strength capabilities of various populations. Laubach (1976) sum-
marized each of these studies in a review of the literature. He
concluded that average female strength rangés between 35 and 84
percent of average male strength, depending on the nature of the
test and specific muscles involved. Averaging the results of all
nine studies, women were found to demonstrate only about 64 per-
cent of the strength men demonstrate. Mean values however do not
reflect the variability of strength within each gender. When
this is accounted for, the problem becomes more complex as dis-
cussed below.

Recently, Keyserling, et al., (1978) summarized the isometric
strength of 1,239 workers in rubber, aluminum, steel and electron-
ic component industries as shown in Table 5.1. Attempts to pre-
dict these six isometric strengths based on individual worker
height (S), body weight (W), age (A), and gender (G) revealed that
anthropologic measures are not good predictors of strength. Cal-
culated coefficients of determination (R2) indicate that these
variables rarely explain more than one-third of the population
variance as shown in Table 5.2.

Gender, however, is an important factor in predicting strength,
with females being weaker than males. (Note the negative regres-
sior. coefficients involving gender.) Also, taller, heavier work-
ers are stronger (positive regression coefficients for the S$ x W
interaction) than their counterparts and body weight is detrimental
to strength with increasing age.

An important point with this table is that unexplained variability
between (among) and within (test-retest) particular individuals
was quite large (ranging from an average standard deviation of

6.4 kg for high far lifting to 29.0 kg for leg strength). Con-
sequently, it would be imprudent to use these anthropologic vari-
ables alone to predict how any particular individual would fare

on a particular strength demanding task.

69


NiGSkhi

TECHNICAL REPORT

Work Practices Guide
for Manual Lifting

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service

Centers for Disease Control

National Institute for Occupational Safety and Health


Strength Testing

Many industries have jobs that require occasional lifting, wherein
for a few seconds a high amount of force is required. Common ex-
amples are loading stock into a machine, picking-up a tote pan
full of parts, lifting a broken machine component during mainten-
ance, or replacing the dies in a press. The activities are often
not frequent enough, or they can be paced by the worker so that
endurance, in the cardio-pulmonary sense, is not the limiting
factor. Rather, one is concerned with the brute force required

to successfully and safely accomplish the task.

Recent studies have indicated that such force requirements
increase both the frequency and severity of musculoskeletal pro-
blems, and particularly low-back pain incidents in industries

of various types (Hult, 1954; Chaffin, et al., 1977; Miner, et al.,
1978). These studies and others have substantiated the need to
more carefully select and place employees on jobs requiring high
forces. What follows is a description of the development of iso-
metric strength tests for this purpose. It is presented as a
better example application of the preceding criteria.

The first step in developing a selection program is to determine
whether such is applicable. This determination often occurs in
two phases.. First, the medical and/or safety functions should
perform a statistical analysis of. musculoskeletal problems in the
organization and determine if these problems are particularly
prevalent and/or severe. As reported in Chapter 1, a recent anal-
ysis of compensation injuries in the state of Arkansas disclosed
that approximately one-third of all injuries and illnesses report-
ed in all industries were musculoskeletal strains and sprains
(Bureau of Labor Statistics, 1976). Some industries, however,
report a prevalence of over 60%. Once it is decided that the
prevalence and/or severity data are excessive, then a second phase
of analysis is initiated. This involves a further statistical
analysis of the injury data to determine if certain jobs (known

to require high exertions) have higher injury frequency rates and
severity rates than the more sedentary jobs in the plant.

At this point, a job analysis as outlined in Chapter g should. be
initiated to determine actual job requirements. These data
should be gathered in the systematic way described in Chapter 8
and made part of the job description record.

Without a specific physical description of the job it is not possible
to develop a yalid selection/placement procedure. Variations in

work postures will result in specific stresses on the musculoskel-
etal system. A test of one type of strength selected for its
administrative simplicity may not predict the type of strength
required on the job if the test does not also reflect the most
strenuous postures required on the job. Given that the job strength
requirements are documented for those jobs having high musculo-
skeletal problems, then valid tests of employees can be developed.

95


Tichauer, E.R. The Biomechanical Basis of Ergonomics. John Wiley
and Sons, Inc., 1978.

Tichauer, E.R.; Miller, G. and Nathan,I. "Lordosimetry: A New
Technique for the Measurement of Postural Response to Mater-
ials Handling." Am. Ind. Hyg. Assoc. J. 34:1~-12, 1973.

Tilley, P. “Is Sacralization a Significant Factor in Lumbar Pain?"
Jd. Am. Osteop. Assoc. 70:238-241, 1970.

Troup, J.D.G. "Mechanical Factors in Spondylolisthesis and Spon-
dyolysis." Clin. Orthop. 117:59-67, 1976.

Troup, J.D.G. and Chapman, A.E. "The Static Strength of the Lum-
bar Erectores Spinae." J. Anat. 105:186, 1968.

Troup, J.D.G. and Chapman, A.E. "The Strength of the Flexor and
Extensor Muscles of the Trunk." J. Biomech. 2:49-62, 1969.

Troup, J.D.G.; Roantree, W. V. and Archibald, R.M. "Survey of
Cases of Lumbar Spine Disability: A Methodological Study."
Medical Officers Broadsheet. National Coal Board, Feb. 1970.

Troup, J.D.G.; Hodgson, S.; Shannon, H.S. and Chapman, M. "Lum-
bar Spinal Radiology in an Epidemiological Survey of 326
Dock Workers." Unpublished Data, 1974.

U.S. Department of Labor. "Teach Them to Lift." Safety in Indus-
try Series. Bull. 110, Wage and Standards Admin. , Bureau
of Labor Standards, Revised, 1970.

U.S. Department of Labor. Handbook of Labor Statistics. 1977.

VanCott, H.P. and Kincade, R.G. Human Engineering Guide to Equip-
ment Design. U.S. Government Printing Office, Washington,
D.C., 1972. Revised Edition.

VanderWalt, W.H. and Wyndham, C.H. "An Equation for Prediction
of Energy Expenditure of Walking and Running." J. Appl.
Physiol. 34:559-563, 1973.

Voorlichtingsbureau voor de Voeding. "Nederlandse Voedingsmid-
delentabel." Voeding. 19:63, 1958.

Westrin, C.G. "Low Back Sick-Listing: A Nosological and Medical
Insurance Investigation." Scand. J. Soc. Med. Suppl. 7,
1973.

White, W.M. “The Compensation Back." Appl. Therap. 8:871-874,
1973.

Whitney, R.J. "The Strength of the Lifting Action in Man." Ergo-
nomics. 1:101-128, 1958.

sz)


many accidents. Human engineering data and recommendations shoudl
be made as the basis for design or purchase of a new fork-lift
truck. In its operation, many mishaps can be avoided by using
“safe practices" in training the new operator, in monitoring the
operator's performance, and by enforcing safe practices by positive
(reward) and negative (punitive) control measures (Lovested, 1977).

Again many of these problems can be alleviated by the intelligent
application of ergonomics/human factors principles. The environ-
ment in which the industrial truck operates can have a large effect
on its efficiency and safety. Drury and Dawson (1974) evaluated
control performance of five powered fork trucks and found that
considerable lateral room was required even to drive them consist-
ently in a straight line. For drivers concentrating totally on
vehicle control, an aisle width of 0.5 m greater than truck or load
width was required. In more realistic situations where the opera-
tor must monitor the load stability, plan his/her course through a
factory and be alert to avoid other traffic much greater clearances
are needed.

For human powered vehicles, Drury, Barnes and Daniels (1975) found
that large wheels gave easier handling and that, again, adequate
clear aisle widths had to be provided. Handles should be about 1
meter above the floor for maximum force exertion. With any human
powered vehicle flatness of floors is essential and special care
must be taken to avoid sudden floor steps of even a few millimeters
as they can cause large increases in the force necessary to push

or pull the vehicle.

120


a. work surface materials

b. surface coating

Cc. floor condition

d. floor angle

e. shoe sole/heel composition and contact area
£30 shoe style

The general recommendation is to adjust shoes and work-
ing surfaces to give a coefficient of static friction
of at least .4 and preferrably .5.

Unnoticed changes in surface friction can also cause
accidents. These can be reduced by:

a. ensuring that different surface materials or coat-
ings have transition zones between them,

b. clearly marking any surface friction change,

Go using good housekeeping procedures to reduce trans-
ient changes in surface friction such as spills,
worn spots, loose or irregular floors.

Because of the forces transmitted from the container to
the body when lifting, good work/floor coupling is
essential in controlling accidents and injuries result-
ing from foot slippage.

The Visual Environment

While manual materials handling operations rarely demand the fine
visual discrimination of delicate assembly work, they do require
control of the visual environment for optimum performance and
safety. The task of lifting involves vision of the container,
the workplace around the container and the surface on which the
operator stands. An illuminance of 150 lux (14 foot-candles) in
each of these areas is a recommended minimum.

Types of lifting can have a large effect on visual performance,
particularly in the areas of depth perception and surface texture
perception. Differences in contrast or differential illumination
can be used to make stairs or changes in walking surfaces easily
discernible. Low, angled illumination is recommended for enhanc-
ing surface texture to warn operators of changes in shoe/surface
friction. Depth perception errors can be controlled by changing
reflectance of the container and task to provide additional con-
trast. Color contrast can be used on edges of steps, loading docks
and ramps when the. consequences of misperception are severe.

150


CONTENTS

DN o}-5 ob of Co) oe v
List of Illustrations.
List of Tables....... see
FOLEWOrd. cee eeeeee
Acknowledgement... ccccccccccccccenccceccccscccccsesccanccce xV

Chapter 1: JUSTIFICATION AND SCOPE......cccccccscceccccces al
Justification and Rationale for this Guide............ 1
Exposed Population.. 0000 seeee eee 4
recy eis CRESS 000000000 00000000 000090000000000000000 7

Chapter 2: BASIS FOR GUIDE: EPIDEMIOLOGICAL APPROACH..... 9
Short Term Effects of MMH on Health.......eeececeseeee 9
Long Term Effects of MMH on Health.. 10
EEG 1815 G5G000q000000000000000000000 ola,
Job Risk Factors.... 14
Personal Risk Factors eee 16
SUMMALY. . 2c cee ee ccceccccccccsccccsccccvesccsscccccnses 22

Chapter 3: BASIS FOR GUIDE: BIOMECHANICAL APPROACH....... 23

Occupational Biomechanics of Load Handling... 23
Low Back Stress. . 27
Posture Effects..... 31
Asymmetric Lifting... 34
Dynamic Lifting...... 34

Biomechanical Design Criteria..cssccccnccvccccvecccece £315)

Chapter 4: BASIS FOR GUIDE: PHYSIOLOGICAL APPROACH....... 41

Dynamic Exercise........2eeeeeee 00000 0000 41
Isometric ExerciSe....... ccc cee eee een ees 0000000 44
Physiological Responses to Lifting Weights.. cease 45
Determination of Work Capacity Limits....... 550 48
Modeling Task and Personal Variables........ 900 60

Physiological Design Criteria........ cece cee cece ewe eee 64

iii


Table A.2 (cont.)

LOWER {UPPER

182


Isometric strength tests are preferred due to the safety criterion.
In an isometric test the subject is required to slowly increase the
force exerted until they reach a level which "feels" safe. No spe-
cific feedback or challenges are given during the testing. This
procedure has been proposed in an AIHA Ergonomic Guide as being a
safe and reliable procedure (Chaffin, 1975). It has been used in

a number of industrial studies (see Chapter 5) testing over 3000
individuals and no injuries have been reported with the above
procedure.

As to whether such testing is a valid indicator of potential risk
of future injury, two longitudinal studies have been performed.
Collectively, these have involved nearly 1,000 workers in both a
light and heavy products industries (Chaffin, 1974; Chaffin, et al.,
1978). These studies have revealed that both frequency and sever-
ity rates of musculoskeletal problems were about three times
greater for those workers who were placed on jobs that required
physical exertions above that demonstrated by them in the isometric
strength tests, when compared with workers placed on jobs having
exertion requirements well below their demonstrated capabilities.

The practicality of strengh testing depends on the sophistication
of the medical and personnel functions. An isometric strength
testing apparatus can be purchased for between one and four-thou-
sand dollars, depending upon the sophistication and reliability
required. A space in the medical department must be provided which
is private. A nurse or medical technician needs to be trained

to perform the tests following the directions proposed in the

AIHA Ergonomics Guide (Chaffin, 1974). Instructions and admin-
istration will normally require between 14 and 30 minutes depend-
ing on the number of tests.

Perhaps more important than the time and capital outlay for the
testing apparatus, however, is the need to have the jobs evaluated
in a way that dictates which types of strength tests are applicable.
This requires a close coordination of the medical function and the
work practice or industrial engineering functions. The latter
groups must perform a rigorous evaluation of the high incidence
jobs and document the actual strength requirements, even on "oc-
casional lifting tasks". In so doing, they must also assure that
the tasks so documented cannot be easily modified to reduce the
physical demands. In other words, the tasks must be truly in-
herent to the processes, and as such, require excessive capitol
expenditure to redesign.

Aerobic Capacity Testing

The physical endurance of the worker becomes critical when the job
requires much high frequency, whole-body activity. When this is
the case, it is important that the individual involved can sus-
tain the required level of activity for an extended period of time
without fatigue. To achieve this the individual must be able to
intake enough oxygen and transport it through the blood stream at
a rate sufficient to meet the oxygen demands of the active muscles.

96


Gender

The literature reveals that the gender of a worker may be related
to the risk of overexertion injury. As mentioned in Chapter 1,
both the ILO (1967) and more recently the U.S. Department of Labor
(1970) recommended that women not lift as much as men. It appears
to be accepted that, on the average, a woman's lifting strengths
(primarily arms and torso strengths) are about 60% of a man's
according to Asmussen and Heeboll-Neilson (1962), Chaffin (1974),
Snook and Ciriello (1974b) and Petrofsky and Lind (1974). Further-
more, the biomechanical linkage mechanism (while lifting) may differ
between males and females with respect to the lever systems em-
ployed, as reported by Tichauer (1973). Hence, if asked to handle
a given load, the average women is more highly stressed than the
average man relative to their strengths. However, the range in
the strength of males and the strength of females is very large.
Gender, thus, becomes secondary to the strength factor per se.
Strength as a risk factor will be discussed later.

Brown (1974), in a survey of industrial workers, reports that women
appear to have larger relative numbers of complaints than men

when required to perform heavy, physical jobs. Magora (1970) re-
ports a similar result. In a test of this, Chaffin and Park (1973)
studied both men and women performing equally demanding, light-
to-moderate load handling jobs and reported equal incidence of
low-back pain cases. However, the women in this study who per-
formed moderate materials handling jobs were stronger than the
women on the lighter jobs (i.e., an unknown selection process

was operating).

Age

As with gender, age has often been considered before placing people
on jobs requiring the manual handling of materials. In practice,
advanced age if often used in restricting a person from load
handling jobs. In fact, this is mainly based on speculation,
namely that older workers have diminished capacity to withstand
physical stresses (Aberg, 1961). The literature indicates the
greatest incidence of low-back pain (LBP) occurs in the 30 to 50
year old group (Herndon, 1927; Hult, 1954; Kosiak, et al., 1968,
Magora and Taustein, 1969; Rowe, 1969 and Brown, 1973). Whether
this is because older workers are not as likely to be exposed to
the injury producing stresses of manual materials handling, or
whether only those older workers who have survived a rigorous
history of earlier stresses remain in the workforce is not clear.
It appears, however, that heavy physical work, even when performed
in the twenties, can cause accelerated rates of injury (Blow and
Jackson, 1971). Clearly, age and aging have a complex effect

on many attributes necessary for workers to safely handle heavy
loads.

17


predictions may overestimate capability (and under-
estimate stress on the low back, for example).

For additional information on this biomechanical strength predic
tion model, refer to Garg and Chaffin (1975). For an example ap-
plication of the approach to industrial job analysis, refer to
Chaffin, Herrin, Keyserling, and Garg (1977). This model was
applied to the various situations described in the recommendations
chapter. The comparative results are presented in the Appendix.

DYNAMIC STRENGTH

Another method for measuring the capacity of an individual to lift
has been referred to in the literature as psychophysical strength.
This is actually a misnomer since isometric strength (as discuss-
ed in the preceding section) is also psychophysical. Dynamic
strength is a more appropriate name for this type of measurement
even though time dependent forces and accelerations are seldom
measured.

The measurement of dynamic strength using the "psychophysical
approach" has been extensively used by several researchers (e.g.,
Snook, 1976, 1978; Snook and Irvine, 1968, 1969; Snook and Ciriello,
1974; Ayoub, et al., 1973, 1976, 1978; Strindberg and Peterson,
1972; Switzer, 1962; and Emanuel, et al., 1956) for determining
different load handling capacities of individuals or groups of
individuals.

Essentially, the subject is civen control of one of the task vari-
ables, usually the weight cf the object being handled. All other
variables such as frequency, size, height, distance, etc., are
controlled. The subject then monitors his own feelings of exer-
tion or fatigue, and adjusts the weight of the object accordingly.
The work tasks are made as realistic as possible with subjects
tested in repetitive dynamic lifting tasks for several hours.

In each of these studies, subjects lifted industrial tote boxes
with handles. The subject varied the weight of the box by adding
or subtracting lead shot. In an attempt to minimize visual cues,
each box contained a false bottom. The subject was aware of the
false bottom, but never knew how much lead shot it contained. The
amount of weight in the false bottom was randomly varied.

Subjects were instructed to work on an incentive basis, working
as hard as they could without straining themselves, or without
becoming unusually tired, weakened, overheated or out of breath.
Several days of training sessions were usually required to allow
subjects to gain experience at monitoring their own feelings, and
adjusting the object weight.

The results of the seven manual handling studies conducted by
Liberty Mutual Insurance Co. were recently summarized by Snook

76


Table 5.2: Prediction equations using anthropometry (Keyserling,
et al., 1978)

2 sD.
STRENGTH i >R' 4SIUN EQUATION R* ERROR

Arm Lift ¥ = 23,8 - 15.26 + .00162SxW - .00303WxA +294 11.5
Torso Lift ¥ = 11.4 - 14.7G + .00309SxW + .00263SxA - .00844WxA +204 19.4
Leg Lift ¥ = 61.5 - 41.2G + .09422SxW - ,0109WxA +379 29.0
High Far Lift Y 6.23 + .00117Sx¥ —- 100571SxwWxe <302 6.44
Floor Lift - 50.9 - 69.86 + 9 55z5xW - .9000302SxWxA + .00877SKAxG .246 23.2.
High Near Lift Pooh .2 S108 = ash 6 2G - .0000189SxWxA +307) 14.5

S saa) nerot om,

2 wudy ean ke

A: ag. ears

G = gender (O=ma =, -=fenate

A similar study by Kamon and Goldfuss (1978) of 457 male and 137
female industrial workers produced comparable results as illus-

trated in Figure 5.1. The back extension measures reported here
are not directly comparable to the results of Keyserling, et al.,

(1978) due to methodological differences. The elbow flexion data
are comparable to Tables 5.1 and 5.2. In both studies, unexplained

differences between individuals were foughly 1/3 of the mean values.
STATIC STRENGTH MODELS

Very few predictive models of human strength are available in the
literature. Most models are regression models which are very
poor in terms of interpolation and extrapolation from experimental
data. The primary reason for the lack of models is the complexity
of the human body.

A computerized, 3-dimensional isometric strength model is reported
by Garg and Chaffin (1975). This model is based on a mechanical
analog of the human body. This analog treats the body segments

as a set of links with masses distributed as dictated from many
past population surveys depicted spatially in Figure 5.2.

Essentially, this model develops resultant torque estimates at
each joint center for specified external forces acting on the
body. These are then compared to the inputted reactive volitional
torques that can be achieved at each joint (i.e., to the muscle

71


ACKNOWLEDGEMENT

The recommendations contained in this 'Work Practices Guide for
Manual Lifting' are based upon not only the results of investiga-
tions by scientists, engineers and physicians, but also on obser-
vations made at the jobsite by many individuals responsible for
the development and implementation of programs in occupational
safety and health.

Contributors to this Guide included the following:

1. Prof. M.M. Ayoub, Department of Industrial Engineering,
Texas Tech University, Lubbock, Texas.

2. Prof. D.B. Chaffin, Center for Ergonomics, The Univer-
sity of Michigan, Ann Arbor, Michigan.

3. Prof. C.G. Drury, Department of Industrial Engineering,
State University of New York (Buffalo), Amherst, New
York.

4. Prof. G.B. Herrin, Center for Ergonomics, The Univer-
sity of Michigan, Ann Arbor, Michigan.

5. Prof. K.H.E. Kroemer, Department of Industrial Engineer-
ing and Operations Research, Wayne State University,
Detroit, Michigan.

6. Prof. A.R. Lind, Department of Physiology, St. Louis
University Medical School, St. Louis, Missouri.

7. #j4JgJ.D.G. Troup, Department of Orthopaedic and Accident
Surgery, University of Liverpool, Liverpool, England.

In addition, S. H. Snook, Liberty Mutual Research Center, Hopkinton,
Mass. contributed to the discussions during the formulative period
of the guide. F. N. Dukes-Dobos and A. Henschel of NIOSH reviewed
the completed manuscript.

Special acknowledgement is made to Prof. Herrin, who made a major
contribution by editing the manuscript for technical content, and
when indicated, by rewriting several of the chapters. The
assistance of E. J. Baise and C. K. Anderson of the University of
Michigan and N. S. Bryant and S. L. Clark of NIOSH was invaluable
in preparing the final manuscript.

D. W. Badger and D. J. Habes coordinated the preparation of this
document and had final editorial responsibility.

XV


CHAPTER 4

BASIS FOR GUIDE: PHYSIOLOGICAL APPROACH

This chapter is concerned with the design of repetitive lifting
tasks which a person can perform without excessive fatigue. Oxygen
consumption, metabolic energy expenditure rate and heart rate are
the physiological measurements which have been suggested most often
for determining the maximum work intensity that can be continuously
performed without accumulating excessive physical fatigue.

It is convenient to consider these responses first as they are
measured in the laboratory and then to place our understanding of
them in the practical conditions of everyday industrial work.

There are two kinds of muscular activity. First, dynamic exercise,
which forms the bulk of everyday activity, can be defined as
exercise in which muscles shorten, causing movement cf the bones
around joints of the skeleton. Exercise such as walking, jogging,
bicycling, etc. comprise almost “pure" dynamic exercise. The
second type of exercise is isometric, when the muscles do not
shorten and there is no movement around the joints; carrying or
holding packages, suitcases, etc. are common examples. The
physiological responses to these two kinds of exercise are quite
different; while dynamic exercise provides a greater expenditure
of energy in daily work, isometric exercise (or static effort)
readily induces muscular fatigue. Daily activity is made up of
both types of exercise and the physiological responses to each
ought to be properly understood.

In the description of physiological responses to exercise given in
this chapter, the individuals are presumed healthy. Iliness or
disease of many kinds can substantially modify those responses.

DYNAMIC EXERCISE

Physiologists measure this kind of exercise as the amount of oxygen
that is used by the muscles. When muscles become active, their
increased metabolism demands an increase in the delivery of oxygen
and foodstuffs if the activity is to continue. These circumstances
call for an increased respiratory function and a greater amount of
blood flow to the muscles. However, these respiratory and cardio-
vascular responses are linearly related to the amount of oxygen

used used by the muscles. In turn, the amount of oxygen used
(aerobic activity; Voz)is linearly related to the amount of external
work done by the muscles. Hence we can judge the severity of the
exercise by the vO2. The skeletal muscles, however, have also the
ability to contract even in the absence of oxygen; that anaerobic
capacity is limited and is called on only when the aerobic metabolism
is insufficient to allow the exercise to continue. When the limit
of the anaerobic metabolism is reached, the muscles are fatigued and
are no longer able to do effective work.

41


Fitzgerald, J.G. "Changes in Spinal Stature Following Brief Per-
iods of Shoulder Loading." Royal Air Force Institute of Av-
iation Medicine, IAM Report No. 514, 1972.

Floyd, W.F. "Physiological Factors Concerned with Weight Lifting."
Ergonomics Res. Soc. Proc. London School of Hygiene and Trop-
ical Medicine, 1958.

Fox, J.G. "Research on the Ergonomics of Fork Lift Trucks."
Paper presented at Conference on Human Factors in Materials
Handling. Ergonomics Research Society, London, 1971.

Frederick, W.S. "Human Energy in Manual Lifting." Modern Mater-
ials Handling. March, 1959.

Garg, A. "A Metabolic Rate Prediction Model for Manual Materials
Handling Jobs." PhD Dissertation, University of Michigan,
1976.

Garg, A. and Chaffin, D.B. “A Biomechanical Computerized Simula-
tion of Human Strength." AIIE Transactions, 7:1-15, 1975.

Garg, A.; Chaffin, D.B. and Herrin, G.D. "Prediction of Metabolic
Rates for Manual Materials Handling Jobs." Am. Ind. Hyg.
Assoc. J. 39:661-674, 1978.

Garg, A. and Saxena, U. "Effects of Lifting Frequency and Tech-
niques on Physical Fatigue with Special Reference to Psycho-
physical Methodology and Metabolic Rate." Am. Ind. Hyg.
Assoc. J. 40:894-903, 1979.

Garrett, J.W. "The Adult Human Hand: Some Anthropometric and
Biomechanical Considerations." Human Factors. 13:117-131,
1971.

Garry, R.C.; Passmore, R.; Warnock, G.M. and Durnin, J.V.G.A.
"Expenditure of Energy and the Consumption of Food by Miners
and Clerks." Med. Res. Council Spec. Rep. Ser. 289, Her
Majesty's Stat. Off., 1955.

Gimlett, D.M. "The Back X-Ray Controversy." J. Occup. Med. 14:
785-786, 1972.

Givoni, B. and Goldman, R.F. "Predicting Metabolic Energy Cost."
Jd. Appl. Physiol. 30:429-433, 1971.

Givoni, B. and Goldman, R.F. "Predicting Heart Rate Response to
Work, Environment and Clothing." J. Appl. Physiol. 34:201-
204, 1973.

Grandjean, E. Fitting the Task to the Man. Taylor and Francis, Ltd.,

London, 1969.

162


Models developed by Poulsen (1970), McDaniel (1972), Dryden
(1973), Knipfer (1974), Ayoub and McDaniel (1973), Ayoub, et al.,
(1973), Ayoub, et al., (1976a), Ayoub, et al., (1976b), and
McConville and Hertzberg (1968) have their obvious limitations.
They are applicable to only one or two height levels for lifting
in the sagittal plane and are developed by collecting data at
only one frequency of lift. Some of these limitations have been
overcome in the models developed by Ayoub, et al., (1978) and
Mital, et al., (1978), but still the models are for lifting in
the sagittal plane.

Some disagreement between the results of these studies can be
observed. According to McDaniel (1972) and Dryden (1973), males
and females should not be used together in the same model to pre-
dict the acceptable weight of lift. Their conclusion was dif-
ferent from the one drawn by Knipfer (1974). According to his
results, the combined model predicts the lifting capacity as well
as the individual models. According to Ayoub, et al., (1978) and
Mital, et al, (1978) the combined models predicted lifting capac-
ity better than individual models.

PSYCHOPHYSICAL DESIGN CRITERIA

Table 5.7 combines the most recent studies of Snook (1978) ana
Ayoub, et al., (1978) in predicting average lifting capacity of
75 percent of industrial women and 25 percent of industrial men:

These quartiles were selected to represent the inherent variabil-
ity within the industrial population. Based on the epidemiologi-
cal evidence presented in Chapter 2, the majority of low back
injuries were shown to occur on jobs that were not acceptable to
more than 75 percent of the population (Snook, 1978). Therefore,
if the workforce was predominantiy women, this percentile specifi-
cation should be protective. For an all male workforce this limit
would be overly restrictive. Further, were individuals tested for
strength capability, an even greater capacity could be expected,
The choice of a 25th percentile male is arbitrary, but assumed
reasonable based on the reliability of available testing methods.

The values presented in Table 5.7 were arrived at by adjusting
Snook's data (1978) for box size and frequency and then combining

it with the lifting capacity data generated by Ayoub, et al., (1978).
A linear box size (horizontal location of the hands in the sagittal
plane) and frequency effect were assumed for interpolation and
extrapolation. These values were further adjusted to show a

linear frequency effect.

For the floor to shoulder height, floor to reach height, ard knuck-
le to reach height levels, the lifting capacity values recommended
by Ayoub, et al., (1978) were used for frequency adjustment since
no other data exist for these height levels. The standard devia-
tion assumed in Table 5.7 is the larger of the values generated by

83


floor to knuckle: 15 cm and 50 cm vertical
knuckle to shoulder: 75 cm and 125 cm vertical
shoulder to reach: 150 cm and 175 cm

A horizontal hand location (in the sagittal plane) of 46 cm is
assumed. The complementary cumulative probability of the stan-
dard normal distribution is recorded in the table as the expected
percent capable for the given set of conditions. The values in
Table 5.7 for males were, likewise, used to obtain the percent
capable for the MPL in Table A.2.

ISOMETRIC % CAPABLE

The table value is obtained by entering the vertical and horizon-
tal hand locations and the AL into the biomechanical model report-
ed by Chaffin, et al., (1978). The following postures were also
assumed:

deep squat, squat or stoop: 15 cm and 50 cm vertical
squat or stoop: 75 cm vertical
stand: 125 cm, 150 cm and 175 cm vertical

The minimum and maximum female percent capable for the various
postures are entered in Table A.1. Lateral hand separation was
assumed to be 20 cm. It should also be noted that this biomechan-
ical model assumes average male and female anthropometric dimen-
sions. The same procedure was used for the MPL (Table A.2) but
values represent the percent of the male workforce capable.

BACK COMPRESSION

Tables A.1 and A.2 present upper and lower bounds on static back
compressive force obtained with the same input to the biomechani-
cal model used for (isometric % capable) above. The larger value
is the maximum back compression obtained for the various postures
and the lower value is the minimum back compression for the given
set of conditions.

Once again, the upper and lower bounds on back compression for the
AL represent biomechanical model output for the female working
population (Table A.1) and bounds for the MPL (Table A.2) represent
male back compression estimates. Preferred and least desirable
postures are not necessarily the same for males versus females or
strength versus back compression.

METABOLIC RATE

The Garg, et al., (1978) equation presented in Chapter 4 was used
in the estimation of the expected metabolic rate (Kcal/min.).
Female 50 percentile body weight was used when calculating the
rate corresponding to the AL, and 50 percentile male body weight
for the rate corresponding to the MPL. Also, basal metabolic

176


Drury, C.G.; Barnes, R.E. and Daniels, E.D. “Pedestrian Operated
Vehicles in Hospitals." Proc. AIIE Annual Conv., Washington,
D.C., 1975.

Dryden, R.D. "A predictive Model for the Maximum Permissible
Weight of Lift from Knuckle to Shoulder Height." PhD Dis-
sertation, Texas Tech University, 1973.

Dukes-Dobos, F.N. "Rationale and Provisions of the Work Practices
Standard for Work in Hot Environments as recommended by NIOSH."
In: Standards for Occupational Exposures to Hot Environments,
M.S. Horvath and R.C. Jensen, Eds. DHEW(NIOSH) Publication
No. 76-100, 1976.

Durnin, J.V.G.A. and Passmore, R. Energy, Work and Leisure.
Heineman Educational Books, London, 1967.

Edholm, 0.G. The Biology of Work. McGraw Hill, New York, 1967.

Eisler, H. "Subjective Scale of Force for a Large Muscle Group."

J. Exper. Psych. 63:253-257, 1962.

Ekblom, B.; Astrand, P.O.; Saltin, B.; Stenberg, J. and Wallstrom,
B. "Effects of Training on Circulatory Response to Exercise."

J. Appl. Physiol. 24:518-527, 1968.

El-Bassoussi, M.M. "A Biomechanical Dynamic Model for Lifting in
the Sagittal Plane." PhD Dissertation, Texas Tech University,
1974.

Emanuel, I.; Chafee, J. and Wing, J. "A Study of Human Weight
Lifting Capabilities for Loading Ammunition into the F-86
Aircraft." U. S. Air Force (WADC-TR 056-367), 1956.

Evans, F.G. and Lissner, H.R. "Biomechanical Studies on the Lum-
bar Spine and Pelvis." J. Bone Joint Surg. 41A:218-290, 1959.

Evans, W.O. “A Titration Schedule on a Treadmill." U.S. Army
Medical Research Laboratory, Report No. 525, Ft. Knox, Ky,, 1961.

Evans, W.O. "The Effect of Treadmill Grade on Performance Decre-
ment Using a Titration Schedule." U.S. Army Medical Research
Laboratory, Report No. 535, Ft. Knox, Ky., 1962.

Parfan, H.F. "The Effects of Torsion on the Lumbar Intervertebral
Joints: The Role of Torsion in the Production of Disc. Degen-
eration. J, Bone Joint Surg. 52A:468-483,1970.

Fiorini, G.T. and McCommond, D. "Forces on the Lumbo-Vertebral
Facets." Ann. Biomed. Eng. 4:354-363, 1976.

161


For Arm Lifting (Kcal/lift):

AE = 10-2).062 BW (Fy - .81) + (3.1L - .52 Sx L) (V2 - Vz))
for .81 < Vy < V2

For Stoop Lifting (Kcal/lift):
AE = 1072 (0.325 BW (0.81 - Vj) + (1.41L + 0.76 S x L)
(V2 - V1)) for Vy < V2 < 0.81
For Squat Lifting (Kcal/lift):
dE = 102 (0.514 BW (0.81 - Vy) + (2.191 + 0.62 S x L)

(Vz < Vg < 0.81

where:
BW = Body weight (kg)

V, = Vertical height from floor (m); starting point for
lift and end point for lower

V2 = Vertical height from floor (m); end point for lift
and starting point for lower

L = Weight of the load (kg)

S = Gender (1 for males; 0 for females)

= .924 VW if standing erect

= .028 if standing in bent position (Aberg, 1968

Application of the model to predict metabolic rates on 48 dif-
ferent industrial jobs showed a simple correlation of .95 with
a standard error of 10.2% (Garg, et al., 1978). Simplifying
the above equations based on an average male body weight (BW =
77 kg), lifting from the floor (V] = 0) to bench height (V2 =
-81) and assuming a desired average metabolic rate (Eq) of 5.2
Kceal/min. the relationships between load, frequency, and body
posture can be estimated, in untrained subjects,

62


CHAPTER 3

BASIS FOR GUIDE: BIOMECHANICAL APPROACH

The general concern in occupational biomechanics is to, determine
with given precision what a person can physically (mechanically)
do. This concern leads one to ask more basic questions regarding
the individual's health status (i.e., history of injury, disease,
nutrition, etc.) and what specific tasks the person is required

to physically perform in a job. In the industrial setting, this
means that the person's physical capabilities must be assessed
along with the physical demands of a prospective job. In addition
to the simple ability to perform, biomechanics is concerned with
those physical attributes of the individual and job that have been
found to produce potential harm to the musculoskeletal system. As
discussed in the first two chapters of this Guide, injury statistics
have resulted in a major research emphasis being placed on under-
standing how the act of lifting loads adversely affects the health
of a person's low back. This chapter, therefore, concentrates

on the biomechanics of the low-back as a basis for a load handling
limit.

OCCUPATIONAL BIOMECHANICS OF LOAD HANDLING

It is a well-established fact that the stresses induced at the
low-back during manual materials handling are due to a combination
of the weight lifted and the person's method for handling the
load. Specifically, the load held in the hands as well as the
person's body masses (when acted on by gravity) create rotational
moments or torques at the various joints of the body. The
skeletal muscles are positioned to exert forces at these joints
in such a manner that they counteract the moments due to the load
and body weight. From the mechanical stress standpoint, it is
unfortunate that the muscles are positioned as they are, since
they act through relatively small moment arms. This means that
they can produce large motions with small degrees of shortening
but for any external load operating on the body, high muscle and
joint forces are produced.

For example, consider the major elbow joint flexor muscles (i.e.,
the brachialis and biceps brachii) illustrated in Figure 3.1. For
the static conditions (such as holding an object),

Ty = T, (i.e., the muscle and load torques are equal).

23


Soule, R.G. and Goldman, R.F. "Energy Cost of Load Carried on the
Head, Hands or Feet." J. Appl. Physiol. 27:687-690, 1969.

Stevens, J.C. and Cain, W.S. “Effort in Muscular Contractions
Related to Force Level and Duration." Perception Psychophy-
sics. 8:240-244, 1970.

Stevens, S.S. "The Direct Estimation of Sensory Magnitudes-Loud-
ness." Am. J. Psych. 69:1-25, 1956.
Stevens, S.S. "The Psychophysics of Sensory Function." Am. Sci.

48:226-253, 1960.

Stevens, S.S. Psychophysics-Introduction to its Perceptual,
Neural and Social Prospects. John Wiley and Sons, ENCloy,,
New York, 1975.

Stewart, S.F. "Pre-employment Examination of the Back." J. Bone
Joint Surg. 29:215-221, 1947.

Strindberg, L. and Peterson, N.F. "Measurement of Force Percep-
tion in Pushing Trolleys." Ergonomics. 15:435-438, 1972.

Sury, R.J. "The Stimulation of a Paced Single-State Work Task."
Int. J. Prod. Res. 4:125-140, 1965.

Switzer, S.A. "Weight- Lifting Capabilities of a Selected Sample
of Human Males. Aerospace Med. Res. Lab. , Report No. AD-
284054, Wright Patterson Air Force Base, Ohio, 1962.

Tauber, J. "An unorthodox Look at Backache." J. Occup. Med. 12:
128-130, 1970.

Taugher, N.J. "Back Injuries." Technical Report, State of Wis-
consin, Dept. of Industry, Labor and Human Relations, 1973.

Thordsen, M.L.; Kroemer, K.H.E. and Laubach, L.L. "Human Forces
Exertions in Aircraft Control Locations." Aerospace Med.
Res. Lab. Tech. Report, AMRL-TR-71-119, Wright Patterson
Air Force Base, Ohio, 1972.

Tichauer, E.R. "Some Aspects of Stress on Forearm and Hand in
Industry." J. Occup. Med. 8:63-71, 1966.

Tichauer, E.R. "A Pilot Study of the Biomechanics of Lifting in
Simulated Industrial Situations." J. Safety Res. 3:98-115
1971.

Tichauer, E.R. "Ergonomic Aspects of Biomechanics." In: The
Industrial Environment: Its Evaluation and Control. USDHEW/
PHS/CDC/NIOSH, 1973.

172


= 20F

uv

oO

3 g
r£ [
ag '5P
se
28 +t

38
ZS r
zo 10F
aw q
SS ft
ay
Oo g
ze
33 5p

5 F

3 L

2 .

= L

fe)

0-250 250-450450-650 650+
PREDICTED COMPRESSIVE FORCES
ON L5/S, DISC
(kg)

Figure 3.9: Relation Between LBP and Compressive Force (Chaffin
and Park, 1973).

lOOp
FEMALE BACK COMPRESSION
350 kg
80;
Beok
- \ (25
fs y = 150
© ~--- 175 om,
w : Vertical Location
= 40 of Load
20-F
fe) 1 1 4 i
15 30 45 60 75

HORIZONTAL LOCATION OF LOAD (cm)

Figure 3.10: Task Variables Producing 350 kg Female Back
Compression.

35


Container Design

From biomechanical considerations, containers should be as small
as possible. In particular every biomechanical model or formula
shows that L5/S] disc pressure is minimized when the center of
gravity of the container is as close to the spinal column as pos-
sible (e.g., Tichauer, 1971; 1978; Ayoub, et al., 1978) as dis-
cussed in Chapter 3. Compactness also allows a load lifted from
the floor to move between the worker's legs, decreasing spinal
disc pressure.

Any factors which decrease the predictability of the container's
response to applied forces will contribute to human error in
manual handling. Examples are loads with an unexpected center

of gravity (perhaps offset towards one face of the container) or
those with a shifting center of gravity. Baffles, dividers or
packing should be used to keep the center of gravity in a constant
position.

The shape and size of the container are affected by anthropometric
dimensions of the work as well as biomechanical considerations.
The method of lifting and/or carriage determines the body posture
adopted and thus the critical dimensions for safe handling.

For narrow containers one posture adopted is carriage by one hand
at the side of the body, as with a suitcase. Containers carried

in this way should be as narrow as possible as the maximum ac-
ceptable weight is directly related to container width. McConville
and Hertzberg (1966) showed that for male military personnel:

Maximum Acceptable Weight (kg) = 35.52 - [0.169

x Width (cm) ]

With weights carried in one hand.in this way an equal and opposite
torque about the center line of the body is required for lateral
balance. This contributes to the spinal stress so that it is
usually preferable to carry two lighter containers, one in each
hand, for improved balance.

For containers carried at the side of the body the main anthropo-
metric requirement is that the arm can be fully extended downwards

so that there are no unnecessary static tensions in the arm muscles.
Maximum height to handle center, if the container is to be carried

at arm's length, would be 72.5 cm for a 5th percentile (short) male and
65 cm for a 5th percentile female.

For containers carried in front of the body, it is usual to grip
the container on each side, either along the base or along the
forward edge, and pull the container against the frontal surface
of the trunk, thereby supporting part of the weight with body/

104


From this, they generated contour maps of acceptable forces while
in these postures based on a maximum of 90 mm Hg of intra truncal

pressure.

What is evident from the preceding section is that a biomechanical
criterion (back compression tolerance) can be converted into
practical recommendations for acceptable lifting task descriptions.
This criterion and those of the following chapters were used to
establish the guidelines for lifting which are presented in detail
in chapter 8.

In terms of the biomechanics of lifting it can be concluded that:

aba

Lifting a 5 kg compact load (wherein the mass CG of the
load is within 50 cm of the ankles) could create compres-
sive forces sufficient to cause damage to older lumbar
vertebral discs.

As the load mass center of gravity is moved horizontally
away from the body, a proportional increase in the com-
pressive force on the low-back is created. Thus even
light loads need to be handled close to the body.

When a load is lifted from the floor, additional stresses
are exerted on the low-back due to the body weight moment
when stooping to pick the load up. Thus heavy loads
should not be stored on the floor, but should be raised
to about standing knuckle height (minimum 50 cm) to

avoid the necessity for stooping over and lifting.

The postures used to lift loads from the floor can exert
a complex and relatively unknown effect on the stresses
of the low-back during lifting. Specific instructions

as to the safe posture to use will be necessarily complex,
reflecting such factors as leg strengths, load and load
size. Until such complexities are better researched,

it is recommended that instructions as to lifting pos-
tures be avoided.

Lifting loads asymmetrically (by one hand or at the side
with the torso twisted) can impart complex and potentially
hazardous stresses to the lumbar column. Such acts

should be avoided by instructions and workplace layouts

which permit the worker to address the load in a symmetric
manner.

The dynamic forces imparted by rapid or jerking motions
can multiply a load's effect greatly. Instructions to
handle even moderate loads in a smooth and deliberate
Manner are recommended.

40


Table 8.2: ers Table.

AVERAGE VERTICAL LOCATION (cm) (in)
V> 75 (30) V < 75 (30)

Standing Stooped
1 hour 18 15
PERIOD
8 hours 15 12

The above equations for the action limit (AL) and maximum permis-
sable limit (MPL) represent a multiplicative factor weighting for
each task variable. The first factor

H Factor = (15/H)

represents the importance of the horizontal location (H). If H =
15 cm this factor is 1 and no adjustment for horizontal location
is necessary. If H = 75 cm, this factor is 15/75 = .20 meaning
the AL is reduced from 40 to 40(.20) = 8 kg.

The factor for vertical location involves the absolute deviation
of V from 75 cm (approximate knuckle height). For V = 75, the v
factor is

V Factor = (1-.004|v-75|)
= (1-.004(0)) =1
For V = 15,
V Factor = (1-.004|15-75|)
= 1-.004(60) = .76
Likewise for V = 135,
V Factor = (1-.004/135-75|)

il

1-.004(60) = .76

127


Clearly, dynamic actions that result in high inertial forces are
more difficult for one to control. Therefore, it is reasonable
to require of people who are engaged in manual materials handling
that they move the loads in a smooth and well-planned manner.
Further, as part of this concern is the need to provide good foot
tractions and hand grips on such loads to avoid any possible slips
and/or falls.

BIOMECHANICAL DESIGN CRITERIA

The biomechanics of lifting and handling loads provides one basis
for why certain musculoskeletal overexertion injuries: and illnesses
develop. Specifically, biomechanical studies indicate that the
low-back is vulnerable to continual overstress damage during even
moderate load handling, but that symptoms may not manifest them-
selves until later in life. In essence, existing knowledge sup-
ports a "wear-and-tear" hypothesis of low-back pain, which requires
that load handling activities be limited and carefully specified.

Figure 3.9 illustrates the observed incidence rates for low back
pain related to predicted back compressive forces on the L5/S; disc
(Chaffin and Park, 1973). Based on this industrial study of 400
workers and earlier compressive tolerance cadaver studies of
Evans and Lissner (1959) and Sonoda (1962) (see Figure 3.6) it is
apparent that jobs which place mor n_6 mpressive force
on the low-back are hazardous to all but the healthiest of workers.
ne i imit. This will not
necessarily be protective for most individuals over 50 years of
age or other susceptible populations. However, based on current
knowledge of injuries in industrial work a lower limit is not
supported at this time. This is, in part, due to historical self-
selection mechanisms which have precluded weaker individuals from
performing rigorous MMH.

To convert low back compression values into load limit recommenda-
tions, computerized biomechanical models which allow simultaneous
analyses of the stresses placed on the many linkages and joints of
the body during lifting are needed. Chaffin (1972) and Garg and
Chaffin (1975) have reported a Static Sagittal Plane Lifting (SSPL)
model in the literature. As the name infers, this particular model
has been developed to evaluate various static situations, such as
when one is holding a weight or pushing or pulling on a non-
moving container. The large static component of a lift enables

the SSPL model to be useful in establishing safe lifting limits.
Figure 3.10 illustrates the trade-offs between horizontal (H)

and vertical (V) locations of a load which would preduce 350 kg
compressive force cn the L5/S] disc of the average female based on
the most recent model of Chaffin, et al., (1978). The assumed
female anthropometry was reported by Dempster (1975). Horizontal
location is measured forward of the body centerline from the mid-
point between the ankles. Vertical location is measured from the
floor or foot sole. Likewise, Figure 3.11 illustrates the same
combinations which would produce 650 kg compressive force on the

36


15-—
|e -
E a Acceptable Limit for Men
oe L
o
= Lt
>
+ 1o-—
Oo o-L
= Lb
<
oO -L
Fa
g Acceptable Limit for Women
a Lb
<
Oo Ff
72)
> -E
xr
a Ss
° po tall po dtl po tial riii I

2 4 6810 50 100 600 1000 5000 10,000
WORKING TIME (minutes)

Figure 4.10: Recommend Maximum Capacities for Continuous
Work.

4. The primary task variables which influence metabolic
rate during lifting are

A) load handled, L

B) vertical location at beginning of lift, V (and
consequently posture)

Cc) vertical travel distance, D
D) frequency of lift, F.

The guideline recommendations summarized in Chapter 8 combine
the above criteria with the biomechanical criteria of the
preceding chapter and the psychophysical criteria of the next
chapter. Estimates of the metabolic rates for the various
working conditions described in Chapter 8 are provided in
Appendix A. These estimates are based on the model of Gargy
et al., (1978) assuming an average male body weight of 77 kg
for the upper limit and average female body weight of 62 kg
for the lower limit.

65


LINES DEPICT EQUAL
LIFTING STRENGTHS (kg)

200 »
30
100

HORIZONTAL DISTANCE
FR!

OM
FORWARD ANKLE TO HANDS
{cm)

VERTICAL DISTANCE FROM FLOOR TO HANDS (cm)

Figure 5.4: Predicted Lifting Strength of Large/Strong Male

the muscle groups which limit performance on each task.

(Chaffin, 1974).

Finally,

it predicts the percentage of the male and female populations
that could be expected to perform each job activity. The model
has several assumptions which limit its usefulness:

1.

The model selects the "best" posture in terms of maxi-
mal strength. Thus, it does not reflect additional

stresses a person may receive due to poor posture
selection.

Body weights and link lengths are based on 50th per-
centile anthropometry for men and women. Therefore,

model predictions may be inaccurate for unusually large
or small populations.

Lifting, may require certain amounts of dynamic strength
depending upon acceleration, deceleration and speed of
movement. This biomechanical model is based only on
static strength capabilities. The relationship between
static and dynamic strength is not well understood.
Therefore, if the model is used to simulate a highly
dynamic task, (e.g., one with jerking actions) the

75


Two further observations from these early cadaver studies are note-
worthy. First, the discs themselves (if healthy) do not herniate.
Instead, the cartilage end-plates’ that distribute the compression
loads to the bodies of the vertebral segments fail, as described
by Armstrong (1965). Second, the large variation in strength of
the cadaver columns may indicate that the cartilage end-plates of
some people were already weakened by prior stresses, with result-
ing microfractures and scarring. If true, this would contribute
to the disc degeneration that now is acknowledged as being neces-
sary before the more common and most serious discogenic low-back
problems can develop. In other words, evidence indicates that
repeated compressive stresses of life (and lifting in particular)
can be sufficient enough to cause microfractures in the cartilage
end-plates and subchondral bone of the vertebral bodies which (it
is hypothesized) would alter the metabolism and necessary fluid
transfer to the disc. If this occurs,'a degenerated capability
of the discs to withstand further compression loads would develop.
The end result of this process is that the annulus fibrosus bulges
or ruptures, causing pressure on the adjacent nerve roots, as
shown in Figure 3.7.

‘Sr-Spinal Nerve and
Vessels

* rectoy Spinae

ries \ eos.

Longitudinal
Lig.
Si Spinal Nerve . Post. Long Lig

pase Nucleus(Disploced)

Ss,
Sy
Bee

—

»

Spinal Vessels:

Figure 3.7: Displaced degenerated disc exerting pressure on spinal
nerves (Chaffin, 1975).

30


tho The crane cab, and all controls, must be designed for
good visibility, and safe operation, Sell (1977) show-
ed how an integrated cab for an overhead crane could be
designed.

2. Of course, no crane should be loaded beyond its rated
load capacity, the load including the weight of all
auxiliary handling devices such as hoist blocks, hooks,
and slings.

cS Load hooks, chains, ropes, and the hoisting device itself
must be clearly marked with the maximum allowable load.

4. Loads must not be carried over people, and may not be
moved while a person is on the load or hook.

Sts The operator must be thoroughly trained. He must not
leave his position at the controls while the load is
suspended.

6. Standard signals should be used between the crane opera-
tor and the signal man on the ground.

Most of the materials handling accidents investigated by Coleman,
et al., (1978) were connected with falling, slipping, or bumping

of the load. The major factors in such accidents were inadequate
communications between the crane operator, the hitcher, and the
signal man. Often the visibility from the cab was insufficient,
the controls inadequate, and the overall design required tiring
body positions. Simple ergonomics principles of control positions
and directions of movement, workplace layout and visibility are
readily available (e.g., McCormick, 1976; VanCott and Kinkade, 1972)
and should be used in design or redesign.

Hoistsand particularly air-balance hoists are useful at the work-
Place where heavy components can be moved with only small (control)
forces having to be supplied by the human operator.

Industrial Trucks

Human factors and safety design weaknesses found by Coleman, et al.,
(1978) concerned missing or inadequate safety devices, lack of
standardization in braking, reduced visibility, cramped driver
compartments, seats not damping transmission of road impacts and
vibrations, and not providing sufficient body support.

Hazards often encountered in the operation of powered and hand
trucks include;

abo contact with the moving parts of the truck,
2. falling of the load, particularly at corners and on
inclines,

118


The risks to health of unskilled MMH: This should be
rooted in local experience and the MMH accident pattern
of the organization concerned;

The basic physics of MMH: Every MMH worker should possess
at least a subjective understanding which can be acquired
using simple models (levers, beam-balances, pulleys,

etc.) to illustrate:

a. the principles of levers;

b. the difference between the force needed to
resist gravitational force on a load and
the work needed to raise it;

c. the work needed to move a load horizontally;

d. the work needed to change the direction of
motion;

e. momentum and kinetic energy;

£. Newton's 3rd Law of Motion.

The effects of MMH on the body: The basic functional
anatomy of the spine and the muscles and joints of the
trunk and limbs is easily taught to trainees in terms
of their practical experience, and with reference to
basic physics. It is easy to demonstrate muscular act-
ivity while muscles are being stretched or lengthened
isometrically; how the rib cage is constrained by push-
ing and carrying; how the shoulder, abdominal and back
muscles contribute to lifting. The increase in intra-
thoracic and intra-abdominal pressures during MMH is
not difficult to understand because of the familiarity
of breath-holding when making an effort.

Individual awareness of the body's strengths and weak-
nesses: The first practical lesson in MMH which all
trainees must learn is how much they can handle com-
fortably; and where, in relation to their bodies, their
strengths and weaknesses lie.

How to avoid the unexpected: There is a need to recog-
nize the physical factors which might contribute to an
accident, and eliminate them. For example:

a. is the load free to move and not stuck?

b. is it a weight that is comfortable to
handle alone or should help be sought?

c. are lifting aids available?

d. has the load proper handles to grasp or can
they be provided?

e. is protective clothing indicated?

f£. is the area for MMH clear of obstruction?

g. is the floor clean, dry and non-slip?

h. is the area for setting the loads down clear?

100


4, floor angle (Harper, Warlow & Clark, 1961; found higher
coefficients of friction were needed on increasingly
steeper slopes).

Bo shoe sole/heel composition and contact area (rubber
soles and certain synthetic soles are better than lea-
ther under dry conditions, but differences reduce or
even reverse under wet conditions; Irvine, 1967; Kroemer,
1971, 1974; Sigler, 1943; Safety Sciences, 1977).

6. The style of shoe (Harper, Warlow & Clarke, 1961, found
that shoes with high or narrow heels are the most hazard~
ous). In a related area Tichauer (1966) found potential
problems of biomechanical stresses from lifting in
higher heels.

The general recommendation is to adjust shoes and working surfaces
to give a coefficient of static friction of at least 0.4 and pre-
ferably 0.5. Unnoticed changes in surface friction are implicated
in many accidents. Going from a less slippery floor to a more slip-
pery one produces slips, the opposite change produces trips and
mis-steps. These unnoticed changes can be reduced by:

abo ensuring that different surface materials or coatings
have transition zones between them.

Zo clearly marking any surface friction change.

Eo using good housekeeping procedures to reduce transient
changes in surface friction such as spills, worn spots,
loose or irregular floors.

These recommendations are of particular importance in manual mate-
rials handling where any handling other than direct lifting in-
volves horizontal inertial forces transmitted from the container
to the body. Such forces require increased frictional forces to
prevent foot slippage. The carriage of weights also affects the
body's learned reflexes for recovering from a slip or trip both by
changing the normal weight distribution of the body and by pre-
venting the arms from being used to regain balance or recover from
the fall. Finally, if a fall does occur, the container becomes
another moving mass in close proximity to the falling operator,
with potential for both crushing and puncturing the body.

THE VISUAL ENVIRONMENT

While manual materials handling operations rarely demand the fine
visual discrimination of delicate assembly work, they do require
control of the visual environment for optimum performance and
safety.

The total amount of light needed is easier to specify than the
type of light. The task of manual materials handling involves

109


and the ASME haye adopted a decimal classification system with
nine major classes. The four most useful classes within a factory
are:

1.000 Conveyors

2.000 Cranes and hoists

3.000 Positioning equipment
and 4.00 Industrial vehicles

Positioning equipment is used to transfer material from workplace
to materials handling equipment or vice versa. It include& manip-
ulators, dumpers, up-enders, positioning tables, lifts, jacks and
transfer machines. As such positioning equipment provides an
alternative to the unaided human operator at the workstation it-
self where many manual materials handling problems occur.

At the workplace simple job aids can often facilitate materials
handling tasks. Examples are:

abo Hooks. The worker should be trained in the use of hand
or packing hooks so that they will not glance off hard
objects. If the hook is carried in the belt, the point
must be covered.

Go Bars. The major hazard in the use of a crow bar is that
it may slip. The point or edge should have a good "bite".

3. Rollers. Rollers are often used to move heavy and bulky
objects. The principal hazard is that fingers or toes
may be pinched or crushed between a roller and the floor.

4, Jacks. All jacks should be marked to indicate the safe
load they can support. The surface onto which a jack
is placed must be level and clean and be sturdy enough
to support the load. After the load is raised, substan-
tial horses or blocking should be placed under it for
support. Workmen using jacks should wear safety shoes
and instep guard protection because handles may slip, or
parts may fall.

5. Platforms. Platforms are useful for loading and unload-
ing, provided that the load is maintained at a convenient
height for lifting and handling.

6. Trestles. These and other supports may be used for
maneuvering long loads on the point of balance, or for
readjusting the grip or carrying posture.

The other three types of materials handling equipment are used es-

sentially between workstations. Their characteristics, functions
and uses have been well summarized by Apple (1971) as follows.

114


The average vertical location is

ve QO + 40 + ue + 120 + 160 _ 80 om
The average vertical travel is
p = SO +10 + 30 + 70 + 110 _ 54 om

5

For the total job then

AL = 40 (15/35) (1-.004(5)) (.7+7.5/54) (1-12/18)

40 (.43) (.98) (.84) (.33)

I

4.7 kg

MPL = 14 kg

Since the carton weight (5 kg) is nominally above the AL, admin-
istrative controls may be required. The analysis suggests that
the problem with this job is not so much strength (at issue in

the individual task analysis) but endurance. Since a number of
simplifying assumptions were made in this analysis a more detailed
metabolic analysis of such a job may be warranted before imple-
menting administrative controls. Such an analysis is described

in detail by Garg, et al., (1978).

Example 5: Continuous High Frequency Lifting, Variable Tasks

Lifting tasks of this type are typical in warehousing, shipping
and receiving activities where there are many different sized
loads, of varying weights that are lifted at varying frequencies.
As a simple example, consider a job with the set of 3 tasks des-
cribed in Figure 8.14.

For highly variable jobs (such as this one) this Guide is most
limited. Following the procedure of the preceding example, each
task should first be examined individually. The recommended AL
and MPL for each task is as follows:

TASK 1: AL 40 (15/40) (1-.004(75)) (.7+7.5/75) (1-1/12)

W

40 (.38) (.70) (.80)(.92) = 7.8
MPL = 23.4

142


Bo X-rays present a radiation hazard, particularly for fe-
males who cannot be given adequate gonadal protection.

In summary, caution must be exercised in using the lumbar spinal
radiograph as a selection tool. If so, it should be performed by
an experienced physician after a thorough interview and examina-
tion, and with full knowledge of the MMH job concerned.

Physical Assessment of Workers

What is needed is an objective means for assessing the physical
capabilities of a worker who is being considered for a certain
job. In providing such an assessment certain medical, social and
economic criteria must be met, these are:

ako Is it safe to administer?

Bo Does it give reliable, quantitative values?

So Is it related to specific job requirements?

4, Is it practical?

Sh Does it predict the risk of future injury or illness?

As we have seen, the low back X-ray fails to some degree in each

of these criteria, particularly in those relating to safety and
reliability. As an alternative, worker strength testing and

aerobic capacity testing have been proposed as means to meet these
objectives. If carefully administered they appear to satisfactorily
meet each of the above criteria as discussed in Chapter 6.

Training

The importance of training in manual materials handling in reducing
hazard is generally accepted. What is lacking, however, is a clear
definition of what the training should be and how it should be
taught. Most controlled studies of training have shown it to be
ineffective in reducing accidents and injuries related to lifting.
Despite this, however, some form of training in manual materials
handling jobs is likely to continue. What follows is a suggestion
of what a training program should include.

1. What should be taught.

The aims of training for safety in lifting should be:

a. To make the trainee aware of the dangers of care-
less and unskilled lifting.

b. To show them how to avoid unnecessary stress.

146


DISCLAIME

Mention of company names and products does not constitute endorse-
ment by the National Institute for Occupational Safety and Health.

DHHS (NIOSH) Publication No. 81-122

NIOSH Project Officer: Donald W. Badger, Ph.D., Applied Psychology
and Ergonomics Branch


PHYSICAL STRESS JOB ANALYSIS SHEET

DEPARTMENT__“48 81 cAT/0A/ pate _2//8/80
JOB TITLE 2Wew PRESS = ANALYST’S NAME _£7S.
TASK OBJECT eee TIONN TASK
DESCRIPTION | WEIGHT | yoy | FREQ} AL/MPL|] REMARKS
Ave Max

cm cm} cm cm

Loap Srock | z0| 20 | 53}36| 53|/c0| oO 7.2 [21-6]| New work
Prechte Needed

Figure 8.9: Job Analysis for Example 1.

Interpreting this activity in terms of the critical variables:
H factor = 15/H = 15/53 = .28

D factor = .7 + 7.5/(160-38) = .76

V factor = 1 = .004 (75-38) = .85

F factor = 1 - 0/15 = 1.0
Therefore

AL = 40(.28) (.76) (.85) (1.0)= 7.akg

MPL

i}

3(7.2) = 21.6 kg

In this case, lifting the 20 kg reel in this way would be stress-—
ful for most people and at least strong administrative controls
would be required if not engineering controls.

136


(Wilkins, et al., 1957; Redfield, 1971; Harley, 1972; Montgomery,
1976) if not condemned out of hand (Houston, 1977). At a con-
ference on this topic in 1973, sponsored by the American College

of Radiology, the American Academy of Orthopaedic Surgeons and

the Industrial Medical Association, low back X-rays without prior
clinical examination were deplored. It was agreed that no worker
should be rejected solely on the basis of radiological appearance
and that the conference recognized that low back X-rays had yet to
be proved as having reliable predictive value. In a recent American
Occupational Medical Association (AOMA) association affairs commit-
tee report (1979), it was concluded that "lumbar spine X-ray exami-
nations should not be used as a routine screening procedure for back
problems, but rather as a special diagnostic procedure available to
the physician on appropriate indications for study."

LaRocca and Macnab (1969), Rowe (1969), Troup, et al., (1974) and
Magora and Schwartz (1976) have found no statistically significant
differences in the incidence of ra-iological abnormalities between
workers with known history of back or sciatic pain and those without.
One study by Redfield (1971) reported a lower incidence of back pain
in a group of workers who had been identified as radiologically of
‘high-risk' than in a group with normal radiological appearance. For
the majority of industrial purposes, however, pre-employment radio-
logical screening cannot be justified on statistical grounds and due
attention must be paid to the hazards of radiation.

The criteria upon which radiological screening is deemed necessary
thus depend on the definition of an abnormally high risk of back
injury. To take an extreme example from the armed forces: pilots
may have to be ejected from fighter aircraft in an emergency, and
this carries a high risk of spinal fractures - some of which damage
the spinal cord. Experimental work has shown that the presence of
some radiological abnormalities (for example, Schmorl's nodes, which
can safely be ignored in most clinical situations) may adversely
affect the capacity of the spine to withstand the acceleration on
ejection (Kazarian, 1978). Such risks can reasonably be defined as
unacceptable and radiological screening is well justified. Moreover
exceptionally high radiographic standards are needed to achieve the
definition required (for example, an enema may be needed before
being X-rayed if the image is not to be obscured by flatus).

As far as industry is concerned, there may be jobs entailing a high
risk of back injury due to MMH, but it is not possible to define.a
criterion for the need for radiological screening. Cases must be
assessed on their own merits and with due regard to current radio-
logical opinion and radiographic practice. When low-back X-rays are
considered justifiable, the informed consent of the individual is
essential. (It should be noted that gonadal protection cannot be
adequately achieved in women for this purpose and thus a higher risk
for them must be considered).

Even where low-back X-rays are deemed acceptable, there remains the
problem of interpretation. An Ad Hoc Committee on Low Back X-rays

93


On-the-job measurement of oxygen utilization is the most
straight-forward method for determining the metabolic energy
requirements of a particular person on a particular job.
However, on-the-job measurement of oxygen utilization is
sometimes difficult due to interference of measuring equip-
ment with the normal work methods. Handling methods, work
operations, weight and size of working material and the
particular workers may be continually changing (Aberg, et
al., 1968), and individual oxygen uptake measurements made
today may not be valid later.

On the other hand, extensive tables of metabolic energy
expenditure estimates for more than 1000 different activities
are available in the literature. Table 4.1 illustrates the
average energy costs for a small set of selected activities
(Durnin and Passmore, 1967). Such studies provide estimates

of the metabolic energy expended by the "average" person who is
performing complex manual activities under different working
conditions such as unloading coal cars, handling boxes,
stapling, loading corrugated cartons (Davis, et al., 1969),
working in a hot environment, construction work, etc.

Table values, however, provide only a rough approximation of
the metabolic cost of a given job. Such values are specific
to particular work situations employed at the time of measure-
ment and do not (for the most part) reflect the variable
effects of important personal and task parameters such as
body weight, postures, object weight, object size, travel
distance, etc. Without more detailed task descriptions it is
difficult to interpolate or extrapolate such values.

A third group of studies relate metabolic energy expended by
a person to the physical measures of the activity. Primarily
through regression and analysis of variance models, empirical
relationships between metabolic energy expenditure rates and
one or more of the physical parameters of the job have been
modelled (Frederik, 1959); Cotes and Meade, 1960; Grimby and
Soderholm, 1962; Garg, 1976; Aquilano, 1968; Aberg, et al.,
1968; Hamilton and Chase, 1969; Soule and Goldman, 1969;
Givoni and Goldman, 1971; Kamon and Belding, 1971; Snook, 1971;
Chaffin, 1972; Van Der Walt and Wyndham, 1973; Garg, et al.,
1978). In general, it is noted that minor-changes in the
physical parameters that are commonly used to describe

the manual activity result in significant changes in
metabolic energy expenditure predictions as will be
illustrated in the following section.

Variables Affecting Metabolic Rate

In order to develop a predictive model of metabolic rate for
lifting a number of variables must be considered. Table 4.2

53


WEIGHT (KG)

Figure 3.13:

WEIGHT (KG)

Figure 3.14:

60

2
°

n
°o

LEG LIFT

700 KG

600 KG
600 KG
4.400 KG

DISTANCE OF LOAD CENTER
OF GRAVITY FROM THE SPINE (cm)

Combination of Weight and Horizontal Location of
Loads Resulting in from 400 to 700 kg.
Force for Leg Lifts.

>
°

N
°o
4

BACK LIFT

700 KG
600 KG
600 KG

ieee) KG

35 «465 «66506857585

DISTANCE OF LOAD CENTER
OF GRAVITY FROM THE SPINE (cm)

Compressive

Combination of Weight and Horizontal Location of
Loads Resulting in from 400 to 700 kg.
Force for Back Lifts.

39

Compressive


Houston, C.S. "Pre-employment Radiographs of Lumbar Spine." J.
Canad. Assoc. Radiol. 28:170, 1977. ~

Hult, L. "Cervical, Dorsal and Lumbar Spinal Syndromes." Acta
Orthop. Scand. Suppl. 17, 1954.

International Labour Organization. "Manual Lifting and Carrying."
Int. Occup. Safety Health Infor. Sheet #3, Geneva, 1962.

International Labour Organization. "Maximum Permissible Weight
to Be Carried by One Worker." ILO Office, Geneva, 1964-1967.

Irvine, C.W. "A New Slipmeter for Evaluating Slippery Walkways."
Materials Research Standards. 7:535-542, 1967.

Jackson, A.; Kirwan, E.0.G. and Sullivan, M.F. "Pathogenesis and
Treatment of Lytic Spondylolisthesis at the L 4/5 Level,"
Paper presented at the International Society for the Study of
the Lumbar Spine, Utrecht, 1977.

Jones, D.F. "Back Strains: The State of the Art." J, Safety Res.
3:38-41, 1971.

Jorgenson, K. and Poulson, E. "Physiological Problems in Repeti-
tive Lifting with Special Reference to Tolerance Limits to
the Maximum Lifting Frequency." Ergonomics. 17:31-40, 1974.

Kamon, E. and Belding, H.S. "The Physiological Cost of Carrying
Loads in Temperate and Hot Environments." Human Factors.
13:153-161, 1971.

Kamon, E. and Goldfuss, A.J. "In-Plant Evaluation of the Muscle
Strength of Workers." Am. Ind. Hyg. Assoc. J. 39:801-807,
1978.

Kazarian, L.E. "Dynamic Response Characteristics of the Human

Vertebral Column." Acta, Orthop. Scand. Suppl. 146, 1972.

Kazarian, L.E. "Creep Characteristics of the Human Spinal Column."
Orthop. Clin. N. Amer. 6:3-18, 1975.

Kazarian, L.E. "Standardization and Interpretation of Spinal
Injury Criteria." Aerospace Med. Res. Lab. Tech. Review,
AMRL-TR-75-85. Wright Patterson Air Force Base, Ohio, 1978.

Kazarian, L.E. and Graves, G.A. "Compressive Strength Character-
istics of the Human Vertebral Centrum." Spine. 2:1-14, 1977.

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4:219-228, 1961.

164


Lind et al (1977)

207 50% Vo,max
= |
€
~ L
2
= r nN
tok Snook (96) sonite
S L N
a Lind et al (1977)
a fr 35% Vo,max
gt nN
wf -N

‘oe
Fe ea Po mn mn
10 20 30 40

WEIGHT OF BOX (kg)

Figure 5.8: Comparison of Physiological and Psychophysical
Criteria.

Two major points can be drawn from this illustration. First, for
low frequency lifting, capacities are limited by strength rather
than endurance criteria. Second, endurance is a function of work
duration. For continuous 8 hour lifting, the metabolic criteria
of Chapter 4 need to be applied to avoid fatigue. For occasional
high frequency lifting the psychophysical limits developed in this
Chapter are more appropriate.

88


individual factors; the latter include the psychosocial stresses
associated with accidents at work (Hirschfeld and Behan, 1963).
This is a subject to which scientific attention has only recently
been turned. It concerns the mechanical and pathological factors
which affect the dynamic response-characteristics of the spine
and thus its capacity to withstand trauma. The strength of the
tissues of the spine is inversely related to the duration of
applied stress (Pery, 1957; Kazarian, 1972; Kazarian and Graves,
1977; Lamy, 1978). The dynamic response-characteristics vary
with time, partly due to the "creep-effects" which take place
whenever the compressive loading exceeds the osmotic pressure in
the tissue. For example, a shoulder load of as little as 9 kg
(20 lbs.) on healthy young adults caused measurable losses of
vertebral height within 20 minutes, the loss of height being
greater in the morning than in the evening. Recovery of height
on removal of the load took about 10 minutes (Fitzgerald, 1972).

A similar effect is produced by vibration, “vibrocreep" being an
acceleration by vibratory stress of the creep effect induced by
static loading (Kazarian, 1972). Prolonged loading of the spine
due to postural or vibratory stresses leads first to a reduction
in height of the vertebral column; to a consequent change in the
dynamics and kinetics of the apophyseal joints and of the liga-
ments and muscles which control them; to a reduction in the com-
pliance of the spinal unit (Kazarian, 1975); and to changes in
the transmissibility of stresses along the spine (Mertens, 1978).
These changes alter the susceptibility to injury which according-
ly varies throughout the working day in a way which depends on
the pattern of spinal stress to which the worker is exposed.

The presence of degenerative changes in the disc affects its de-
formation under load (Rolander, 1966). The rate of creep depends
on the grade of degeneration. Degenerated discs display a higher
creep rate and a greater deformation which reaches a state of
equilibrium more rapidly (Kazarian, 1975). However, there is no
way in which the onset of disc degeneration can be detected in
the living. It is only visible radiologically at a comparatively
late stage. Though the process is usually symptom-free, its
presence (with the associated changes in dynamic response to load-
ing) must be accepted as a potential but unseen "conditioning"
factor which alters susceptibility to injury. Kazarian (1975)
found that recovery from creep-effect was inversely related to
the duration of loading, taking as long as 20 hours. With a more
rapid creep rate and greater deformation in the degenerated spinal
unit, it could be deduced that for a given loading duration the
degenerated segment takes longer to recover. It must therefore
be concluded that some levels of postural stress may have a chronic,
cumulative effect. Likewise exposure to vibration and other forms
of kinetic stress while under static load may not allow a full
recovery overnight.

13


CHAPTER 2

BASIS FOR GUIDE: EPIDEMIOLOGICAL APPROACH

This chapter presents a survey of recent epidemiological litera-
ture regarding short and long term health effects of MMH. Since
lumbar spine disability has received much attention in research,
this evidence is examined in detail. The factors which modify
risk of injury are divided into job and personal risk factors.
The characteristics of the job which contribute to risk are dis-
cussed in terms of weight handled, size of the load and frequency
of lifting. Personal risk factors include gender, age, anthropo-
metry, lift technique, attitude, training, and strength. The
potential for preventive measures through selective matching of
personal and job factors is also examined.

SHORT TERM EFFECTS OF MMH ON HEALTH

There are both short and long term health effects attributed to
MMH. Short term effects include traumatic injury and fatigue.
Traumatic injury to the body such as lacerations, bruising or
fractures often arise during MMH due to

1. sharp or rough surfaces on the materials handled;

2. materials being dropped;

3. workers being struck by swinging loads, and other
moving materials not under their control;

4. workers slipping and falling;

5. workers colliding with unseen objects;

6. mechanical stresses induced in the musculoskeletal
system leading to sprained joints or torn muscles.

For injuries to the limbs and those which are superficial, the
evidence is usually well documented because cause and effect are
often simple to diagnose. Musculoskeletal injuries (especially
to the lower back) are less clear cut and the extent of trauma
is seldom defined. The interpretation of such injuries therefore
depends mainly on the mechanism of injury and this (due to inex-
perienced, incomplete or subjective reporting) is difficult to
analyze.

Traumatic back injuries may arise from the unexpected situation,
the load being unexpectedly heavy or light or stuck, and from
blows on the back or slips and falls (Troup, 1965; Troup, et al.,
1970; Manning, 1971; Magora, 1974). In terms of the cost of

9


unexplained variance (about 27%) remains eyen when five or six
attributes are combined (Chaffin, et al., 1977).

The exact nature of the physical task required of a person dictates
which physical attributes are important. In one task, functional
reach overhead may be important. In another, lifting, pushing,

or pulling force capabilities are essential. To designate which
one is most relevant to selection, it is necessary to evaluate
carefully the job physical requirements as outlined later in
Chapter 8. In this same regard, it is a legal requirement that
such tests be job related to assure that they do not discriminate
against women, minorities, aged workers, or the physically disabled
(Miner, et al., 1978).

Is the test practical? To be used, a selection procedure must be
practical. What is necessary in one industry may not be needed in
another. Smaller industries may be able to construct a set of
carefully controlled tests of a person's physical capability while
the person is learning the job during the first couple of days or
weeks on the job. Larger industries may need more standardized
tests under more clinical settings to assess large numbers of job
applicants. These should meet the following practical conditions:

alo Require minimum hardware expense.
20 Have hardware capable of simulating different job conditions.
So Require minimum time to administer.

4, Require minimum instruction and learning time.

Does the test predict risk of future injury or illness? This cri-
terion 1s probably the most difficult one to meet. It requires con-
tinual evaluation of new selection methods using epidemiological
studies in industry. Injury and illness data supporting any se-
lection and placement procedure are necessary. Once a procedure is
chosen, careful evaluation of its potential effect on injury and
illness data should be instigated to assure that it meets the
desired goal of reducing health and safety problems. Such an eval-
uation must consider the degree of matching of a worker's physical
capabilities (as measured by the proposed test) with the job physi-
cal demands. After a period of time, injury frequency and severity
rates can then be compared for both those that are well matched

and for those that are not well matched.

Radiological Screening

There has been considerable zeal amongst a number of occupational
medical advisors for routine lumbar spinal radiography as a pre-
requisite for employment in heavy manual work (Stewart, 1947;
Colcher and Hursh, 1952; Becker, 1955; 1961; Kosiak, et al., 1966;
McGill, 1968; Bimlett, 1972; Moreton, 1974).: Elsewhere the enthus-
iasm waned and it is considered by many to be unwarrantable

92


CONCLUSION

It has been traditional to bemoan the existence of widely diver-
gent recommendations for maximum weights which can be safely
lifted. These divergent recommendations have actually been due
to the differences in what is accepted as a reasonable criterion
by different scientific disciplines. This Guide has attempted
to integrate the conclusions of four distinctive disciplines with-
in the broad field of Ergonomics into one set of recommendations.
Obviously, the recommendations do not satisfy any of the criter-
ia perfectly and considerable qualification of the specific as-
pects of lifting were required in order to arrive at any
.consensus.

The convergence between the methodologies, however, overshadows
the differences. The fact that these technologies agree on the
major factors which limit lifting ability and even point to
similar absolute magnitudes is strong evidence that the guidance
is sound. This guidance will only be effective, though, if it
is carefully studied and applied within your workplace.

153


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160


isometric strength tests and were then monitored for one year for
all medical injury experiences. A sharp increase in the mean low
back pain incidence rates (by a factor of 3:1) was observed for
those jobs populated by individuals who had not demonstrated
strengths equal to or exceeding that required by their jobs.

Because of the importance of this result, a second longitudinal
study (Chaffin, et al., 1977), was undertaken to determine if the
results were reproducible. This study included another 550 workers
in both light and heavy industries. All persons were strength
tested as described earlier before being placed on their jobs.

Again, the incidence rate of back pain episodes was found to be
almost three times higher in the overstressed group than in the
understressed. For strains, sprains, dislocations, and fractures
involving other than the back, strength alone did not correlate
well. However, for frequent lifting an increased incident rate
and severity rate resulted. Furthermore, contact type incidences
such as lacerations, bruises, and abrasions of a traumatic nature
increased in frequency and severity for weaker workers placed on
high strength demanding jobs. It was concluded that

“overstressing a person beyond their demonstrated
strengths cannot be tolerated by a person's musculo-
skeletal system, especially when such exertions are
performed more often than about 100 times each week.
This appears true whether the mediaal costs are measured
in terms of back injuries or more generally in terms
of musculoskeletal or trauma related injuries."

Snook (1978) surveyed the strength requirements on Liberty Mutual
Insurance Company policyholder jobs. From a sample of 191 compens-
able low-back injury claims, it was revealed that

"...approximately one-quarter of policyholder jobs
involve manual handling tasks that are acceptable

to less than 75% of the workers (in terms of strength
required); however, one-half of the low back injuries
were associated with these jobs. This indicates that
a worker is three times more susceptible to low back
injury if performing a manual handling task that is
acceptable to less than 75% of the working population.
This also indicates that, at best, two out of every
three low back injuries associated with heavy manual
handling tasks can be prevented if the tasks are design-
ed to fit at least 75% of the population. The third
injury will occur anyway, regardless of the job. The
other low back injuries not associated with heavy
manual handling tasks will also occur. Therefore, it
can be concluded that the proper design of manual
handling tasks can reduce up to one-third of indus-
trial back injuries."

21


Laubach, L.L. "Comparative Muscular Strength of Men and Women:
A Review of the Literature." Aviat. Space Environ. Med. 47:
534-542, 1976.

Laubach, L.L.; Kroemer, K.H.E. and Thordsen, M.L. "Relationships
Among Isometric Forces Measured in Aircraft Control Locations."
Aerospace Med. 43:738-742, 1972.

Laubach, L.L. and McConville, J.T. "The Relationship of Strength
to Body Size and Typology." Med. Sci. Sports. 1:189-194,
1969.

Lawrence, J.S. "Disc Degeneration: Its Frequency and Relation-
ship to Symptoms." Ann. Rhum. Dis. 28:121-138, 1969.

Lehmann, G. Praktische Arbeitsphysiologie. Verlag, Stuttgart, 1953.

Lind, A.R.; McNicol, G.W. and Donald, K.W. "Circulatory Adjustments
to Sustained (Static) Muscular Activity." Proceedings, Sym-
posium Physical Activity in Health and Disease, Universitets-
forlaget, Oslo, 1966.

Lind, A.R. and McNicol, G.W. "The Cardiovascular Responses to
Holding and Carrying Weights by Hand and by Shoulder Harness,"
with Special Reference to Stretcher Carrying." J. Appl.
Physiol. 25:261-267, 1968.

Lind, A.R.; Petrofsky, J.S. and Betz, W. "A Comparative Study of
Metabolic, Respiratory and Cardiovascular Characteristics of
Fatigue During Lifting Tasks and Bicycle Ergometry." Final
Report, DHEW(NIOSH) Contract CDC 00-74-8., 1977.

Lind, A.R.; Williams, C.A. and Petrofsky, J.S. "Physiological
Responses of Women to Repetitive Lifting and an Examination
of Factors Responsible for Fatigue." Final Report, DHEW(NIOSH)
Contract 210-77-0044, 1979.

Lovestead, G.E. "Top 10 Fork Lift Accidents." Nat. Safety News.
16:123-127, 1977. ~

McCall, I.W.; O'Brien, J.P. and Park, W.M. "Induced Pain Referal
from Posterior Lumbar Elements in Normal Subjects." Paper
presented at the International Society for the Study of the
Lumbar Spine, San Francisco, 1978.

McConville, J.T. and Hertzberg, H.T.E. “A Study of One-Handed
Lifting." Aerospace Med. Res. Lab. Tech. Report AMRL-TR-
66-17, Wright Patterson Air Force Base, Ohio, 1966.

McConville, J.T. and Hertzberg, H.T.E. "A Study of One-Handed
Lifting: A Final Report." Ergonomics. 11:297-304, 1968.

166




DEPARTMENT OF HEALTH AND HUMAN SERVICES
PUBLIC HEALTH SERVICE
CENTERS FOR DISEASE CONTROL
NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH
ROBERT A. TAFT LABORATORIES
4676 COLUMBIA PARKWAY, CINCINNATI, OHIO 45226

OFFICIAL BUSINESS
PENALTY FOR PRIVATE USE, $300

Special Fourth Class—Book
POSTAGE AND FEES PAID
U.S. DEPARTMENT OF HHS:
HHS 396,

DHHS (NIOSH) Publication No. 81-122


b. A 350 kg (770 1b) compression force on the L 5/8, disc
can be tolerated by most young, healthy workers- Such
forces would be created by conditions described by the
AL.

ce. Metabolic rates would exceed 3.5 for most individ-
uals working above the AL.

dad. Over 75% of women and over 99% of men could lift
loads described by the AL.

Thus, properly analyzed lifting tasks may be of 3 types:

1. those above the MPL should be viewed as unacceptable
and require engineering controls

2. those between the AL and MPL are unacceptable without
administrative or engineering controls

3. those below the AL are believed to represent nominal
risk to most industrial workforces.

To illustrate this point, Figure 8.1 shows the three regions and
boundaries defined for infrequent lifting (F < .2) from the floor
(V = 15 cm [6 in]) to knuckle height (D = 60 cm [24 in]).
Depending on the size of the object, in terms of horizontal hand
location, the maximum weight which can be lifted can be determined

(Ib) (Kg)
2005
8OF
7 Body Interference
i Limit
150+
8 60r
le 71 Hazardous
= E Lifting
4 1004 Conditions
LS 40h
2 4 Functional Reach
> 5 Limit ! .
Bo | _ Maximum
20 lh ted
7 Action
Limit
he
o+ o 0 (cm)
(in)

10 20 30
HORIZONTAL LOCATION OF LOAD

Figure 8.1: Maximum Weight versus Horizontal Location for Infre-
quent Lifts from Floor to Knuckle Height

125


Montgomery, C.H. “Pre-employment Back X-Rays." J. Occup. Med.
18:495-498, 1976.

Mooney, V. and Robertson, J. "The Facet Syndrome." Clin. Orthop.
115:149-156, 1976.

Moores, B. "A Comparison of Work Load Using Physiological and
Time Assessment." Ergonomics. 14:61-69, 1970.

Moreton, R.D. "The Role of Preplacement Roentgen Examination in
Evaluation of Low Back Pain." South. Med. J. 67:1105-1110,
1974.

Moreton, R.D.; Winston, J.R. and Bibby, D.E. "Value of Preplace-
ment Examination of the Lumbar Spine." Radiology. 70:661-
665, 1958.

Morris, J.M.; Lucas, D.B. and Bresler, B. "Role of the Trunk in

Stability of the Spine." J. Bone Joint Surg. 43A:327-351,
1961.

Munchinger, R. "Manual Lifting." International Labour Organiza-
tion, Geneva, 1962.

Murphy, R.W. "Nerve Roots and Spinal Nerves in Degenerative Disc
Disease." Clin. Orthop. 129:46-60, 1977.

Murrell, K.F.H "Man in His Working Environment." In: Ergonomics.
Chapman and Hall, London, 1969.

Nachemson, A. "Low Back Pain, Its Etiology and Treatment." Clin.
Med. 78:18-24, 1971.

Nachemson, A. and Elfstrom, G. Intravital Dynamic Pressure Measure-
ments in Lumbar Discs. Almqvist and Wiksell, Stockholm, 1970.

Napier, S. "The Prehensile Movements of the Human Hand." J. Bone
Joint Surg. 38B:902-913, 1956.

National Safety Council. Accident Facts. 1975.

Nordgren, B. "Athropometric Measures and Muscle Strength in Young
Women." Scand. J. Rehab. Med. 4:165-169, 1972.

Park, K.S. "A Computerized Simulation Model of Postures During
Manual Materials Handling." PhD Dissertation, University of
Michigan, 1973.

Parvi, V and Virolainen, M. "Roentgenographic Findings of the Lum-
bosacral Spine in Preemployment Examination of Lumbermen with
Special Reference to Spondylolisthesis." Scand. J.Work Environ.
Health. 1:40-44, 1975.

168


terms, the reverse is true because the men are so much stronger
than women. As an extreme example, at their average MVC, 36 kg,
the endurance of women is only 3 sec whereas the same tension
represents only 70% MVC for men who hold that tension for an
average of 36 sec, or 12 times longer. At the other end of the
tension/endurance curve, a tension of 7.5 kg represents 15% MVC
for men and can be held for an average time in excess of 30 min
(the absolute time is not known). In contrast, 7.5 kg represents
21% Mvc for women and the average endurance time is about 10
minutes.

The type of muscle examined is doubtless an important determinant
of endurance, but there is insufficient data to provide realistic
comparisons with hand-grip contraction. The muscle mass will
undoubtedly affect the oxygen uptake but has little effect on the
cardiovascular responses, which provide the most dramatic differ-
ences in physiologic changes when compared to those in dynamic
exercise. In rhythmic exercise there is a large increase in heart
rate but little increase in blood pressure, whereas the increase
in heart rate is modest with a large increase in blood pressure
during isometric exercise.

The physiological responses to isometric exercise are superimposed
on those due to rhythmic exercise when the two kinds of activity
are carried out simultaneously. But insufficient information is
available about intermittent exercise to characterize precisely
that the circumstances that induce fatigue. Current evidence
indicates a duration of contractions:rest ratio of 2:1 will induce
fatigue at high tensions such as 60% MVC whereas a ratio of 3:1 is
necessary to induce fatigue at low tensions (25% MVC).

PHYSIOLOGICAL RESPONSES TO LIFTING WEIGHTS

1. Strength of Lifting

Claims concerning the ‘physiological or anthropometric correla-
tes to lifting strength are varied. Whitney (1958) made it
clear that in most lifting actions, the body weight, which acts
as a counterbalance, must have a proportional relationship (as-
sociated, of course, with the center of gravity of the body) to
the lifting strength. He showed that the most important factor
which affected the isometric strength in a lifting action was
the distance of the feet from the object to be lifted and that
other features of the lifting procedure such as the "derrick"
or "knee" action, or the type of grasp used, had only small
effects on the strength exerted. Poulsen (1970) came to the
conclusion that the maximum load that can be lifted correlated
well with the maximum isometric strength of the back muscles;
it must be noted, however, that the isometric strength of the
back muscles was measured with a dynamometer that did not
directly duplicate the lifting task. Recent proposals concern-

ing "safe" weights to be lifted in a wide range of positions have

45


TRAINING OF WORKERS

Training for safety in MMH has been in practice in several European
countries since the Second World War. A pioneer in this field T.
McClurg Anderson (The Institute of Kinetics, Glasgow) advanced the
concept of “human kinetics" in the avoidance of unnecessary stress.
It was widely adopted by a number of government departments and in-
dustries in the United Kingdom and forms the basis for industrial
training courses run by the Royal Society for the Prevention of
Accidents. Comparable modified courses are held for recruits in
many industries and instructors' courses are run by organizations
such as the British Safety Council. The Back Pain Association has
recently published an instructors' manual, "Lifting in Industry."
In France, the Institut Nationale de Recherche et de Securitie
publishes "Gestes et postures de securitie dans le travail"; and
similar courses are offered throughout Europe and Scandinavia.

The importance of training in manual materials handling in reducing
hazard is generally accepted. The lacking ingredient is largely

a definition of what the training should be and how this early ex-
perience can be given to a new worker without harm. The value of
any training program is open to question as there appear to have
been no controlled studies showing a consequent drop in the MMH
accident rate or the back injury rate. Yet so long as it isa
legal duty for employers to provide such training or for as long

as the employer is liable to a claim of negligence for failing to
train workers in safe methods of MMH, the practice is likely to
continue despite the lack of evidence to support it. Meanwhile,

it may be worth considering what improvements can be made to exist-
ing training techniques.

Training Aims
The aims of training for safety in MMH should be:

abe to make the trainees aware of the dangers of careless
and unskilled MMH;

2 to show them how to avoid unnecessary stress and

3. to teach them individually to be aware of what they can
handle safely.

Instructors must be well versed in the sciences basic to MMH and

in Safety Engineering, and be practically experienced and skilled
in all types of manual handling. The content of the courses should
be suited to the educational background of the trainees. It is
important not to underestimate people's practical intelligence

and understanding.

The course should cover the following aspects:

99


illustrated as in Figure 4.8. This figure demonstrates the
cost associated with moving the body from a squat versus
stoop posture. This may be one of the reasons why workers
often employ the stoop posture (minimizing energy expenditure)
for a fixed work output.

The relationship between load and frequency is illustrated
in Figure 4.9. This study (Snook, 1971) demonstrated

the effect of work load (load x frequency x distance

of lift) on metabolic rate for different loads based on 30
male workers. For a fixed work capacity criterion (such
as 5.0 Kcal/min) it is apparent that a greater amount of

work can be accomplished if heavier loads are lifted at
slower paces.

oO &
ZE Ff Stooped Lift Posture
- w
32 4
w
° S 9+ A
SZ =F
go 7
$2 ‘|
g5 '
e> 5r SAL
* Cm Squat Lift Posture°~~ _,
4
3k
IL
1L L 1 L 1
fo) 5 ike) 15 20 25

LOAD (kg)

Figure 4.8: Estimated Maximum Frequency of Lift with Two
Postures (Adapted from Garg and Herrin, 1979).

63


In most previous studies, jobs have been simply classified as
heavy, medium or light work. When referring to low back stresses,
a job classified as light or sedentary by traditional criteria
(i.e., caloric costs to perform the job) still may require the
person to lift 30 to 50 kilograms a few times during a work shift.
Depending on the size of the object and body postures assumed,
these infrequent acts could produce injurious mechanical stresses
on the low back.

Studies by Chaffin and coworkers confirm the relation between weight
handled on a job and musculoskeletal injuries. The first study
(Chaffin and Park, 1973) monitored 400 workers for a one-year period.
It was concluded that the

"lifting of loads greater than about 35 pounds (16 kg)
when held in close to the body, or equivalent condi-
tions, such as 20 pounds (9 kg) between 25 and 35
inches (64 and 89 cm) in front of the body, would be
potentially hazardous for some people."

This conclusion was based on both on-the-job and off-the-job low-
back pain incidence rates.

In a follow-on study (Chaffin, et al., 1977) of 550 workers for

a two-year period it was found that heavier jobs (in terms of
maximum load lifted) also resulted in increased severity of in-
juries in terms of total lost workdays or medical work restric-
tion days. In general, load handling of less than about 20
kilograms resulted in relatively few incidents of a severe strain
or sprain diagnosis, but the heavier load handling jobs were
associated with more severe sprains, joint dislocations and bone
fractures.

Ayoub, et al., (1978) established job severity indices for workers
based upon job demands which include weight handled, box size and
frequency of lift. In a study of 220 males and 24 females employed
on 63 lifting jobs, they found an increase in the incidence and
severity of musculoskeletal disorders as these job severity indices
increased.

Location/Size of Load

The physical dimensions of the load handled are important from a
biomechanical, physiological, and psychophysical point of view.

In studies referred to above, Chaffin and co-workers combined the
weight, horizontal and vertical location of the object handled into
an index of job stress referred to as a "lift strength rating."

The value of this rating ranged from zero, when little or no lifting
was involved, to 1.0 where the lifting was such that only a very
strong person could perform the job due to excessive weight or
awkward posture.

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List of Tables (cont.)

Table 5.7: Maximum recommended weights based on dynamic
SCTENGth. oeceeccrcccccccccccacccsccccccsccce

Table 8.1: Emphasis on task variables by alternative
APPLOACHES. eee ceccccccccccccvcccccccecccces

Table 8.2: Fhaax Tables. seceeeccecerecer ee eccececnsccees

84

123
129


15.9 kg
Tr 22.7 kg
29.5 kg

Weight of Container

METABOLIC RATE (kcal/min)
a
T

10 20 30 40 50 60 70 8
WORK LOAD (kg—m/min)

Figure 4.9: Effect of Workload on Metabolic Rate (Snook, 1971).

PHYSIOLOGICAL DESIGN CRITERIA

Based on the preceding discussion of the metabolic costs
associated with repetitive work it is recommended that:

1. For occasional lifting (for one hour or less) metabolic
energy expenditure rates should not exceed 9 Kcal/min
for physically fit males or 6.5 Kcal/min for physically
fit females (see Figure 4.10). This is based on indus-
trial population aerobic capacity estimates discussed
earlier in this chapter adjusted for working time. (It
does not, however, take into account, Lind$ observations
that the VOj max Kcal/min values for men and 10.5 Keal/min
values for women) .

2. Likewise, continuous (8 hour) limits should not exceed
33% of aerobic. capacity or 5.0 Keal/min and 3.5 Kcal/min
respectively. These guideline limits do not reflect the
increased metabolic rates which would be associated with
overweight or deconditioned workforces.

3. Personal attributes of age, gender, body weight, etc.
are insufficient to accurately predict work capacity for
any particular individual, although such data are suf-
ficient for making predictions of group averages.

64


Table 5.1: Maximal voluntary isometric strength (kilograms)

MALES RETEST FFMALES:
ASSUMED SAMPLE ‘COEFF. OF PE OLULAT LOMAS TILE) SAMPLE CUEFF. OF POPULATION STILE
TEST DISTRIBUTION SIZE VARIATION 10 25 50 75 90 SIZE VARIATION 10 25 50 75 90
Arm Lift: Normal 1052 07 23°31 «#39 48 #56 *187 08 9 15 22 28 34
Torso Lift es 1052 09 - 34 Go 6q 77 187 10 13°17 #24 «33 44
Leg Lift Normal 638 os D 69 ms 114 134 133 _ Ba 40 wD 64
Tene PNT Log Normal 309 209 16 19 £3 28 34 35 a ’ 7 13° le 19 ;
Floor Lift Normal 309 208 59° 74 9: 108 123 a .08 32 44 56 69 80
High Near wift Normal 309 208 35° 44 oD 66 76 35 7 GO 4 29° 36 «42
i
t
(1) ARM LIFT (2) TORSO LIFT (3) LEG LIFT
90° V=38cem H=38 cm V=38cm H=0 cm
1 |
(4) HIGH FAR LIFT (5) FLOOR LIFT (6) HIGH NEAR LIFT
V=1I52 cm H=5l cm V=15 cm H=25cm V=iS2cm H=25cm

70


List of Illustrations (cont.)

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

4.1:

4.2:

4.3:

4.4:

4.5:

4.6:

4.7:

4.8:

4.9:

4.10:

Bo ab8

30238

Slr sis

5.4:

5.5:

5.6:

Estimated Population Aerobic Capacities
for U.S. Men and Women (Chaffin, 1972).....

Physical Work Capacity and Working Time
(Bink, 1962 and Bonjer, 1962)..............

Metabolic Rate for Different Postures
(Brown, 1971)... .. cece ee ee cece ccc eee eens

Effect of Technique and Workload on Meta-
bolic Rate (Garg and Saxena, 1979).........

Energy Expenditure versus Work Pace
(Hamilton, 1969)... .. ccc cece cee cece eee

Effect of Pace on Metabolic Rate
(Aquilano, 1968).............. 90000000G0000

Energy Efficiency with Weight and Range
of Lifting (Frederik, 1959)................

Estimated Maximum Frequency of Lift with
Two Postures (Garg and Herrin, 1979).......

Effect of Workload on Metabolic Rate
(SHOCK MELOM +) eteteleletetedelehslelsleleleletcleletetcicioieieteteleiche

Recommended Maximum Capacities for
Continuous Work....... eee eee eee cece eee e eee

_ Strength Differences between Male and

Female Industrial Workers (Kamon and
Goldfuss, 1978)... ... cece ccc cen c cnc ccnecces

Linkage Representation (Garg and Chaffin,

OMS} ob o00000 99000000000000050G0G00G00000000

Body Angles Used to Depict Posture (Gary
and Chaffin, 1975)...... 900000000000G0040000

Predicted Lifting Strength of Large/
Strong Male (Chaffin, 1974)................

Classification of Lifting Height...........

Maximum Recommended Weights Based on Male
Isometric Strength... ... cece cece ec wee eee eee

viii

49

52

57

57

58

59

61

63

64

65

72

73

74

5
80

86


the individual's aerobic capacity after adjustment for work period
duration to determine the relative stressfulness of the job.

Aerobic capacity testing is most appropriate for jobs that inyolve
relatively continuous, dynamic work. For activities such as lift-
ing, holding and carrying a significant amount of static work is
also performed. Thus, the above testing methods and comparisons
may not be very accurate. Preliminary studies indicate that more
research is needed before good evaluation techniques for the meta-
bolic cost of mixed static and dynamic tasks will be available.

At present, applying capacity estimates based on dynamic testing
to jobs with both static and dynamic tasks will not necessarily

be protective.

In terms of practicality, numerous variations in aerobic capacity
testing procedure exist. The primary differences are in the
sophistication of hardware required and the time needed to com-
plete the test (Kamon and Ayoub, 1976). As would be expected,

the major trade-off is between accuracy and cost. Procedures can
take anywhere from fifteen minutes to approximately an hour and
involve nothing more than a simple step device or require relative-
ly elaborate heart rate monitoring and gas analyzers.

SELECTION OF WORKERS - SUMMARY

The assessment of a worker who performs manual lifting must in-
clude specific tests of physical capability. Such tests must meet
certain criteria to be accepted as medically, economically, and
legally justified. The medical and economic justification for
such tests is recognized by all concerned with controlling the
excessive costs and human suffering associated with overexertion
strain/sprain injury and illness in industry. The legal basis

for such tests is not well established, and will not exist until

a documented history of success or failure is developed in the
courts for different types of tests.

It is hoped that as various tests are developed by medical depart-
ments, the criteria stated in the first part of this section will
be of assistance. Strength testing and aerobic capacity testing
have been implemented as part of medical examinations in a number
of large industries as experimental medical procedures. Further
studies are needed to validate and refine the necessary procedures.

98


STATIC STRENGTH

The usefulness of any test of human performance is inherently
limited by the reliability and repeatability of the measurement
technique. Strength is no exception. It is susceptible to many
influences which can affect the outcome of the measurement.
Following a review of the literature by Kroemer and Howard (1970),
it was recognized that there was little uniformity in either the
techniques used in assessing strength or in the statistical
methods used to report the results of studies. Due to the lack
of consensus on methodology, an ad hoc committee of experts first
held a series of meetings in 1972 for the purpose of proposing

a strength testing standard (Caldwell, et al., 1974). The recom-
mendations of this group were later adopted as an "Ergonomics
Guide for the’ Assessment of Human Static Strength" by the Ameri-
can Industrial Hygiene Association (Chaffin, 1975). This guide
describes the use of static tests for the measurement of human
strength.

Static strength is defined as:

"...the maximal force muscles can exert isometrically
in a single voluntary effort." (Roebuck, Kroemer, and
Thompson, 1975).

There are several advantages in using this technique of strength
assessment.

ako The technique is relatively simple. The subject is ask-
ed to assume a particular body posture and to exert a
force against a stable resistance. As a result, the
position of the subject's joints are under the control
of the experimenter and only one measurement is required,
namely the magnitude of the exertion. On the other
hand, dynamic strength tests involve body motion. The
positions of the subject's joints are no longer con-
trolled and, therefore, should be continuously monitored.
Furthermore, the velocity and acceleration of various
body members need to be measured. Instead of recording
and analyzing a single result, many data points must
be considered in the dynamic analysis of strength
(Kroemer and Howard, 1970; Caldwell, et al., 1974).

Jo Subjects are at minimal risk of injuring themselves
during this type of test since the exertion is isometric
and completely voluntary. They are requested to slowly
increase their exertions, and to stop if any abnormal
discomfort is felt.

So The measurement is repeatable with a high degree of re-
liability, (test-retest coefficient of variationson the
order of 14 percent are reported by Chaffin, et al.,
1977).

68


‘Table A.1 (cont.)

ISOMETRIC BACK
3. CAPABLE COMPRESS. .
UPPER | LOWER|UPPER | LOWER

179


(1964) proposed some radiological criteria for job-placement based
on a consensus of opinion (since there were no reliable data on the
predictive rating for the observed abnormalities). Such data are
still nonexistent. It is impossible to quote a value for the
increased probability of back trouble for each abnormality. One
partial exception is the case of scralisation studied extensively
by Tilley (1970). He studied lost work-time due to back pain in
7,236 workers who had pre-employment X-rays: 1013 (14%) had
sacralisation and as a group their average lostwork-time of 3.4
days/year did not differ from the normal. When the group was
subdivided, some types of sacralisation were associated with less
than the average lost work-time (e.g., bilateral sacralisation and
fusion - 2.4 days/year) and others with aboveaverage (e.g., pseudar-
throsis one side and fusion the other - 4.6 days/year). Even in
this case there is hardly an adequate basis for advising anyone not
to take a given job based solely on this positive sign.

Spondylolisthesis is more generally assumed to be a 'high-risk'
abnormality, particularly in the younger worker (Parvi and
Virolainen, 1975). However, in none of the many published series
in which its incidence is recorded has there been any attempt to
differentiate by cause [i.e., spondylolytic, dysplastic, degenera-
tive, etc., (Wiltse, et al., 1976); by vertebral level, despite the
widely varying prognostic implications [clinically, those at L4/5
are more likely to have neurological dysfunction than those at
L5/S1' (Jackson, et al., 1977); or, in the case of spondylolysis, in
terms of its structural stability (Hirabayashi, et al., 1972; Troup,
1976].

A third aspect of the interpretation of lumbar spinal radiographs
concerns the size and shape of the spinal canal and intervertebral
foramina. Computer-assisted tomography is a technically elegant
way of assessing these dimensions. There is evidence that those
with sciatic pain or symptoms due to stenosis have narrower canals
and foramina than the normal population (Porter, et al., 1980). The
phenomenon is not one of general narrowness in all dimensions
because there is evidence that the different measurements (mid-
sagittal diameter, interpedicular distance, pedicular length or
foraminal AP, and interfacetal distance) are independent variables
(Troup, et al., 1974). The predictive significance of these
measurements to back or sciatic pain is not yet known, though Troup,
et al., (1974) found an inverse relationship between previous
sciatic pain and AP foraminal diameter.

In summary, with present knowledge of the prognostic significance
of radiological abnormality, employment advice should be given

with caution, and then only by an experienced physician or surgeon
after a thorough interview and examination, and with full knowledge
of the postural, kinetic and handling stresses of the MMH job
concerned.

94


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79


Sigler, P.A. "Relative Slipperiness of Floor and Deck Surfaces."
Building Materials and Structures Report. National Bureau
of Standards, BMS, 100, July, 1943.

Smith, L.E. and Edwards, D.K. "Prediction of Muscular Endurance
from Individual Differences in Initial and Post Training
Increments from Static Strength." Human Factors. 10:345-
350, 1968.

Snook, S.H. "Group Work Capacity: A Technique for Evaluating
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Snook, S.H. "The Effects of Age and Physique on Continuous
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Snook, S.H. "Psychophysiological Indices-What People Will Do."

In: Safety in Manual Materials Handling. C.G. Drury, Ed.
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Snook, S.H. "The Design of Manual Handling Tasks." Ergonomics
Society Lecture given at the Cranfield Institute of Technol-
ogy, Bedfordshire, England, 1978.

Snook, S.H. "The Design of Manual Handling Tasks." Ergonomics.
21:963-985, 1978.

Snook, S.H. and Ciriello, V.M. "The Effects of Heat Stress on
Manual Handling Tasks." Am. Ind. Hyg. Assoc. J. 35:681-685,
1974a.

Snook, S.H. and Ciriello, V.M. "Maximum Weights and Work Loads
Acceptable to Female Workers." J.Occup. Med. 16:527-534,
1974b.

Snook, S.H. and Irvine, C.H. "Maximum Acceptable Weight of Lift."
Am. Ind. Hyg. Assoc. J. 28:322-329, 1967.

Snook, S.H. and Irvine, C.H. "Maximum Frequency of Lift Accept-
able to Male Industrial Workers." Am. Ind. Hyg. Assoc. J.
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Snook, S.H. and Irvine, C.H. "Psychophysical Studies of Physiol-
ogical Fatigue Criteria." Human Factors. 11:291-300, 1969.

Snook, S.H.; Irvine, C.H. and Bass, S.F. "Maximum Weights and
Workloads Acceptable to Male Industrial Workers." Am. Ind.
Hyg. Assoc. J. 31:579-586, 1970.

Sonoda, T. "Studies on the Compression, Tension, and Torsion

Strength of the Human Vertebral Column." J. Kyoto Prefect
Med. Univ. 71:659-702, 1962.

abejat


Wickstrom, G.; Nanninen, K.; Lahtinen, M. and Riihimaki, H.
"Previous Back Syndromes and Present Back Symptoms in
Concrete Reinforcement Workers." Scand. J. Work Environ.
Health. 4, Suppl. 1:20-28, 1978.

Wilkins, G.F.; Schilling, J.W. and Showalter, E.J. "Industrial
Back Injuries-Bell System." Am. Med. Assoc. Arch. Ind.
Health. 15:91-100, 1957.

Williams, C.A.; Petrofsky, J.S. and Lind, A.R. "Physiological
Responses of Women During Lifting Exercise." Submitted for
publication to the Eur. J. Appl. Physiol., 1980.

Wiltse, L.L.; Newman, P.H. and McNab, I. "Classification of
Spondylolysis and Spondylolisthesis." Clin. Orthop.117:
23-29, 1976.

Wisconsin Bureau of Research and Statistics. Work Force 1980,
Occupational Projections. Wisconsin Department of Industry,
Labor and Human Relations, 1974.

Women's Bureau. 1975 Handbook on Women Workers. Employment
Standards Administration, U.S. Department of Labor, Bull.
297, 1975.

Workmen's Compensation Division, Department of Industry, Labor
and Human Relations, State of Wisconsin, Workmen's Compen-
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Woodson, W. "Good Human Engineering Is Possible Using
off the Shelf Component Products." Human Factors. 13:141-
152, 1971.

Wyke, B. "Neurological Aspects of Low Back.Pain." In: The
Lumbar Spine and Back Pain. M.I.V. Jayson, Ed. Sector
Publishing Co., London, 1976. pp 189-256.

174


2)

there was a reduction on hand-grip isometric endurance
(measured after the hour's lifting) linearly related
to the weight of the box; electromyographic evidence
showed more fatigue generated in the forearm than in
the back muscles during lifting. (Lind et al., 1977;
Petrofsky and Lind, 1978a).

When women lifted boxes by the bent leg method from the
floor to a bench 50 cm high (24 inches),

1)

2)

the evidence from oxygen uptake, heart rate, and
forearm electromyograms showed that fatigue occurred
at approximately 50% of the VO2 max measured for the
weight of box being lifted (i.e., at levels corre-
sponding to some 40% VO. max for bicycle ergometry
at box weights up to 6.8 kg (15 1b).

there was a reduction of hand-grip endurance after
the hour's lifting which was linearly related to the
weight of box. That reduction was not as great as
in the data from men. (Lind et al., 1979; Williams
et al., 1980).

Relationship of Vo to weight of box and frequency of

lifting. For all the data available, irrespective of the
origin of the lift and its vertical travel, there is

1)

2)

3)

a linear increase in Vo2 with the rate of lifting
boxes of the same weight.

a linear increase in V05 with the weight of box at
the same rate of lifting.

a greater VO2, when the weight of box and the rate

of lifting are constant, when the lifting is performed
by the bent-leg method when compared to the cantilever
method. (Aquilano, 1968; Hamilton, 1969; Brown, 1971;
Snook, 1971; Lind et al., 1977; Miller, et al., 1977;
Lind and Petrofsky, 1978a; Lind, et al., 1979; and ~
Williams, et al., 1980).

Relationship of Vo> with point of origin and height of

travel of lifting.

1)

2)

There are comparable data on VO2 from experiments in
which the boxes are lifted from the floor to a height.
of 50 to 90 cm (20 to 35 inches) when the weight of
box and the rate of lifting are constant. The vari-
ation in average VO2 can be attributed to different
sizes of box or the method of lifting.

The energy cost of lifting boxes from "table" height,
50 to 90 cm (20 to 35 inches) to a height of 100 to
120 cm (40 to 47 inches) is substantially lower than
lifting from the floor when the weight of box and the
rate of lifting are kept constant.

47


Age and Sex

The age and sex of an individual have a profound effect on

the VO2 max. The decline of VO2 max with age for both sexes,
can be expressed as a linear function (cf. Astrand and Rodahl,
1977; Hodgson and Buskirk, 1978). At age 60 years, an individ-
ual will have, on average, a reduction of about 30 to 40% of
the VO2 max he had at 20 years; that is, there is a loss of
about 10% in aerobic capacity each decade. The VO2 max at any
given age for women averages about 70% of that for men.

Body Weight and Body Fat

The Voy max of men who are not fat is linearly related to the
body weight. The same is true for women but the relationship
has a lower slope, particularly noticeable after puberty. The
difference between men and women may well be due to sex
differences in fat content of the body.

When the VO2 max is expressed in terms of unit body weight, the
influence of age is not diminished but the variance of the
results is much reduced. Treatment of the results in this way
also reduces the influence of sex for a given age. When the
amount of body weight that is fat is measured, the VO2 max can
be expressed as oxygen uptake per unit of "lean" body weight.
That procedure further reduces the variance of the results for
men and women at given ages. It diminishes but does not
completely abolish the sex difference in VO2 max and does not
influence the proportional loss of VO2 max with age.

Type of Exercise

The type of exercise used in the assessment of VO2 max
influences the results. The dominant feature influencing the
measurement is the mass of the muscles involved in the exercise.
Thereby, running uphill provides values which are marginally
greater (5 to 8%) than running on the horizontal or bicycling.
Bicycling itself is reported to yield a slightly higher VO2 max
at 60 rpm than at higher or lower values of rpm. Arm cranking
or bicycling with both legs while simultaneously cranking with
both arms does not result in a VO2 max greater than that found
in running uphill; there appears to be an optimum mass of
muscles which provides a VO2 max.

This matter gains importance when industrial work is considered.
It has been shown that the energy cost of lifting boxes of the

43


An added complexity in determining the exposed worker population
stems from the continually changing mix of personal risk factors.
As suggested by the ILO (Table 1.1) both gender and age of the
work force modify risk. Figure 1.1 depicts the total employment
by gender in the U.S. from 1966 through 1976. An analysis of
such trends by the Women's Bureau of the U.S. Department of
Labor (1975) stated:

“The number of women employees increased at a much
faster rate than men workers did. Women made up 39%
of the total employment in 1974 compared to 34% in
1964.
four major industry divisions that showed the fastest

growth:

U.S. LABOR FORCE (millions)

Most of the increase in women workers was in

Services: 2.8 million increase

Government: 2.6 million increase

Wholesale & Retail Trade: 2.2 million increase
Manufacturing: 1.4 million increase"

o
{e)

iM) &
(e} {e)

{o)

Figure 1.1:

Men Employed ey eee
fn ng 0 OT OT OT

°

Women Employed

L n re ae

1968 1970 19721974 1976

YEAR

Employment in United States by Gender, 1966-1976.
(DOL, 1977).


ASYMMETRIC LIFTING

The preceding biomechanical discussion has considered relatively
symmetric and smooth lifting of loads. Symmetric lifting, wherein
the load is held with both hands in front of the body, is believed
to be the most common method of handling a heavy load. This
method equalizes the stresses bilaterally on the musculoskeletal
system allowing a person to utilize their muscle strengths most
effectively. There are situations, however, wherein moderate
loads may be lifted asymmetrically. Unfortunately, the hazards
of such lifting postures have not been documented in controlled
field studies, but biomechanically one must be concerned. An
asymmetric lift, which has the person bring the load up along the
side of the body, causes not only a lateral bending moment on the
lumbar column but, because oflordodic curvature of the column,
produces a rotation of each vertebra on its adjacent vertebra.

One laboratory study by Farfan, et al., (1970) indicates that
disc degeneration most often involves the annulus fibrosus, which
is the structure that provides 40-50% of the torsional resistance
to twisting of the lumbar vertebrae. With disc degeneration, this
torsional resistance can be reduced to less than one-half its
normal strength, thus providing a significant injury potential.

In addition, the asymmetric loading of the musculature of the
back could produce a concentrated stress of sufficient magnitude
to strain a specific muscle of the many muscles required to sta-
bilize the column. In general, it must be concluded that lifting
of loads along the side of the body is to be avoided. A person's
arm and shoulder strengths are not well enough developed to lift
heavy weights in an asymmetric fashion. Moderate load lifting,
however, may be attempted using a side lift, and therefore in-
structions and job redesign often are indicated to reduce the
stresses associated with such lifts.

DYNAMIC LIFTING

Another limitation regarding the present state of knowledge con-
cerns the dynamics of load lifting. One investigation by Park
(1973) disclosed that the lifting of loads between 6.5 kg (15 lbs.)
and 23 kg (50 lbs.) from the floor to an erect carrying position
(load against the front of the upper legs) resulted in an acceli-
eration effect that added between 15% and 20% to the static load
100 msec after the beginning of the lift. Furthermore, with fast
motions the ability of the somatic nervous system to coordinate
the many muscles necessary to stabilize the spinal column is
stressed. Electromyographic studies by Donish and Basmajian
(1972), Tichauer (1971), Morris, Benner, and Lucas (1962)

have only recently begun to identify the temporal complexities
involved in coordinating the recruitment of the back muscles.

It is hypothesized by Brown (1973) that some low-back problems
are related to muscle fatigue, which further inhibits coordina-
tion of the back muscles. Tichauer (1966) suggests that unanti-
cipated motions due to trying to catch falling or tossed objects
can cause low-back injuries.

34


400

350 O-——0 Back Lift

a—a Leg Lift

250

o
=
= 200
a Compressive
é 4150 Force

100 -

pols Shear Force

° 1 1 L L mn L 1 1
2 4 6 & 4.0 41.2 44 4.6

Time in Seconds

Figure 3.12: Changes in Compressive and Shear Forces on the Low
Back for a Leg Lift and Back Lift During the Lifting
Action

lift, the compressive force reaches a maximum after about 200 msec.
Figures 3.13 and 3.14 illustrate the combination of weight and
horizontal location of the load lifted which result in compressive
forces of from 400 to 700 kg on the Ls5/S, disc for leg (squat)
lifts and back lifts (stooped), respectively. For each figure
average male anthropometry and body segment weights are assumed.
Horizontal location of the load with respect to the spine is
determined using the equation H = (W/2 + 20) cm, where W = width
of the object away from the body.

Davis and Stubbs (1977a, 1977b, 1978), in a three-part series,
expressed safe levels of manual forces for young males in various
standing, squatting, sitting, and kneeling postures based on
biomechanical considerations. Morris, et al., (1961) and Davis

and Troup (1964) noted a correlation between magnitudes of forces
acting on the lower spine during manual activity and the magnitudes
of intra-abdominal pressure. Using this relationship, Davis and
Stubbs monitored intra-abdominal pressure with a swallowed radio
pill on 200 male soldiers while they performed the aforementioned
activities. ,

38


CHAPTER 5

BASIS FOR GUIDE: PSYCHOPHYSICAL APPROACH

A minimal requirement for performing any manual materials handling
task is sufficient strength to exert the required force. Strength
in this context is defined as the maximum voluntary force a per-
son is willing to exert in a single attempt. Endurance (capacity)
is the force a person is willing to repeatedly exert for an ex-
tended period of time without "feeling fatigued".

In this chapter, human strength will be expressed in psychophysi-
cal terms. Unlike the biological concepts of fatigue and tissue
tolerance; psychophysics is concerned with human acceptance of
pain or discomfort during an exertion under normal conditions.

PSYCHOPHYSICS

Psychophysics is a very old branch of psychology that is concerned
with the relationship between human sensations and their physical
stimuli; very rarely is this a one-to-one relationship. Accord-
ing to modern psychophysical theory (Stevens, 1960), the strength
of a sensation (S) is directly related to the intensity of its
physical stimulus (I) by means of a power function: S = kITM,

The constant (k) is a function of the particular units of measure-
ment that are used. When plotted on log-log coordinates, a power
function is represented by a straight line, with the exponent (n)
being equal to the slope of the line. Exponents have been experi-
mentally determined for many types of stimuli, for example, 3.5
for electric shock, 1.3 for taste (salt), and 0.6 for loudness
(binaural). Of interest here is the perception of muscular effort
and force, both of which have been found to obey the power law,
and both with an exponent of approximately 1.6 (Borg, 1962; Eisler,
1962). Stevens and Cain (1970) found that the exponent for dura-
tion of hand grip is about half the exponent for force of hand
grip.

Psychophysics has been applied to practical problems in many areas.
For example, the scales of effective temperature, loudness, and
brightness were developed with psychophysical methodology (Hough-
ton and Yagloglou, 1923; Stevens, 1956, 1960). Psychophysics has
also been used by Borg (1962, 1973) in developing ratings of per-
ceived exertion (RPE); by the U.S. Air Force in studies of lifting
(Emanuel, et al., 1956; Switzer, 1962); by the U.S. Army in studies
of treadmill walking (Evans, 1961, 1962); and in the development
of effort scales (Caldwell and Smith, 1967; Caldwell and Grossman,
1973).

67


PHYSICAL STRESS JOB ANALYSIS SHEET

DEPARTMENT. DATE
JOB TITLE ANALYST’S NAME
TASK oByecT |HAND LOCATION | task

Origin [Destination
DESCRIPTION | WEIGHT | U9)

Ave Max | om cm] cm cm

FREQ AL] MPL]} REMARKS

Figure 8.2: Example Coding Form.

determined by measuring the distance from the floor to
the point at which the hands grasp the object. The co-
ordinate system is illustrated in Figure 8.3.

These measures are repeated for the ending point of the lift (in
the lifting position) and all four values are recorded on the job

analysis sheet.

It these four values vary from task to task (e.g., stacking cartons
on top of each other), the job must be separated into elements and

each element evaluated. Examples later will suggest ways to handle
such situations.

To be precise, the H value should be measured as described in
item 2. However, a convenient rule of thumb is H = (W/2+15) cm
or (W/2+6) in, where W = the distance of the load away from the
body measured in the horizontal axis (fig. 8.3).

3. Frequency of lift - this should be recorded on the job
analysis sheet in average lifts/min. for high frequency
jobs. A separate frequency should be entered for each
distinguishable job task.

131


objects on the floor rather than on shelves. Of course, bio-
mechanical low back compression (Chapter 3) and cardiovascular
(Chapter 4) criteria would deem this least desirable.

CRITERIA FOR GUIDELINE

It is concluded, regardless of the approach taken to evaluate the
physical stresses of lifting, that a large individual variability
in risk of injury and lifting performance capability exists in the
population today. This realization requires that the resulting
controls be of both an engineering and administrative nature. In
other words, there are some lifting situations which are so ha-
zardous that only a few people could be expected to be capable of
safely performing them. These conditions need to be modified to
reduce stresses through job redesign. On the other hand, some
lifting conditions may be safely tolerated by some people, but
others, particularly weaker individuals, must be protected by an
aggressive selection and training program. To specifically define
these conditions two limits are provided based on epidemiological,
biomechanical, physiological, and psychophysical criteria.

alo Maximum Permissible Limit (MPL)

This limit is defined to best meet the four criteria:

a. Musculoskeletal injury rates and severity rates
have been shown to increase significantly in
populations when work is performed above the
MPL.

b. Biomechanical compression forces on the Ls/S; disc
are not tolerable over 650 kg (1430 lb) in most workers.
This would result from conditions above the MPL.

@o Metabolic rates would exceed 5.0 Kcal/minute for
most individuals working above the MPL.

d. Only about 25% of men and less than 1% of women
workers have the muscle strengths to be capable
of performing work above the MPL.

25 Action Limit (AL)
The large variability in capacities between individuals
in the population indicates the need for administrative
controls when conditions exceed this limit based on:
a. Musculoskeletal injury incidence and severity rates

increase moderately in populations exposed to
lifting conditions described by the AL.

124




GUIDELINE LIMITS

With the

large number of task variables (5 in this case) which

modify risk during lifting it is virtually impossible to provide

a simple yet accurate procedure for evaluating all possible jobs.
This problem is further complicated by the need to satisfy four
separate criteria (epidemiological, biomechanical, physiological,
and psychophysical). The following guideline is the simplest form
known which best satisfies the four criteria.

In algebraic form:

AL (Kg)
AL (1b)

MPL

40(15/H) (1-.004|V-75|) (.7+7.5/D) (1-F/Fpay) “metric units

or
90(6/H) (1-.01|V-30|) (.7+3/D) (1-F/Fya,)-U.S. Customary units

3 (AL)

where H = horizontal location (centimeters or inches)
forward of midpoint between ankles at origin
of lift

V = vertical location (centimeters or inches) at
origin of lift

D = vertical travel distance (centimeters or inches)
between origin and destination of lift

F = average frequency of lift (lifts/minute)

Pax = maximum frequency which can be sustained
{see Table 8.2)

For purposes of this Guide, these variables.are assumed to have
the following limits.

alo

H is between 15 cm (6 in) and 80 cm (32 in). Objects
cannot, in general, be closer than 15 cm (6 in) without
interference with the body. Objects further than 80 cm
(32 in) cannot be reached by many people.

V is assumed between 0 cm and 175 cm (70 in) representing
the range of vertical reach for most people.

D is assumed between 25 cm (10 in) and (200-v) cm[(80-v) in].
For travel less than 25 cm, set D = 25,

F is assumed between .2 (one lift every 5 minutes) and

Fmax (See Table 8.2). For lifting less frequently than
once per 5 minutes, set F = 0.

126



There is an upper limit to aerobic metabolism, often called the
aerobic capacity (or in physiological jargon, the VOz max) which
varies considerably from person to person, influenced by a number
of factors which are described below. Usually, the muscles'
anaerobic metabolism is called into play when the exercise demands
50% or more of the VO2 max. As the severity of the exercise
increases above that level, the greater is the proportion of
anaerobic metabolism for the muscular activity and, because the
amount of anaerobic metabolism is finite, the duration of exercise
before fatigue occurs varies inversely with increasing severity of
the activity. Obviously, it is important to know what the Vo max
is for an individual or a group of individyals and also to avoid
prolonged work at levels greater than 50%,VO2 max if fatigue is to
be avoided (mn fact, lower percentages of VO2 max have been con-
sidered limiting, in some studies, as is discussed below). Factors
which influence VO2 include:

1. Level of vo2 max ("fitness")

It is well established that when men and women engage in a

regimen of relatively severe exercise on a controlled repetitive

basis, they are able to improve their physical performance.
Their aerobic work capacity may increase by amounts reported to
be between 5 and 25%; the increase appears to depend on the
frequency, duration and severity of the training regimen and
also on the degree of "fitness" of the individuals concerned
(cf. Astrand and Rodahl, 1970).

The level of activity of the bulk of the population is labelled
as "sedentary". In the last few decades surveys have been

completed “on the proportion of time spent in different activities

in a variety of occupations; in a number of these surveys,
information has also been collected on activities during non-
working hours. In light industries, much of the work done
involves low levels of energy expenditure. Except in "heavy"
industries such as mining, steel working, etc., the work seldom
reaches or exceeds 50% VO2 max and then only intermittently.

In these heavy industries, the work level yields an average
daily oxygen requirement of about 1 1 02/min. (metabolic
energy equivalent = 5 Keal/min). Periodically, tasks occur at

much higher levels, of 2 1 02/min (10 Kcal/min) or even as high
as 3 1 02/min (15 kKeal/min) .- These tasks are short, lasting
only for several minutes, and to provide an average daily level
of 1.0 1 95/min, are offset by rest periods along with periods
of work at low levels of oxygen usage. Even in "heavy" indus-
tries, however, the increasing use of automation tends to reduce
the energy expenditure of the work. However, workers in such
jobs can be regarded as "active" (or trained) in some degree

and they have a greater aerobic capacity than their "sendentary"
counterparts in most industries.

42


ABSTRACT

Overexertion in the workplace accounts for a large number (and in
some industries the majority) of disabling injuries. Most of these
injuries involve the act of manually handling materials. This
Guide summarizes a wealth of research and presents recommendations
to control the various types of hazards associated with the unaided
act of symmetric (two-handed) lifting of an object of known weight
and size. Quantitative recommendations regarding the safe load
weight, size, location and frequency of handling are presented.
Factors which mitigate these recommendations are also discussed
(e.g., worker training, physical fitness, strength, workplace
layout, load handles, etc.). The Guide includes recommendations
regarding both the selection and training of workers who must per-
form manual materials handling activities as well as presenting
some engineering guidelines for the design of workplaces where
Manual materials handling is performed.


Kelly, F.J. “Evaluating Musculoskeletal Capabilities- A Practical
Approach." J. Occup. Med. 17:266-269, 1975.

Keyserling, W.M.; Herrin, G.D. and Chaffin, D.B. "An Analysis of
Selected Work Muscle Strength." Proceedings of the Human
Factors Society 22nd Annual Meeting, Detroit, 1978.

Khalil, T.M. “An Electromyographic Methodology for the Evaluation
of Industrial Design." Human Factors. 15:257-264, 1973.

Knipfer, R.E. "Predictive Models for the Maximum Acceptable
Weight of Lift." PhD Dissertation, Texas Tech University,
1974.

Konz, S.; Dey, S. and Bennett. C. "Forces and Torques in Lifting."
Human Factors. 15:237-245, 1973.

Kosiak, M.; Aurelius, J.R. and Harfield, W.F. "Backache in Indus-
try." J. Occup. Med. 8:51-58, 1966.

Koyl, L.F. and Hanson, P.M. "Age, Physical Ability and Work
Potential." National Council on Aging, U.S. Manpower Admin-
istration and U.S. Department of Labor, 1969.

Kraus, H. “Prevention of Low Back Pain." J. Occup. Med. 9:555-
559, 1967.

Kroemer, K.H.E. "Push Forces Exerted in 65 Common Working Posi-
tions." Aerospace Med. Res. Lab. Tech. Report, AMRL-TR-68-
143, Wright Patterson Air Force Base, Ohio, 1969.

Kroemer, K.H.E. “Human Strength Terminology, Measurement and
Interpretation of Data." Human Factors. 12:297-313, 1970.

Kroemer, K.H.E. "Foot Operation of Controls: Speed of Activation
and Exertion of Force to Pedals, Perception, Speed and Ac-

curacy of Leg and Foot Motion." Ergonomics. 14:333-361,
1971.
Kroemer, K.H.E. and Howard, J.M. "Towards Standardization of

Muscle Strength Testing." Med. Sci. Sports. 2;224, 1970.

Kroemer, K.H.E. "Horizontal Push and Pull Forces." Appl. Ergo-
nomics. 5:94-102, 1974.

Lamy, C. "Hydraulic Behavior of Vertebral Body." Paper presented
at the International Society for the Study of the Lumbar
Spine, San Francisco, 1978.

LaRocca, H. and MacNab, I. "Value of Pre-employment Radiographic
Assessment of the Lumbar Spine." Canad. Med. Assoc. J. 101:
383-388, 1969.

165




1Or
| * Standard a4
w al © Incentive fpe
ire 2
E2 i 6
a
ze ef
as L
ge a
3s '
w &)
fr] Pr ivi ah ay
io 14 18 22 4 8 12 16
PACE IN CARTONS/MINUTE PACE IN CARTONS/MINUTE
TASK 1-FLOOR TO WAIST-HIGH TABLE TASK 2-FLOOR TO WAIST-HIGH TABLE
4.5 kg CARTON 11.4 kg CARTON

o
WAS AGA

a
UA WA

+

ENERGY EXPENDITURE
(reat (net) /minute)

¥,
8 12 16 20 4 8 12 16
PACE IN CARTONS/ MINUTE PACE IN CARTONS/MINUTE
TASK 3- FLOOR TO HEAD-HIGH TABLE TASK 4-FLOOR TO HEAD-HIGH TABLE
4.5 kg CARTON 11.4 kg CARTON
WwW
«
2
Ee 5
ary ;
ae 4
a€£ 3
BS
2
ae
2X
w 1 s
Soo bh
14 18 22 26 8 12 16 20
PACE IN CARTONS/ MINUTE PACE IN CARTONS/ MINUTE
TASK S- WAIST-HIGH TO HEAD-HIGH TASK 6-WAIST-HIGH TO HEAD-HIGH
TABLE 4.5 kg CARTON TABLE 11.4 kg CARTON
Figure 4.6;

Effect of Pace on Metabolic Rate (Aquilano, 1968).

59


One of the most common types of reported back injury is of sudden
back pain while stooped to lift. In such cases there is no way
to distinguish the long term effects of MMH from the short term
effects of postural stress of injury, given our present knowledge.

JOB RISK FACTORS

Many aspects of the physical act of manually lifting a load have
been identified as potentially hazardous to a person's musculo-
skeletal system. Among those cited in a review of the litera-
ture by Herrin, et al., (1974) are:

1. Weight - force required

2. Location/Site - postiion of the load center
of gravity with respect to the worker

3. Frequency/Duration/Pace - temporal aspects of
the task in terms of repetitiveness of handling

4. Stability - consistency in location of load
center of gravity as in handling bulky or
liquid materials

5. Coupling - texture, handle size and location,
shape, color, etc.

6. Workplace Geometry - spatial aspects of the task
in terms of movement distance, direction,
obstacles, postural constraints, etc.

7. Environment - factors such as temperature,
humidity, illumination, noise, vibration,
frictional stability of the foot, etc.

To date only the first three aspects have received sufficient
attention in lifting injury research to form a strong basis for
guidance.

Weight Lifted

The weight of the material handled is perhaps the most obvious
factor which modifies risk of injury. Hult (1974), Kosiak (1968),
Lawrence (1969), Magora (1969, 1970) and Rowe (1969, 1971) are

but a few of the authors who have found that the frequency of

low back injuries is greater in "heavy" than in "light" industries.
Many of these injuries occur when individuals lift objects which
are familiar in size, shape and weight (Herndon, 1927; Brown,
1958). This is one reason some researchers believe the expendi-
ture of time and effort in the training and education of industrial
workers in good lifting technique has been to little or no avail

in back injury abatement (Brown, 1972; Snook, 1978).

14


oO
T

w he}
= |00- Back Straight .-7
EE “7
O> 907
zs
ws. 80-
x3
We 7OPL
2
5S 60
ws
G~ 50F
40P 0"
o” Free Style
30F
a
ig a as I
10 20 30 40
LOAD (kg)

Figure 4.3: Metabolic Rate for Different Postures (Brown, 1971),

METABOLIC RATE (kcal/min)

LSS

L
30 40 50 60 70 80 90

MAXIMUM ACCEPTABLE WORKLOAD
(kg-m/min)

Figure 4.4; Effect of Technique and Workload on Metabolic Rate
(Garg and Saxena, 1979).

57


Notice that the H factor is most critical in this case (H factor =
-28). If possible, the operator should load the machine from the
side (i.e., grasping the reel by its perimeter). This would allow
the reel to be brought closer to the body (i.e., H = 20 cm for
example). With this preferred work practice,

AL = 40(.75) (.76) (.85) (1.0) = 19.4

MPL = 3(19.4) = 58.2

For all practical purposes the activity is now within the capabil-
ities of most people. Alternative engineering controls might in-
clude elevating the delivery of reels (above the floor) thus
improving both the V and-.D factors.

Examples 2 and 3: H Not Constant

Evaluation of jobs where the H value is not constant throughout
the lift should be approached with caution by the analyst. The
most common error is to exaggerate H, making a job seem more
difficult than it actually is.

For example, a compact load is lifted from the floor to a point 125
em high as illustrated in Figure 8.10. Due to workplace constraints
the object must be placed on a moving conveyor at a distance H=80
cm but the lift is unconstrained up to waist height (about 100 cm).
In most cases, the load can be lifted close to the body up to this
point and then through transfer of momentum be placed on the con-
veyor with little difficulty. Using an H value of 80 cm greatly
overestimates the strength requirements of this task. As a general
rule of thumb, the analyst should use the H value at the origin

(in this case close to the body) to determine the weight limit

and not the H value for the end point of the lift. However, a
fragile load that must be carefully handled throughout the lift
may well require the strength capabilities of lifting the load
from the floor to the end point at an H value equal to that of

the end point!

The estimation of horizontal location as W/2 + 15 cm for body
clearance is not appropriate with horizontal obstructions such as
illustrated in Figure 8.11. In this case, the appropriate hori-
zontal dimension is 60 cm at the origin. Presumably the load is
not fragile, nor need it be retrieved from the destination. If
it must be retrieved the horizontal location at the destination
will become the origin for a subsequent lift!

Example 4: Occasional High Frequency Lifting of Constant Weights
Suppose that for a period of one hour, a person unloads palletized

cartons each weighing 5 kg stacked 5 high onto a conveyor as illus-
trated in Figure 8.12. In this case the vertical origin location

137


Christensen (1955) proposed that if work is performed with

an energy expenditure rate below 50 percent of a person's
aerobic capacity, excessive fatigue will not occur. Astrand
(1960) tested subjects with a large variation in aerobic
capacities (from 11.2 to 26.0 Kcal/min.) and found that when
subjects were performing at 50 percent of their aerobic
capacities for eight hours, their heart rates increased to a
level of 120 to 135 beats per minute. She observed that, if
the aerobic capacities had not been measuréd (and energy
expenditure rates adjusted accordingly) the heart rate in the
older subjects (with limited aerobic capacities), if required
to perform at the same energy expenditure rates as the younger
subjects, would have incurred a heart rate increase of 30
beats/minute in the first hour of work. Thus the older
subjects would have rapidly approached their predicted maximum
heart rates of about 165 beats per minute.

Studies by Lehmann (1953) and Bink (1962, 1964) indicated that

a work load of 5.2 Kcal/minute is the maximum energy expenditure
rate that should be expected for an eight hour workday. Lehman
assumed a mean age of males of 35 years, with 2500 Kcal to be
available during an 8 hour day.

Legwork at levels above 5.0 Kcal/minute in well-trained
individuals has been found to cause increased levels of blood
lactate (Ekblom, et al., 1968). This is further evidence

to indicate that the metabolic demand for oxygen within the
muscles is not completely fulfilled when the task is at higher
levels than 33 percent of the aerobic capacity of the person,
even with highly dynamic work. Additional support for using

a 33 percent value rather than the higher 50 percent of aerobic
capacity is suggested by Snook and Irvine (1969). They found
that when healthy industrial men were allowed to choose the
amount of repeated lifting acceptable for eight hours, they
chose a level that produced a heart rate averaging 112 beats
per minute, which would be equivalent to about 5.0 Kcal/minute
of energy expenditure.

A more recent industrial study by Rodgers (1976) supported a
lower (33 percent) capacity in concluding:

"We have observed that most people will select a
level of effort that keeps them within the 33% of
maximum capacity guideline and will also integrate
other factors such as:

- the biomechanical aspects of materials to be
handled--grasping characteristics, size, etc.

- environmental characteristics of the workplace--
heat, hours of work, chemical agents, pacing, etc.

- the individual's physical fitness level

50


SCOPE OF GUIDE

The variety of work methods, loads handled, frequency of exertions
and worker characteristics which modify risk of injury in manual
materials handling is virtually limitless. Careful reviews of
the knowledge base regarding the hazards of manual materials
handling (Herrin, et al., 1974; Drury, 1978) reveal that many
facets of the problem still remain inadequately researched. The
act of lifting in particular, however, has been extensively
studied by many researchers (over 600 literature citations) to
the extent that the hazards are reasonably well understood.

There are basically four approaches or criteria for establishing
this Guide:

1. epidemiological

2. biomechanical

3. physiological, and
4. psychophysical.

In the next four chapters of this Guide each approach and the
implied set of criteria are examined in detail to provide a
basis for the recommendations that follow.

Epidemiology as a science is concerned with identification of the
incidence, distribution, and potential controls for illness and
injuries in a population. Chapter 2 surveys a number of factors
identified in epidemiological studies which tend to modify risks
of overexertion injuries with particular emphasis on the incidence
and severity of low-back pain in industry.

Biomechanical approaches outlined in Chapter 3 show that the
musculoskeletal structure (particularly the low-back) of some
individuals can be overstressed when lifting compact loads of
moderate magnitudes even occasionally. Further, the complex
stresses to the body during manual materials handling can be pre-
dicted given a careful documentation of the specific handling
task.

Physiological studies of the human body's metabolic and circulatory
responses to various loads (especially with high frequency of

load handling) are presented in Capter 4. Conditions which will
not result in excessive physiological strain or ftaigue for a
majority of workers are presented.

Psychophysical studies are designed to quantify the subjective
tolerance of people to the stresses of manual materials handling.
These studies, outlined in Chapter 5, indicate that a large vari-
ation exists in the working population's acceptnace of typical
load handling conditions, and that combinations of certain weights
and work methods are much more objectionable than others. Further,
strain/sprain injuries are more prevalent for jobs requiring

7


Fy (EFFECTIVE
MUSCLE

20 kgs

Figure 3.1: Illustration of Muscle Mechanics (Chaffin, 1975).

In terms of forces and moment arms, then

FY x 4. = 20 kg x x.

M M L
With average male anthropometry ly = 5 cm and ry = 35 cm.
Therefore

Fu = 140 kg

The weight of the forearm and hand would cause an additional 34
kg-cm moment for a total muscle force of 174 kg. Thus, simply
holding a given load in the hands requires more than 7 times
greater elbow flexion muscle force due to the mechanical disad-
vantage of the muscles.

To apply this concept, consider that a 20 kg (44 1b.) object must
be lifted with both hands (10 kg in each) from the back of a shelf
placed at about shoulder height. Figure 3.2 illustrates the pos—
ture. Note first that the elbow is extended, which reduces the
flexor muscle moment arm to about 25% of its former value (i.e.,
Ty now is about’1.2 cm for an average man).

24


a. Begin with a poster campaign drawing attention to
the need to do something about lifting accidents
and back injuries, changing them every few days.

b. Organize a training session for managers and super-
visors.

Go Present the training course to small groups of
employees.

d. Have the plant doctor and the team of instructors
tour the plant, discussing any points put to
them by the workers.

e. Continue to make plant tours at regular intervals.

In summary, there is a vital need for training to be both in the
classroom and on the site, to involve all levels of workers
including executives, and to be monitored routinely throughout
the factory in order for the program to be effective.

ERGONOMIC/ENGINEERING CONTROLS

More important than proper selection and training in the long term
prevention of accidents and injuries related to lifting, is pro-
viding a safe ergonomic environment in which to work. This includes
factors of the mechanical environment (such as container design,
human/container coupling design, worker/floor surface coupling),

the visual environment and the thermal environment.

Mechanical Environment

oho Container Design. From the biomechanical considerations
discussed in Chapter 3, containers should be as small as
possible. A compact load minimizes the compressive
forces applied to the L5/S] disc because the load center
of gravity can be brought close to the spinal column -
oftentimes between the legs, which reduces disc pressure
even more. Another important factor in container design
is the load center of gravity. Baffles, dividers or
packing should be used to keep the center of gravity in
a constant position because any shifting of the load
could contribute to human error in manual handling.

Anthropometry plays an important part in recommending
the maximum dimension of a container. If the forward
edge is to be reached, the length should not exceed

70 cm (males) or 65 cm (females). To prevent interfer-—
ence with forward vision, the maximum height of the
container above handle position should be 83 cm (males)
and 80 cm (females). Also handles should be placed so
that the whole container is above hip height when the
elbows are extended.

148


same weight and at the same frequency is substantially greater
when the legs are bent to assist lifting than when the trunk
only (cantilever) is bent (Brown, 1971; Garg, 1976). The vo
max of lifting boxes by the cantilever method is much lower
than for bicycling (Petrofsky and Lind, 1978). With boxes of
15 kg or more, the Vo max of lifting with the bent-leg

method is the same as in bicycling but when the boxes weighed
8 kg or less, the ¥ max is lower than the response to
bicycling (Lind and Riliians, 1979).

Depending on the nature,and severity of the task, selection of
workers based on their VO, max is clearly desirable. But in many
industrial circumstances, it is impractical to provide this

kind of selection procedure for individuals.

ISOMETRIC EXERCISE

The methods for assessing physiological responses to isometric
exercise are different from those described for dynamic exercise.
At most tensions, sustained contractions readily induce local
muscular fatigue, from which recovery is slow. For a given group
of muscles, the maximum voluntary contraction (MVC) is first
measured on a dynamometer. The bulk of experiments then report
the endurance time to fatigue when specific sub-maximum tensions
are held. There emerges a clear pattern of relationship between
the sustained tension and the endurance time (when that tension
can no longer be maintained). At 100% MVC the endurance is about
3 to 4 seconds. At 70, 50 and 30% MVC, the endurance is of the
order of 35, 90, and 250 sec, respectively. When the tension falls
to less than about 15% MVC, the contraction can be maintained for
a long time, and results in little fatigue.

The most commonly reported studies involve hand-grip contractions,
for which the MVC for men is about 50 kg, while for women it is 35
kg. There is, however, a wide individual variation in both the MVC
and the endurance of sub-maximal tensions. Several factors
influence both the MVC and the endurance time. Age results in a
reduction of strength, but not as dramatically as it affects WO
max in dynamic exercise. Muscle temperature does not affect the
MVC but, as the muscle temperature increases, there is a reduction
in endurance time. At relatively low tensions (e.g., 30% MVC) this
can amount to some 20 sec for each °C difference in muscle temper-
ature; the muscle temperature can readily vary by 5 or 6°C as a
result of the influence of the environment, clothing, subcutaneous
fat or previous exercise. At high tensions (e.g., 70% MVC) the
difference due to temperature is not so marked.

Women have greater endurance than men at all relative tensions
below 50-60% MVC. But when these results are considered in absolute

44


medical services, productivity losses, etc., these injuries are
the most serious because of the frequency with which they lead
to chronic disability.

Fatigue, on the other hand, is a more prevalent short term health
effect than injury in some occupations, but recovery is more
rapid. Symptoms of fatigue may be respiratory, cardiovascular

or muscular. Respiratory symptoms such as shortness of breath
are seldom a problem in MMH except for the individual with chronic
respiratory disease. In this case the worker finds it difficult
to move materials in postures which restrict the movements of

the rib cage.

Signs of cardiovascular stress are increased heart rate or

blood pressure. Prolonged, sustained muscular activity imposes
postural stresses on the muscles (particularly when bending down
or carrying for any length of time) which leads to an increase

in heart rate and blood pressure. High frequency, repetitive
lifting elevates heart rate and blood pressure. This may be sig-
nificant for the worker who suffers from the effects of hyper-
tension or who has a cardiac disorder.

Muscular fatigue is a common cause of symptoms during and after
MMH. The severity and duration of symptoms depend not only on

the weight and frequency of handling but on the fitness and skill
of the individual. Infrequent sessions of unaccustomed, hard
work are often associated with the deposition of fibrin anda
temporary peritenomyosis (Rais, 1961) which may persist for two

or three days. Postural stress is also a common cause of muscular
fatigue and discomfort at work (Corlett and Bishop, 1976; Wick-
strom, et al., 1978).

LONG TERM EFFECTS OF MMH ON HEALTH

There are few epidemiological studies which relate chronic ex-
posure to fatigue, postural stress, or musculoskeletal injury
with excess morbidity or mortality. One study by Davis and
Jackson (1962) identified a greater incidence of chronic bron-
chitis in those who regularly stooped to lift compared with
those who lifted on the shoulder without flexing the trunk.
Otherwise, the main long term health hazards concern the spine.

Backache due to fatigue (or postural stress), back or sciatic pain
following back injury, and the early onset of degenerative dis-
ease of the spine have been cited as consequences of MMH, However,
distinctions which exist between these "causes" in theory are not
always distinguishable in practice. Prevention requires first,

a definition of the pathological process attributable to MMH,

and second the identification of the causative mechanism. The
epidemiological data are at present insufficient either for a
precise understanding of the pathological changes produced or to
allow distinctions to be drawn even between fatigue, postural
stress and injury. In practice these distinctions are difficult
because: 5

10


analysis sheet. The remarks section may be useful in describing
which factor is most restrictive or limiting for each element.

EXAMPLES

Five examples will now be discussed to illustrate the analysis and
interpretation of this methodology.

Example 1: Infrequent Lifting

Figure 8.8 illustrates a common misconception (or oversight) in
terms of what types of jobs may be physically stressful. A punch
press operator, for example, routinely handles small parts, feeding
them in and out of a press. A cursory view of this task may over-
look the fact that once per shift, the operator is required to load
a reel of supply stock (illustrated shoulder height) from the floor
onto the machine. The reel weighs 20 kg. This activity is docu-
mented in Figure 8.9. Assuming the operator lifts the reel in the
plane shown (rather than on the side of the machine) fhe appropri-
ate horizontal dimension is (75/2 + 15) = 53 cmor (32+ 6) = 21 in.
At the destination V = 160 cm (63 in). Since the activity occurs
only once per shift, F = 0 (no frequency adjustment required).

O cm

Figure 8.8: Example 1, Punch Press Operator.

135


(1978). The maximum acceptable weights for lifting tasks are
given in Table 5.3 for male industrial workers, and in Table 5.4
for female industrial workers. For example, 18 kg is the maxi-
mum acceptable weight for 75% of the male workers lifting a
rather large object (75 cm width) through a 76 cm distance from
the floor to knuckle height once every minute. The equivalent
value acceptable to 75% of female workers is 13 kg.* These
values are based upon a freely chosen lifting posture where
workers are not instructed to lift by any particular technique.
An earlier experiment indicated that workers would not lift as
much weight when required to maintain a straight back and bent
knees. The maximum acceptable weights of lift are based upon
objects with handles located in the middle of the width dimension.

Ayoub, et al., (1978) conducted a similar study to determine and
model the lifting capacity of male and female industrial workers.
Seventy-three male and 73 female subjects were used. Six differ-
ent height levels and four frequencies (2,4,6, and 8 lifts/minute)
were employed. The height levels were: (1) floor to knuckle;

(2) floor to shoulder; (3) floor to reach; (4) knuckle to shoulder;
(5) knuckle to reach; and (6) shoulder to reach as illustrated in
Figure 5.5. The results of the study are summarized in Table 5.5.

A number of other studies are reported in the literature (e.g.,
Emanuel, 1956; Switzer, 1962; Whitney, 1958; etc.). Much of this
earlier literature involves experiments with college students or
air force personnel and will not be presented in detail.

DYNAMIC STRENGTH MODELS

Several researchers have used the psychophysical approach to de-
velop lifting capacity prediction modéls (McConville and Hertzberg,
1968; McDaniel, 1972; Dryden, 1973; Knipfer, 1974; Ayoub and
McDaniel, 1973; Ayoub, et al., 1973, Ayoub, et al, 1976a, Ayoub,
et al., 1976b; Ayoub, et al., 1978; and Mital, et al., 1978).

Table 5.6 summarizes their models for lifting activities based

on the psychophysical approach. These models lead to some inter-
esting conclusions. According to the model developed by McConville
and Hertzberg (1968), a person cannot lift a box which is 60
inches wide. Models developed by Poulsen (1970), McDaniel (1972),
Dryden (1973), Knipfer (1974), Ayoub and McDaniel (1973), Ayoub,
et al., (1973), Ayoub, et al., (1976a), Ayoub, et al., (1976b),
Ayoub, et al., (1978), and Mital, et al., (1978) have one thing
in common: lifting capacity is a function of isometric back
strength. In these models, the indépendent variables do not in-
clude task variables. It should also be noted that most of these
models do not include interactive effects,

*It should be noted here that "width" for the Snook data
means distance of the load away from the body measured in
the horizontal axis (See fig. 8.3).

77


Grimby, G. and Soderholm, B. "Energy Expenditure of Men in Differ-
ent Age Groups During Level Walking and Bicycle Ergometry."
Scand. J. Clin. Lab. Invest. 14:321-328, 1962.

Grinnold, R.E. "A Follow-Up Study of Permanent Disability under
Wisconsin Worker's Compensation." PhD Dissertation, Univer-
sity of Wisconsin, 1976.

Hamilton, B.J. and Chase, R.B. “A Work Physiology Study of the
Relative Effects of Pace and Weight in a Carton Handling
Task." AIIE Transactions, 1:106-111, 1969.

Hanman, B. "The Evaluation of Physical Ability." New. Eng. J. Med.
258:986-993, 1958.

Harley, W.J. “Lost Time Back Injuries: Their Relationship to
Heavy Work and Preplacement Back X-Rays." J. Occup. Med.

14:511-614, 1972.

Harper, F.C.; Warlow, W.J. and Clarke, B.L. "The Forces Applied
to the Floor by the Foot in Walking." National Building Studies
Research Paper 32. Her Majesty's Stat. Off., 1961.

Herndon, R.F. “Back Injuries in Industrial Employees." J. Bone
Joint Surg. 9:234-269, 1927.

Herrin, G.D.; Chaffin, D.B. and Mach, R.S. "Criteria for Research
on the Hazards of Manual Materials Handling." NIOSH Contract
Report, CDC 99-74-118, 1974.

Hill, A.V. "The Heat of Shortening and the Dynamic Constants of
Muscle." Proc. Roy. Soc. London. B126:136-195, 1938.

Himberry, S. -"Kinetic Methods of Manual Handling in Industry."
Occup. Safety Health Series, International Labour Organization,
No. 10, 1967.

Hirabayashi, I.; Ikeda, K.; Tsuchihashi, Z; Morita, T. and Saito,
M. "Spondylolysis and Spondylolisthesis: Pathological Con-
ditions and Therapeutic Indications." J. Jap. Orthop. Assoc.
46:49-68, 1972.

Hirsch, T. “The Billion Dollar Backache." Nat. Safety News. May,
1977.

Hirschfield, A.H. and Behan, R.C. "The Accident Process. I. Etio-
logical Consideration of Industrial Injuries." J. Am. Med.
Assoc. 186:193-199, 1963.

Houghton, F.C. and Yagloglou, C.P. "Determination of the Comfort
Zone." J. Am. Soc. Heating Ventil. Eng. 29:515-536, 1923.

163


It is believed by Rowe (1969) that 70-80% of all chronic low-back
pain will be diagnosed as discogenic after a period of repeated
episodes. At the very least, degeneration and the narrowing of
the disc that results from it will contribute to a more unstable
spinal structure. In this latter regard, Fiorini and McCammond
(1976) state: "Many damaged discs showing radiographical narrow-
ing may not cause discogenic back pain at all if there is no disc
hernation, but may cause facetogenic back pain by causing sublux-
ation and malapposition of the interarticular joints." Some evi-
dence that disc degeneration is accelerated by physical stresses
has been developed by Hult (1954). He reported that narrowing
and osteophyte development of the discs and adjacent vertebral
bodies was 1-1/2 times greater in those people engaged in heavy
physical labor than in sedentary workers.

The implications of this disc degeneration theory are far-reaching.
Most important is that assigning cause for low-back pain cannot be
based simply on the immediate circumstances at the time when the
pain first developed. In fact, most low-back episodes do not
suddenly start with a "jabbing pain," although these cases are
easily remembered and reported by patients and physicians alike.
Rather the symptoms more often are slow to develop, with stiffness,
dull aching pain and finally, incapacitating discomfort, which
occurs possibly hours or even days later. With this in mind, it
is easy to rationalize why the statistics relating a person's
physical acts to the incidence of low-back pain generally are so
poor.

POSTURE EFFECTS

Returning to the biomechanical aspects of manual materials handling,
several general concepts need further definition. First, there
remains the issue of how a person's posture affects low-back
stresses. It already has been shown that if the load is horizon-
tally distant from the torso, large forces can result, even with-
out bending over. Therefore, the most important rule in materials
handling is to ensure that the person is able to bring the torso
as close to the load center of gravity as possible before lifting
it. This often requires having the person squat down beside the
load with the legs straddling it when the load is on or near the
floor, and lifting it between the knees. This assumes, of course,
that the load is small enough to go between the flexed legs easily.

If the load is small, the companion rule regarding keeping the
back near vertical is biomechanically justified, as it reduces

the stresses on the low back due to the torso weight. Unfortunate-
ly, lifting with the legs from a squatting position with the back
vertical (i.e., the classic recommended posture) often is not
possible because the person so instructed does not have the quad-
riceps strength necessary to extend the knees and raise the body
from such a position. In other words, most people, when lifting
weights, lean their torso forward to reduce the moment on the
knees. This is so common in lifting that the quadriceps muscles

31


16+ Back EXTENSION

FREQUENCY

6 |2 18 24 3036 42 48 54 60 66 72 78 84 90 96 102
STRENGTH (kg)

n ELBOW FLEXION
20

FREQUENCY
°

is 2430 36 42 46
STRENGTH (kg)

12

Or

Figure 5.1: Strength Differences between Male and Female Indus-
trial Workers (Kamon and Goldfuss, 1978).

72


McCormick, E,J. Human Factors in Engineering and Design. 4th Ed.
McGraw Hill, New York, 1976.

McDaniel, J.W. "Prediction of Acceptable Lift Capability." PhD
Dissertation, Texas Tech University, 1972.

McGill, C.M. "Industrial Back Problems: A Control Program." J.
Occup. Med. 10:174-176, 1968.

Magnuson, P.B. and Coulter, J.S. “Workmen's Backache." Int. Clin.
31:214-253, 1921.

Magora, A. "An Investigation of the Problem of Sick Leave in the
Patient Suffering from Low Back Pain." Ind. Med. Surg. 38:
398-408, 1969.

Magora, A. "Investigation of the Relation between Low Back Pain
and Occupation. Part I." Ind. Med. Surg. 39:21-37, Nov. 1970.

Magora, A. "Investigation of the Relation between Low Back Pain
and Occupation. Part II." Ind. Med. Surg. 39:28-34, Dec. 1970.

Magora, A. "Investigation of the Relation between Low Back Pain
and Occupation. Part 6. Scand. J. Rehab. Med. 6:81-88, 1974.

Magora, A. and Schartz, A. "Relation between the Low Back Pain
Syndrome and X-Ray Findings. 1. Degenerative Arthritis."
Scand. J. Rehab. Med. 8:115-125, 1976.

Manning, D.P. "Industrial Accident-Type Classifications: A
Study of the Theory and Practice of Accident Prevention
Based on a Computer Analysis of Industrial Injury Records."
MD Thesis, University of Liverpool, 1971. °

Marshall, L.L.; Trethewie, E.R. and Curtain, C.C. "Chemical
Radiculitis: A Clinical, Physiological and Immunological
Study." Clin. Orthop. 129:61-67, 1977.

Martin, J.B. and Chaffin, D.B. "Biomechanical Computerized Sim-
ulation of Human Strength in Sagittal Plane Activities."
AIIE Transactions. 4:19, 1972.

Mertens, H. "Nonlinear Behavior of Sitting Human under Increasing
Gravity." Aviat. Space Environ. Med. 49:287-298, 1978.

Miller, J.C.; Farlow, D.E. and Seltzer, M.L. "Physiological
Analysis of Repetitive Lifting." Aviat. Space Environ. Med.
48:984-988, 1977.

Miner, M.G. and Miner, J.B. Employee Selection within the Law.
Bureau of National Affairs, Inc., Washington, D.C. 1978.

Mital, A.; Ayoub, M.M.; Asfour, S.S. and Bethea, N.J. "Relation-
Ship between Lifting Capacity and Injuries in Occupations
Requiring Lifting." Proceedings of the Human Factors Society
22nd Annual Meeting, Detroit, 1978.

167


20 Male Mean

o
oF

- Male. Lower 95%

Nn
T

r Female Mean

Female Lower 95 %

AEROBIC CAPACITY (kcal/min)
+ @
T iu

L
10 20 30 40 50 60 70
AGE (yrs)

(o}

Figure 4.1: Estimated Population Aerobic Capacities for U.S.

Men and Women (Chaffin, 1972).

Because the aerobic capacities in the working population vary
so greatly, persons being considered for jobs requiring high
metabolic demands should be specifically tested. Often aerobic
capacity testing is done in industry on those persons who

have a history of cardiovascular problems. In this sense it

is used to determine the effectiveness of various cardiac
rehabilitation programs, rather than being a tool of preventive
medicine. Only through routine aerobic capacity testing in
industry can objective decisions be made about what job is "too
difficult for certain susceptible groups of people and for
specific individuals. For the present, the recommendations
provided later in this Guide are based on an assumed aerobic
capacity of 15 Kcal/min. for men (cf. Robinson, 1939) and 10.5
(70% of men) for women (cf. Astrand, 1960). These limits may

be too high, however, for a de-conditioned, aging workforce.
Aerobic capacity testing of a broad, cross-section of the

U.S. workforce is needed.

Eight Hour Work Duration

In terms of endurance, a controversy exists in the literature
over appropriate allowances for extended continuous work.

49


fan
Ark? oant
- Rall of foot

Figure 5.2: Linkage representation (Garg and Chaffin, 1975)

group strengths). Figure 5.3 depicts the body angles used to
describe the posture of a person for modeling. For each angle
there are at least two opposing muscle strengths that must be
inputted to the model to act as the limiting reactive voluntary
torques at each joint. These inputted muscle strengths need to
either be measured, or population distribution strengths can be
assumed. The model then allows the manipulation of the external
forces and postures of interest to determine the maximum hand
forces that can be produced by a designated population without
having a joint resultant torque exceed a given joint reactive
torque strength. Thus, the model can be thought of as depicting
the static muscular capability of a person in any posture and
load combination described.

Two other human limitations are recognized by the model for
strength prediction. One is the body balance capability. As

an example it is possible when standing that the external forces
on the body can cause the line of gravity of the total person/
load system center of gravity to be outside the area bounded by
the feet, and hence the person will fall over if a rapid postural
correction is not made. This loss of static equilibrium is as-
sessed for any posture and force combination inputted to the model,

73


These values are quoted without evidence as to their efficacy.
Design Recommendations for Handles

For final recommendations on handle diameter, there is little
agreement in the literature, although diameters from 25 to 38 mm
receive more support than most and thus must, reluctantly, be
recommended. The elimination of sharp edges, seams, ribbing,

and corners appears to be an equally important criteria. The
handle should be textured to provide maximum gripping force, par-
ticularly if other than a pull force is to be exerted. The shape
of the handle is better cylindrical than molded to the contours
of the hand. Tichauer (1966, 1971, 1978) demonstrates the limit-—
ations of such form-fitting handles. It is almost impossible to
design finger grooves into a handle in such a way that they fit a
large percentage of the population. Thus any set of finger grooves
will impair performance for those workers not perfectly fitted.

Handle or hand-hold width should be at least 11.5 cm, with 5 cm
clearance all around the handle to accommodate a 95th percentile
hand. If use with gloves is anticipated, at least 2.5 cm should
be added to these dimensions.

Worker/Floor Surface Coupling

The worker/working surface coupling has been cited as a causative
factor in a large percentage of accidents, particularly trips and
falls (Safety Science, 1977). The major types of accidents of
this type are:

ato slip (loss of traction on work surface)

Ac trip (movement of lower body arrested)

Eo mis-step (putting foot down where there is no support)
These accident patterns account for almost three quarters of work-
surface related accidents. The major mediating variable in this
type of accident is the coefficient of friction between the shoe
sole and the working surface. Pfauth and Miller (1976) review
shoe/working surface friction and find it affected by:

nia work surface material (wood, concrete, steel, tile, etc.;
Irvine, 1967; Kroemer, 1971, 1974; Sigler, 1943).

Bo surface coating (e.g., waxes can both increase and de-
crease friction; Braun and Roemer, 1974).

So floor condition (clean or dirty; wet, dry or greasy; e.g.,
Irvine, 1967; Kroemer, 1971, 1974; Sigler, 1943).

108


A number of studies (Edholm, 1967) have confirmed increased acci-
dent rates away from the comfort zone (18°C to 21°C, 65°F to 70°F)
and a greater effect of temperature for older workers. It is thus
particularly important to ensure that thermal stress does not
contribute to manual materials handling safety problems.

Cold environments are not usually a problem in MMH safety; the
strenuous nature of many of the MMH jobs gives a rate of metabolic
heat production sufficient to prevent hypothermia. However, cotder
environments can lead to reduced dexterity (e.g., Wentzel, 1968)
which could increase human error in MMH. Also colder temperatures
can motivate workers to wear protective clothing, particularly at
the extremities. Powell, et al., (1971) found decreased accidents
at lower environmental temperatures in a warehousing operation, much
of the effect due to the operators wearing their safety gloves in
colder weather for reasons of thermal comfort.

In hot environments the added metabolic ‘heat produced inside the
body by MMH work can result in greater risk of overstrain and heat
illnesses. To prevent excessive exposure to heat, an assessment
must first be made of whether or not a heat problem exists.
Currently, the most accepted method for determining heat load is
decribed in the Threshold Limit Value (TLV) booklet of the American
Conference of Governmental Industrial Hygienists (ACGIH, 1980).
Figure 7.2 shows the lines of permissible exposure to work in hot
environments as developed by Dukes-Dobos and Henschel (Dukes-Dobos,
1976) which was adopted by the ACGIH as the TLV for heat stress.
The values given apply to acclimated workers who are physically
fit.

Various measures are possible for the alleviation of the heat
stress problem. All seek to reduce heat storage by the body,
either by limiting input heat load from the environment, by
limiting metabolic heat generation, or by limiting exposure
duration. In practice this can mean air conditioning, radiant
heat shielding, forced air movement, clothing design to minimize
heat absorption and maximize evaporative cooling, and finally
the design of work-rest schedules to prevent body temperature
from increasing during the work shift.

Other methods of estimating metabolic costs for manual materials

handling tasks are available (Givoni and Goldman, 1971; Garg,
et al., 1978).

oka,


It seems likely that the younger person may not have developed the
requisite abilities to recognize and control the hazards of manual
materials handling as has the older worker. He may be overly
stressing his body, yet may have the strength to withstand the
rigor of the job. On the other hand, the older individual, while
having perfected his skills in handling heavy or cumbersome loads,
is likely to have some diminished physical capabilities. Age must,
therefore, be viewed as a potential risk factor but the exact form
of this risk is not yet fully understood.

Anthropometry

Body weight and stature are two anthropometric attributes with
potentially complex effects on an individual's risk of injury
during manual materials handling. It is generally accepted that
body weight has a direct effect on the metabolic energy expendi-
ture rate of a person while lifting and carrying loads (Kamon

and Belding, 1971; Garg, et al., 1978. Thus, a heavier person
would have a greater metabolic rate and concomitant circulatory
load, which could lead to earlier fatigue (Petrofsky and Lind,
1974), or cardiovascular problems if the person were so predis-
posed. On the other hand, a heavier person is usually stronger
than his lighter counterpart and usually has the mass necessary

to counter-balance the handling of large objects (Snook and

Irvine, 1967; Troup and Chapman, 1969 and Konz, et al., 1973).
Also, Ayoub et al. (1978) found that there appears to be a relation-
ship between body size and ability to lift. No direct link between
back pain or overexertion injuries and worker body weight has been
drawn.

Tauber (1970) indicated that taller people have more low-back
pain incidents than shorter people. Three separate epidemiologi-
cally oriented studies by Hult (1954), Rowe (1971), and Chaffin
and Park (1973) have not been able to support the notion that
either fat or thin or tall or short people are at a significantly
higher risk of low-back injury.

In brief, the selection of people for materials handling jobs

based on their anthropometry is not well justified in terms of
reducing low-back pain incidence rates. There is, however, the

need to specifically consider a person's anthropometry in relation

to the physical characteristics of the prospective workplace in

terms of reach and mobility. All jobs that do not allow for a

large range of anthropometric variation in the population, as

stated in reference books such as VanCott and Kinkade (1972) and
Damon, et al., (1966), should be identified and those specific
function limiting dimensions should be stated in the job descriptions.

Lift Technique
A substantial amount of literature has been published on lift
technique as an individual worker skill for minimizing injury.

Unfortunately, no controlled epidemiological study has validated
any of the contemporary theories on the subject. Proponents of

18


c. To teach the worker individually to become aware
of what he/she can handle comfortably without un-
due effort.

In order for the training to be effective, the instruc-
tors must be well versed in the sciences basid to manual
materials handling and in Safety Engineering. The con-
tent of the course must also be suited to the education-
al background of the trainees.

The course should cover the following aspects:
a. The risks to health of unskilled lifting: Case

histories from the organization concerned provide
the best illustration.

b. The basic biomechanics of lifting: The body as
represented by a system of levers.

c. The effects of lifting on the body: The basic
anatomy of the spine and the muscles and joints
of the trunk; the contribution of intra-thoracic
pressure while lifting.

d. Individual awareness of the body's strengths and
weaknesses: How to estimate one's comfortable

lifting capacity.
e. How to avoid the unexpected: Recognition of the

physical factors which might contribute to an
accident.

$30 Handling skill: Safe lifting postures; minimizing
the load - moment effects; timing for smooth and
easy lifting.

g. Handling aids: Platforms, stages or steps,
trestles, handles, wheels, shoulder pads.

he Warnings: What to be aware of when lifting.

How it Should be Taught.

It is not enough to teach safe lifting practices by
slides or films. The trainee should be practically in-
volved from the start. Therefore, classes must be small.
Training should not be restricted to a classroom. Lift-
ing technique should be demonstrated and practiced at
the work site. Supervisors, as well as trainees, should
be involved in the training program. The following are
the steps to an effective training program:

147


Rigby, L.V. "Why Do People Drop Things?" Quality Progress.
16-19, Sept. 1973. a

Rodahl, K. and Issekutz, B. "Physical Performance Capacity in the
Older Individual." In: Muscle as a Tissue. McGraw-Hill,

New York, 1962. Chapt. 14.

Rodgers, S.H. "Metabolic Indices in Materials Handling Tasks."
In: Safety in Manual Materials Handling. C.G. Drury, Ed.
DHEW(NIOSH) Publication No. 78-185, 1978. pp 52-56.

Roebuck, J.A.; Kroemer, K.H.E. and Thompson, W.G. Engineering
Anthropometry Methods. John Wiley and Sons,Inc.,New York, 1975.

Rolander, S.D. " Motion of the Lumbar Spine with Special Refer-
ence to the Stabilizing Effect of Posterior Fusion: An Ex-
perimental Study on Autopsy Specimens." Acta Orthop. Scand.

Suppl. 90, 1966.

Ronnholm, H.; Harvonen, M.J. and Lapinleimu, V.0o. “Physiological
Studies in the Optimum Rhythm of Lifting Work." Ergonomics.
5:52-53, 1962.

Rowe, M.L. "Low Back Pain in Industry: A Position Paper." J.
Occup. Med. 11:161-169, 1969.

Rowe, M.L. “Low Back Pain: Updated Position." J. Occup. Med.
13:476-478, 1971.

Saari, J. and Wickstrom, G. "Load on Back in Concrete Reinforce-
ment Work." Scand. J. Work Environ. Health. Suppl. 1: 13-
19, 1978.

Safety in Manual Materials Handling. C.G. Drury, Ed. DHEW (NIOSH)
Publication No. 78-185, 1978.

Safety Sciences, Inc. "Collection and Analysis of Work Surface
Accident Profile Data." Draft Final Report, DHEW (NIOSH)
Contract 210-76-0150, 1977.

Salvaterra, P. and Chiusano, R. "Optimum Cylindrical Handles."
Unpublished Report. Industrial Engineering Dept. State
University of New York( Buffalo), 1977.

Sanderson, C.W. "A Study on Handle Design." Unpublished Report.
State University of New York( Buffalo), 1976.

Saran, C. "Biomechanical Evaluation of T-handles for a Pronation-
Supination Task." J. Occup. Med. 15:712-716, 1973.

Sell, R.G. "Ergonomics as Applied to Crane Cabs." In: Human
Factors in Work, Design and Production. Taylor and Francis,
Ltd., London, 1977. pp 1-43,

170




Likewise, the D factor ranges from 1 to .74 as D varies from 0 to
200 cm.

D Factor - (.7+7.5/D)

For D = 0 set D = 25 (the minimum allowed value), then
D Factor = (.7+7.5/25) = 1.0

For D = 200,
D Factor = (.7+7.5/200) = .74

The F factor is a bit more complicated. If the lifting originates
below 75 centimeters (on the average V < 75) and is performed con-
tinuously throughout the day, Fmax = 12 (as given in Table 8.2).
If the observed frequency is 6 lifts per minute (F = 6) then,

F Factor = (1-F/Fhax)

Hl]

1-6/12 = .5

The effective weight which can be lifted is thus halved due to
frequency of lifting required.

Combining factors is illustrated for continuous lifting below
knuckle height. with average H = 20 cm, V = 40 cm, an average dis-
tance of D = 100 cm, at a rate of 6 lifts per minute; then

AL = 40 ( 15/20) (1-.004]40-75|) (.7+7.5/100) (1-6/12)
= 40 (.75) (.86) (.78)(.5) = 10 kg.
MPL = 30 kg.

The mechanics of this example would be exactly the same using the
U.S. Customary form of the equation and the task variable limits
given on page 120. The weight handled on the job could be com-
pared directly with these values. Suppose the load was 35 kg
(above the MPL). In this case, one "engineering control" might
be to reduce the frequency of lifting from 6 per minute to 1 per
minute (admittedly a drastic change). ‘This would increase the
frequency factor from .5 to .92 and consequently the

AL = 18.4, and

MPL = 55.2

128


Example 2, Transfer of Momentum,

Figure 8.10

138


Based on this elemental analysis it can be concluded that Pasks

1 and 2 are the most stressful. In fact, the maximum weights of
40 kg (Pask 1) are above the MPL = 23.4 and engineering controls
(such as mechanical aids or two-person lifting) are warranted.
Note that the average weights in Task 1 (10 kg) are reasonable
with administrative controls. In both cases, the horizontal lo-
cation produces the greatest discount in ability. Task 3 appears
to be nominal in terms of physical stress.

As with Example 4, analysis of the combined elements of the job

can only be approximated by computing weighted averages according
to frequency. In this case, the composite

H=

1(40) + 2(30) + 5(15) _ 22 cm

The coefficients 1, 2, and 5 correspond to each elemental frequency
and 8 is the total number of lifts per minute.

Similarly, the weighted average

vy = LO) + 2(0) + 5(75)
eae

= 47

D= 1(75) + 2135) + 5(25) _ 29

i)
al]

1+2+558

Thus, for the combined tasks:

AL

i]

40 (15/22) (1-.004(75-47)) (.7+7..5/29) (1-8/12)
40 (.68) (.89) (.96) (.33) = 7.7 kg
MPL = 23.1 kg

Since the average weight lifted is 8.1 kg, administrative controls
are required.

It is important to note that this “averaging" of the task descrip-
tors in this case tends to dampen out the large differences between
the tasks. This is true in general (but not always). This error
and the errors introduced by ignoring other carrying, holding,
pushing, and pulling tasks can only be resolved with more detailed
biomechanical, metabolic, and psychophysical evaluations.

144


L5/S, DISC
L5/S, DISC COMPRESSION = 278.5 kg
COMPRESSION =183.3 kg

BODY WEIGHT
ABOVE L/S,
CENTER-OF-GRAVITY

LOAD
CENTER-OF-
GRAVITY

Figure 3.8: Low Back Compression Associated with Two Lifting
Postures (Park and Chaffin, 1974).

acceptable but extreme flexion could predispose the lower back to

injury when the peak load occurs, usually during the first 200 msec
of the lift.

Based on simple quasistatic biomechanical concepts, one must con-
clude that instructions as to lifting postures must reflect con-
cern for the person's strength and mobility as well as the size

of the object to be lifted. Lifting of objects that cannot pass
between the legs should be done with the traditional stooped-over
torso and legs only slightly flexed. Where possible loads should
be reduced in size to allow them to come between the legs. When
this is possible, a squatting leg lift with the back nearly verti-
cal is recommended. Unfortunately, these recommendations are
based on biomechanical considerations only. Controlled field
studies to determine the benefits associated with these recommenda-
tions have not been made. As Brown (1973) and Jones (1971) point
out, much more research is necessary to establish the validity of
any suggested methods of handling loads. For the present, however,
biomechanically based recommendations certainly are worth serious
consideration when counseling a person as to how a load should be
lifted and carried safely.

33


List of Illustrations (cont.)

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure

Figure

5.7:

5.8:

7.13

7.2:

8.1:

8.2:
8.3:

8.4:
8.5:
8.6:
8.7:
8.8:
8.9:
8.10:
8.11:
8.12:
8.13:
8.14:

Maximum Recommended Weights Based on
Female Isometric Strength................05-

Comparison of Physiological and Psycho-

physical Criteria.............+.. 9

Hand Forces and Torques on Handles

Acceptable Heat and Metabolic Loads
(UNelen ge, ala) o00000000000000000000000000nn00

Maximum Weight versus Horizontal Location

for Infrequent Lifts from Floor to

Knuckle
Example

Graphic
Lateral

PERG I5 O50 50000000000 00590000000000000

Corelinye, IPF oO oD D0 000000000 00000000n00

Representation of Vertical,
and Horizontal AxeS..........eeeee0e

Horizontal Factor Nomogram.......sseseeeeees

Vertical Factor Nomogram........ 000000000000

Vertical Distance Factor Nomogram...........

Frequency of Lift Factor Nomogram...........

Example 1, Punch Press Operator.............

Job Analysis for Example 1...........00-e000-

Example
Example
Example
Example

Example

2,
3,
4,
4,

5,

Transfer of Momentum.............

Horizontal Obstruction...........

Depalletizing Operation..........

Analysis Sheet.............0000ee

Highly Variable Job....

ix

87

88
106

ohil4

125
abeyil

132
133
133
134
134
135
136
138
139
140
141
143


LOW BACK STRESS

Perhaps the most important point to be gleened from Figures 3.2
and 3.3 is that when a 20-kg load is held at arm's length, it
produces a large torque at the lumbosacral joint of the back.

For the average man's anthropometry, such a load produces more
than 1200 kg-cm torque. This, in combination with the torso
weight, produces a compression force at the Is/S, disc equivalent
to what holding about a 40-kg load between the knees would pro-
duce. In other words, one does not have to "bend over" to pro-
duce high forces on the low-back structures. A person with strong
arms and shoulders, in particular, can position the body in pos-
tures that greatly multiply an external load's effect on the low
back. The biomechanical consequences to the low back will now
be considered in more detail.

The lumbar spine can be thought of as a set of small links with
flexible articulations (discs) between each. With proper geo-
metric and physiologic data, the forces in each disc during a
specific lifting activity can be predicted. Because the clinical
and biomechanical data indicate the greatest problem to be at the
lower lumbar spine, the L5/S) disc (lumbosacral joint) has been
used to represent the spinal stresses of lifting in earlier
studies by Morris, Lucas and Bresler (1961), Tichauer (1966) and
Chaffin (1969). These models have clearly shown that during
weight lifting, the bending moment at the lumbosacral joint can
become quite large (on the order of 2000 kg-cm when lifting about
50 kg from the floor). To counteract this moment, the muscles

of the low-back region (primarily the erector spinae group) must
exert correspondingly high forces, since they operate on small
moment arms (about 3.8 - 5.0 cm, as referenced by Chaffin and
Moulis (1969) and shown in Figure 3.4).

The high forces generated by the low-back muscles are the primary
source of compression forces on the lumbosacral disc. These
concepts are illustrated in Figure 3.5 for a person holding a
variable load, designated Fy. The graph at the bottom of Figure
3.5 displays the predicted compression forces at the L5/S; disc
for increasing loads held in the position depicted, using a 50
percentile man's anthropometric data and normative abdominal
assistance values (Chaffin, 1969).

The important concept in Figure 3.5 is that even in the “reason-
able" lifting posture depicted, high compression forces are
created in the disc. Direct pressure transducer measurements by
Nachemson and Elfstrom (1970) of the compression forces in the
lumbar discs have confirmed the range of these predicted values.

The maximal amount of compression that can be tolerated by the
lumbar spinal column has been estimated from axial loading com-
pression tests on cadaver columns. Data from separate studies
of this type by Evans and Lissner (1959) and Sonoda (1962) dis-
close large biologic variations in the disc's (and its weight-

27


One major variable has been handle diameter, whose effect has
been measured for different forces and torques and using differ-
ent criteria. Pheasant and O'Neill (1975) measured flexion/ex-
tension torques and found the larger the handle the better, at
least up to 70 mm diameter although maximum shear force at the
handle surface was greatest for a diameter of 30-50 mm. Thrust
forces peaked at about 44 mm diameter. Flexion/extension torque
was maximized using rough knurled surfaces in place of smooth
cylindrical handles. There was no effect of a wide variety of
handle shapes on maximum torques when the effect of diameter was
eliminated.

The pronation/supination torque was measured as a function of
handle diameter by Saran (1973), who found that a 2.5 cm diameter
handle was preferred over either a 1.9 or a 3.2 cm handle. There
were no differences between handle diameters in terms of electro-
myographic (EMG) activity of the muscle groups involved in the
task.

Tasks requiring the production of a push/pull force have been
used to evaluate handle diameter in a number of studies. This is
perhaps the most relevant type of task for designing container
handles. Ayoub and LoPresti (1971) found a relatively flat opti-
mum between about 25 mm and 64 mm diameter when EMG was measured.
However grip forces were optimum for a diameter of 38 mm. Khalil
(1973) measured EMG activity for three diameters of cylindrical
handle, (3.5, 5 and 7 cm) plus an elliptical handle 5 cm long x
3.2 cm wide anda5cm diameter sphere. Of all these handles the
3.2 cm diameter handle was best.

Two unpublished studies both requiring a pull force (Sanderson,
1976; Salvaterra and Chiusano, 1977) have tested cylindrical
handles in different tasks. The former used a holding task and
handles from 1 to 2.5 cm and found an optimum at 1.9 cm. The
latter used both a subjective scale and a change in grip strength
following a one minute holding of a 15 kg container to evaluate
handles from 1 to 5 cm. Optima were found in each case at the
center of the range, 1.9 cm and 3.8 cm respectively.

Other recommendations can be made based on different criteria.

If the hand is to fit the handle with no overlap of fingers and
thumb then Garret's (1971) anthropometric data would suggest 41

mm as a maximum diameter for a 5th percentile male without gloves.
Similarly, guidebook recommendations in human engineering recommend
diameters as follows (Rigby, 1973):

Weight of Item Miminum Diameter
15 lb. 6 mm
15-20 lb. 13 mn
20-40 1b. 19 mm
40 lb. 25 mm

107


What is disturbing in this regard in the prevalance of manual
materials handling occupations in these industries, thus exposing
more women to overexertion hazards. As an example, 8 out of 10
employees in the health services industry in 1974 were women
(Women's Bureau, 1975). As depicted earlier in Table 1.3, this
industry reported the highest proportion of overexertion injuries
(48%) and the higher proportion of strains and sprains (62%).
Presumably a majority of these incidents are attributed to
patient handling by nurses, nurses aides and therapists.

It is clear that the number of women exposed to manual materials
handling is increasing. Further, the workforce is aging. Figure
1.2 depicts this trend for women in particular for the last 30
years. The most rapid expansion of the women labor force has
been in the age group 45 to 54 (Women's Bureau, 1975). The
emergence of these and other susceptible groups into workplaces
which were designed years ago for "strong, healthy, young males"
will undoubtedly exacerbate problems of overexertion injury in
the U.S. A work practices guide is needed to allow future work-
places to be designed safer and more productive for a changing
workforce.

PROPORTION OF WOMEN

wy Bot IN LABOR FORCE
x
© - Ages 7-65
. Age

S 6OF im ges 45-54
< a T)
2 ) %
> 407
Z g
= a
$
iL 20+
° L
xe

) é a,

1940 1950 {!960 1970
YEAR

Figure 1.2: Female Labor Force as Percent of Women by Age
(Women's Bureau, 1975)

6


CHAPTER 1

JUSTIFICATION AND SCOPE

Overexertion in the workplace accounts for a large number (and in
some industries the majority) of disabling injuries. Most of these
injuries involve the act of manually handling materials. This
Guide summarizes research on the hazards of manual materials handl-
ing in industry, and formulates recommendations to reduce the toll
of human suffering and economic burden.

JUSTIFICATION AND RATIONALE FOR THIS GUIDE

For many years manual materials handling (MMH) has been recognized
as a major hazard to industrial workers by authorities in the field
of occupational health and safety. A particular concern has been
shown for women and children performing such acts. In fact,

during the period from 1930 to 1950 almost all the states enacted
laws specifically limiting the weights that women and children
could handle. All of these statutes have been subsequently struck
down as unconstitutional since they discriminate against employment
of all women without recognition of the large variation in capa-
bilities between women.

As late as 1962 the International Labour Organization published

an Information Sheet which stated limits shown in Table 1.1. These
limits were primarily based on inspection of injury and illness
statistics which depicted manual materials handling as contribut-
ing to about a threefold increase in spinal, knee and shoulder
injuries, a tenfold increase in elbow injuries, and about a five-
fold increase in hip injuries (ILO, 1962).

Table 1.1: ILO suggested limits for occasional weight
lifting (kilograms) (ILO, 1962)

Age (years) Men Women
14 - 16 14.6 9.8
16 - 18 18.5 11.7
18 - 20 22.6 13.7
20 - 35 24.5 14.6
35 - 50 20.6 12.7

Over 50 15.6 9.8





CHAPTER 8

RECOMMENDATIONS

This chapter outlines load limit recommendations for lifting tasks.
Sections on how to identify a hazardous lifting job, how to inter-
pret the guidelines and how to apply them to actual lifting situ-
ations are provided. In addition, brief summaries of Chapter 6,
Administrative Controls and Chapter 7, Engineering Controls, are
included.

It is intended that this chapter will provide the Industrial or
Safety Engineer with an easy means to quickly scan the Guide,

pick out important points and apply them to the appropriate situ-
ation. References to earlier chapters are made if a more thorough
and detailed discussion of a topic is desired.

DEFINITION OF A LIFTING TASK

For the purpose of this Guide, a lifting task is considered to be
the act of manually grasping and raising an object of definable
size without mechanical aids (i.e., hoists, conveyors, block and
tackle, etc.). The time duration of such an act is normally less
than two seconds, and thus little sustained exertion is required
(as opposed to holding or carrying activities). The lifting limits
presented in this chapter do not apply to all kinds of lifts. They
are intended to apply only for:

a. smooth lifting

b. two-handed, symmetric lifting in the saggital plane
(directly in front of the body; no twisting during lift)

c. moderate width, e.g., 75 cm (30 in) or less

d. unrestricted lifting posture

e. good couplings (handles, shoes, floor surface)

ido favorable ambient environments.

It is assumed that other manual handling activities such as hold-
ing, carrying, pushing, pulling, etc., are minimal. When not
engaged in lifting activities, the individual is assumed at rest.
The assumed work force is physically fit and accustomed to physi-
cal labor.

The Guide does not include "safety factors" commonly used by en-
gineers to assure that unpredicted conditions are accommodated.

121


As was noted in Chapter 4, when oxygen is not available to the
muscles, lactic acid is produced, contractile abilities rapidly
decrease and eventually the muscles can no longer generate signif-
icart force. Aerobic capacity testing is one means for assessing
this dimension of individual physical fitness (as proposed by the
American Heart Association (1972) and the American Industrial
Hygiene Association (Kamon and Ayoub, 1976)).

A person's aerobic capacity corresponds to the maximal oxygen up-
take rate, which is accompanied by the maximal attainable heart
rate, for that individual. One can determine a person's aerobic
capacity by monitoring the oxygen uptake and heart rate while in-
creasing the work load in a controlled manner. Common devices

for this are motor driven treadmills, stationary bicycles and
benches which are used for stepping. The maximal oxygen uptake

is determined by noting the value at which the oxygen uptake

stops climbing as the work load increases. Since the conversion
factors between oxygen uptake level and metabolic rate have been
established, one can convert the maximal oxygen uptake to the maxi-
mal aerobic metabolism rate (also commonly called the maximal aero-
bic power or aerobic capacity).

Maximal aerobic capacity testing, as described above, is quite
stressful. Often the test is run until the person cannot continue
due to exhaustion. Since this condition may present a serious
medical risk, sub-maximal testing is often employed instead.

Sub-maximal aerobic capacity testing consists of measuring oxygen
uptake and heart rate at two to three work loads ranging between
50% and 75% of the worker's expected aerobic capacity and extrapol-
ating. A population based estimate of expected maximum heart rate
(based on the worker's age, sex and physical fitness) is then used
to estimate the maximal oxygen uptake. This extrapolated value is
then converted to an estimate of aerobic capacity. Since an in-
dividual's maximal heart rate may vary as much as + 20 beats per
minute from tabulated population values, a 10 to 15% error can ac-—
crue for the estimated maximal oxygen uptake (Kamon and Ayoub,
1976). Thus sub-maximal testing is not as accurate as maximal
testing.

Aerobic capacity testing needs to be undertaken with caution, es-
pecially if it involves submitting the individual to a workload
corresponding to his or her maximal aerobic power. Obviously,
maximal testing should only be done under the most careful atten-
tion of a physician. Sub-maximal testing is not as risky but
still should only be undertaken with healthy individuals who have
been ascertained by a physician to have no cardiovascular impair-
ments or indications of infectious diseases including the common
cold.

Metabolic requirements for jobs can be estimated using the methods
described in Chapter 4. These requirements can then be compared to

97


3. Squat lifting requires more work to lift the body
itself than stoop lifting and thus involves a
higher metabolic cost.

Figures 4.3 and 4.4 illustrate the importance of lift
technique on metabolic rate. Brown (1971) demonstrated
that free style lifting is least fatiguing compared to
stooping or squatting as shown in Figure 4.3. Garg and
Saxena (1979) found similar differences, expecially
with frequent low level lifting as shown in Figure 4.4.
(It is important to note that the variables plotted are
quite different in these two figures).

Weight of the Load. In repetitive lifting, mechanical
work per minute can be written as:

Mechanical Work = Load x Frequency x Distance of
Vertical Lift

Therefore, the heavier the load to be lifted, the
greater the mechanical work performed and subsequently
the greater energy expenditure. Frederik, 1959; Snook,
1965; Aquilano, 1968; Hamilton, 1969; Brown, 1971;
Lind, et al., 1977; and Garg, et al., 1978, have each
reported that an increase in load to be lifted results
in an increase in metabolic energy expenditure rate.
Most studies agree that a linear relation between
object weight and metabolic rate is reasonable for
most lifting tasks.

Frequency of Lifting. Frequency is defined as the
number of lifts per minute. The mechanical work done
by the musculoskeletal system is also directly pro-
portional to frequency (as noted above). If all other
factors such as load, range and distance of vertical
lift, speed of lift, technique, etc., are held constant,
the metabolic energy expenditure rate should be directly
proportional to the pace of lifting.

The effect of pace on energy expenditure has been
studied by Anuilane, 1968; Hamilton, 1969; Chaffin,
1972; Snook, 1965; Garg, 1976; and Lind, et al., 1977.
Figures 4.5 and 4.6 illustrate relationships between
weight, lifting frequency and metabolic rate observed
by Hamilton (1969 and Aquilano (1968). Thus the
relationship between work pace and metabolic energy
expenditure appears to be linear.

56


ako There have been few attempts to analyze MMH tasks in
terms of postural and handling stresses because it is
a very expensive task.

Ao Back pain has many possible etiologies, thus, to attri-
bute it to a single causative factor is misleading.

So There appear to be various "conditioning" factors (in-
dividual and environmental) which predispose a back to
injury.

Not surprisingly, many authors have found it epidemiologically
expedient to lump together all reported episodes of back pain
attributable to work irrespective of diagnosis or cause.

BACK PAIN

Back pain can be defined as primary or secondary (Wyke, 1976).
Primary back pain arises directly from the tissues of the back
which are in a state of neurological, mechanical or biochemical
irritation because of fatigue, postural stress, injury or local
pathological change such as degeneration. It can arise from any
of the tissues supplied by nociceptor afferents (i.e., practically
every tissue except for the intervertebral disc and the facets

of the apophyseal joints). Back muscles and fascia, vertebral
ligaments, apophyseal joint capsules, the spinal dura and dural
root sleeves, the adventitia of blood vessels and the periosteum
all receive a nerve supply and may become primary sites for back
pain. The cartilaginous facets of the apophyseal joints, the
end-plates, the nucleus pulposus or the annulus fibrosus (except
where it is in contact with the longitudinal ligament or periosteum)
are not primary sites.

Secondary back pain is caused by a lesion which affects the nerve
supply to the tissues of the back. Thus a mechanical derangement
of the spine (for example, any of the degenerative processes af-
fecting the disc or apophyseal joints) which leads to irritation
or ischaemia of the posterior primary ramus or nerve degeneration
in its fibers may be an indirect or secondary cause of back pain.
This is analogous to the derangement of the lumbar spine (such

as a prolapsed intervertebral disc) which mechanically compromises
a nerve root by stretching it or angulating it from its normal
path and which may cause pain, weakness or-numbness in the lower
limb in the area of distribution of that nerve root (Murphy, 1977;
Marshall, et al., 1977).

Back pain is seldom well localized. When severe, it may be refer-
red down the buttocks and thighs without actual root involvement
(Mooney and Robertson, 1976; McCall, et al., 1978). It cannot

be measured since the severity of pain actually experienced has
no direct relationship either to the intensity of nociceptor
activity or to the underlying pathological process (Wyke, 1976).

il


DISTANCE FACTOR

Figure 8.6:

FREQUENCY FACTOR

Figure 8.7:

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1

100

40

80
30
LIFT DISTANCE

1 10 (em)

50 (in)

Vertical Distance Factor Nomogram.

1,0
09

go 9g gg 9
a a oN @

9

°
ww

bo

iS)

Occasional,
lench Height and Above

Continuous,
Bench Height and Above

Occasional,
Low Lifting

Continuous,
Low Lifting

1
14

7

3

4 5 6 7 8 9 10 H 12 13 iS # ]

AVERAGE LIFTS PER MINUTE

18

Frequency of Lift Factor Nomogram,

134


FOREWORD

Overexertion injuries associated with manual materials handling
(MMH) jobs (unaided lifting, lowering, etc.) continue to plague
industry. The National Safety Council reports that at least

25 percent of all injuries, accounting annually for 12 million
lost workdays and over 1 billion dollars in compensation costs,
can be attributed to such mishaps.

Past attempts to deal with this complex problem have focused
upon adopting rather arbitrary weight limits for lifting loads
or using training procedures which emphasize "correct" lifting
techniques; neither approach has had much effect upon reducing
overexertion type injuries. This Work Practices Guide provides
for a broader examination of the problem, encompassing medical,
scientific and engineering viewpoints, and presents procedures
to gauge the risk of overexertion hazards in MMH jobs and for
risk control.

The first two chapters of the Guide identify the need for
solutions to MMH problems based upon injury data and the exten-
sive portion of the workforce engaged in such tasks. Chapter 2
further establishes this need through a review of epidemiological
reports involving short and long term health effects associated
with MMH tasks and identifies important job and personal risk
factors.

Chapters 3,4 and 5 examine the problem in detail from the
separate viewpoints of biomechanics, work physiology and psycho-
physics, respectively. Each approach develops criteria for
establishing load limits in terms of load weight, horizontal
location, lift distance and frequency of lift.

The next 2 chapters deal with hazard control procedures.
Chapter 6 considers methods for avoiding worker-job mismatches
by selecting workers for jobs based upon work capacity and
physical attributes, and current knowledge of training pro-
cedures which might be employed to make workers aware of good
lifting practices. Chapter 7 examines engineering control
procedures and design criteria dealing with the workplace
environment. Container and handle design, constraints imposed
by the visual and thermal environment and materials handling
system alternatives are considered here.

Chapter 8 incorporates the several approaches into a composite
formulation for recommending load limits given different task
factors. This material will be particularly useful to those
directly charged with plant safety and health responsibilities.

xiii


job ranking should come from medical information (if available)
such as:

medical reports

first aid reports

OSHA 101 forms

~ worker's compensation payments

Things to look for on these reports include:
abo musculoskeletal injuries, particularly back injuries

- overexertion
- strains/sprains
- contact injuries such as:
- lacerations
- bruises
- abrasions
- fractures

Bo what job the injury occurred on so that a total for
each job can be compiled

Oo how much lost time was associated with each injury.
In the absence of medical records, stressful jobs can
be identified by information obtained from line super-
visors or foremen for a particular job, such as:

- is there a high turnover rate?
- is there frequent absenteeism?
- are there frequent sprain/strain complaints?

Analysis Procedures

All data collected should be organized on a "Physical Stress Job
Analysis Sheet" (see Figure 8.2). The exact form is, of course,
optional but it should include background and identification in-
formation for each job such as date, plant name, department,

analyst's name and job title as well as the task descriptors to
be measured. These are:

abs Weight of the object lifted - determined ‘by direct
weighing. If this varies from time to time, note the
average and maximum weights.

Bo The position of the load with respect to the body - this
must be measured at both the starting and ending points
of a lift in terms of horizontal and vertical location.
The horizontal location from the body (H) is measured
from the midpoint of the line joining the ankles to the
midpoint at which the hands grasp the object while in
the lifting position. The vertical component is

130


APPENDIX

This appendix contains a sensitivity analysis of the work prac-
tice guide equation presented in Chapter 8. For a variety of
vertical and horizontal hand locations, vertical travel distances,
and frequencies of lifting, the action limit (AL) and maximum per-
missible limit (MPL) are evaluated in terms of:

ako the expected percent of the workforce capable based on
dynamic strength, Ayoub, et al., (1978) and Snook (1978),

Ag the range of expected percent of the workforce capable
based on isometric strength, Chaffin, et al., (1978),

Eo the lower and upper bounds on predicted static com-
pressive force at the L5/S, vertebral level, Chaffin,
et al., (1978), and

4, the range of predicted metabolic rates according to
Garg, et al., (1978).

In all cases, the AL is evaluated based upon a female industrial
population; the MPL is based upon a male industrial population.

Ranges for the last three indices were obtained by calculating
these values for both the least- and most-desirable posture. For
example, the metabolic expenditure for low lifting in a squat
posture is greater than when lifting in a stoop posture (see
Chapter 4). Therefore, the value corresponding to the squat
posture is least preferred and represents the upper bound on meta-
bolic rate. The lower value corresponds to a stoop posture. No
range is given when the vertical hand location is 81 cm or higher
because it is assumed that only a stand posture would be appro-
priate for lifting. A detailed explanation of the derivation of
values for each column follows.

DYNAMIC % CAPABLE

The table value is calculated by obtaining the standard normal
distribution Z-value for the given vertical and horizontal hand
location, travel distance and frequency suggested in Chapter 5
(Table 5,7). For the Action Limit (Table A.1) the ranges assumed
were as follows:

p75)


In the first study, the incidence rate of low-back pain was strong-
ly correlated with the lift strength rating. In the moderate-
strength-requiring jobs, where a potential hazard first appeared

to exist, weight lifting was an equally serious hazard for both

men and women. In the follow-on study (Chaffin, et al., 1977),

it was observed that

"the more remote the load center of gravity from the
body (due to either the bulk of the object being
handled or the workplace layout), the greater the
frequency and severity of musculoskeletal problems
and contact injuries."

The etiological basis for these findings will be discussed in
Chapter 3.

Frequency

The relation of frequency, duration and pace of lifting to back
injury potential was also studied by Chaffin, et al., (1977).
High frequency load lifting was related to increased injury rates.
In particular,

"the more frequent the lifting of maximal loads on a
job, the greater the frequency and severity rates of
musculoskeletal problems (other than backs) and the
greater the severity of contact injuries."

These results suggest

1. a greater exposure to physical stresses during
repetitive lifting which could accelerate "wear
and tear" in connective tissues,

2. a greater potential for muscle fatigue with
repetitive lifting, and

3. a greater probability of an uncoordinated muscle
action during a lift.

The physiological and psychophysical implications of these results
are discussed in detail in Chapters 4 and 5.

PERSONAL RISK FACTORS

The capacity to perform the physical act of lifting varies con-
siderably not only from individual to individual but within any
given individual over time. Furthermore, the limitations of this
capacity are complex and interrelated. Understanding the rela-
tionship of these characteristics to the resulting risk of injury
to the worker is prerequisite to the development of schemes for
placing people in jobs which do not compromise their health and
safety.

16


Backlund, L. and Nordgren, L. "A New Method for Testing Isometric
Muscle Strength under Standardized Conditions." Scand. J.
Clin. Lab. Invest. 21:33-41, 1968.

Bartelink, D.L. "The Role of Abdominal Pressure in Relieving the
Pressure on the Lumbar Intervertebral Discs." J. Bone Joint

Surg. 39B:718-725, 1957.

Becker, W.F. "Prevention of Back Injuries through Preplacement
Examinations." Indust. Med. Surg. 24:486-490, 1955.

Becker, W.F. "Prevention of Low Back Disability." J. Occup. Med.
3:329-335, 1961.

Berenson, P.J. and Robertson,W.G. "Temperature." In: Bigastronau-
tics Data Book. 2nd Ed., J.F. Parker and V.R. West, Eds. NASA

tics Data Book ®
1973. Chapt..3,

Bernauer, E.M. and Bonanno, J. "Development of Physical Profiles
for Specific Jobs," J. Occup. Med. 17:27-33, 1975.

Bink, B. "The Physical Working Capacity in Relation to Working
Time and Age." Ergonomics. 5:25-38, 1962.

Bink, B. "Additional Studies on Physical Working Capacity in
Relation to Working Time and Age." Proc. Second Internat.
Cong. Ergonomics, Dortmund, 1964.

Blow, R.J. and Jackson, J.M. “Rehabilitation of Registered Dock
Workers." Proc. Roy. Soc. Med. 64:753-760, 1971.

Bobbert, A.C. "Optimal Form and Dimensions of Hand-Grips on Cer-
tain Concrete Building Blocks." Ergonomics. 2:141-147, 1960.

Bonjer, F.H. "Actual Energy Expenditure in Relation to the
Physical Working Capacity." Ergonomics. 5:29-31, 1962.

Bonjer, F.H. "Temporal Factors and Physiological Load." In:

Measurement of Man at Work, W.T. Singleton, J.G. Fox and
D. Whitfield, Eds. Taylor and Francis, Ltd., London, 1971.

Borg, G.A.V. Physical Performance and Perceived Exertion. Ejnar
M. Munksgoard, Copenhagen, 1973.

Borg, G.A.V. "Perceived Exertion: a Note on 'History' and Methods."
Med. Sci. in Sports+ 5:90-93, 1973.

Braun, R. and Roemer, D. “Influence of Waxes on Static and Dy-
namic Friction. Soap, Cosmetics, Chemical Specialties, West
Germany, 60-72, 1974.

Brouha, L. "Role of Climatic and Environmental Conditions in
Weight Carrying." Indust. Med. Surg. 36:257-266, 1967.

157


rate for a bent standing posture was assumed when the vertical
hand position was 81 cm or less (irrespective of the travel dis-
tance), whereas the basal rate for a standing posture was used
otherwise (ive., 82 cm and above). The lower value corresponds
to an assumed stoop lift and the larger value assumes a squat
lift. Obviously, such a range can only be calculated when the
lift begins. below 81 cm. Above 81 cm, the equation for an arm
lift was used to calculate the metabolic expenditure per lift.

For frequencies of .2 lifts per minute, the metabolic rates repre-

sent basal metabolic rates. Horizontal hand location is not ac-
counted for in these estimates.

177


Table 5.7: (cont.)

Horz. Freq.

Female

Height of Lift (cm) (liftfin) 25%ile 50%ile 50%ile
Knuckle to shmilder 30 1 13 14 24 29
2 12 14 23 27
4 12 13 22 26
6 11 13 20 24
8 9 12 18 22
12 9 10 15 18
38 1 12 13 27 31
2 11 13 26 30
4 12 13 24 28
6 11 13 22 27
8 9 11 20 24
12 8 9 14 17
46 1 11 13 21 25
2 11 13 20 25
4 10 12 19 24
6 9 11 18 24
8 9 10 17 21
12 8 9 14 17
Knuckle to reach 30 2 10 13 21 26
4 lo 12 20 22
6 10 12 18 21
8 9 il 17 19
38 2 1l 12 24 27
4 11 12 22 24
6 10 il 20 22
8 10 11 18 21
46 2 13 14 24 28
4 11 13 22 24
6 ll 12 20 21
8 9 il 18 20
Shoulder to reach 30 1 11 12 23 27
2 10 12 22 24
4 10 1L 21 25
6 9 10 19 22
8 7 8 15 18
12 6 6 ala, 14
38 1 10 11 20 24
2 9 Ab 19 22
4 9 10 18 22
6 8 9 17 22
8 6 8 15 18
12 5 6 11 13
46 1 11 12 20 24
2 10 11 18 22
4 9 10 18 21
6 9 10 17 21
8 8 9 15 18
12 5 6 11 13

-)
uw


viewing the relative weights of each factor (.75) (.86) (.75) (.50)
in this case allows a quick evaluation of the relative weights
associated with changes in any factor. Frequency of lifting is
the biggest discounting factor (.5) in this case and should re-
ceive first consideration.

Reducing frequency to 1 lift per minute would lower the stressful-
ness of the job to within the “administrative controls" region.

In this case, 35 kg is between AL = 18.4 and MPL = 55,2. This
should not preclude further "engineering controls". It is import-
ant to realize that the job still cannot be safely performed by
most women or the majority of men. Further reductions in the fre-
quency (factor = .92) would be ineffective since this factor can
only increase to 1.0. The load probably cannot be brought appre-
ciably closer to the body (H = 15 versus H = 20 cm).

The best engineering solution at this point would be to change the
load weight. Halving the weight (from 35 kg to 18 kg) would be
one solution to achieving a task within the capabilities of most
people (18 kg is less than AL = 18.4 kg). An equally acceptable
solution would be to increase the load to 100 or more kg and pro-
vide mechanical lifting aids thus precluding manual handling and
relieving the person of lifting altogether.

JOB PHYSICAL STRESS EVALUATION

The purpose of the job physical demands evaluation is to identify,
quantify and document the physical stresses associated with a given
job. This section outlines how such an evaluation should be per-
formed so that the guidance of the previous section may be applied.

Selection of Analysts

All job analyses should be performed by an individual who has ex-
perience in work measurement and who is thoroughly familiar with
the plant and the jobs done in it. The analyst should know the
prescribed methods of performing these jobs and should be aware

of all tasks (including any irregularly occurring tasks) associated
with the job.

Selection of Employees

Only experienced employees who routinely perform their work accord-
ing to job descriptions and who work at a normal pace should be se-
lected for measurement during the job evaluation. This will assure
that the job description accurately describes the work performed.
Selection of Jobs

Jobs should be rank ordered by incidence and severity rates of

musculoskeletal disorders. Jobs with the highest rank should be
studied first. The primary source used to formulate this sort of

129


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78




Methods for identifying hazardous MMH tasks and evaluating
these jobs in terms of excessive load weight, location and
lift frequency are given in a readily understandable manner,
along with recommendations for appropriate hazard control
procedures.

The Guide does not address all MMH activities. Indeed, its
primary focus is on lifting compact loads with both hands.
For such work, it represents the current state of the art
in risk control and presents a useful starting point to deal

with complex problems presented by other manual materials
handling tasks in industry.

xiv


Table 5.7: Maximum recommended weights based on dynamic
strength (Kg.)

Female Male.
Horz. Freq.

Height of Lift (cm) (liftfin) 25%ile 50%ile 50¢ile 75%ile

Floor to knuckle 30 1 18 20 30 36
2 17 18 28 34

4 14 16 24 28

6 12 14 22 28

8 lat, 13 21 26

12 9 11 18 21

38 1 15 17 27 32
2 11 13 26 31

4 10 13 24 30

6 10 12 22 25

8 10 12 20 24

12 9 10 15 18

46 1 13 16 24 29
2 ll 13 23 27

4 ll 12 21 26

6 11 12 20 23

8 10 ll 18 23

12 8 10 14 17

Floor to shoulder 30 2 11 13 23 27
4 12 13 22 25

6 11 13 20 24

8 11 13 19 23

338 2 12 14 24 26
4 11 13 23 27

6 9 13 22 25

8 10 12 21 25

46 2 ll 13 23 26
4 1l 13 22 25

6 10 12 21 24

8 9 ll 20 25

Floor to reach 30 2 ll 12 21 24
4 10 12 20 24

6 11 11 19 21

8 10 11 18 20

38 2 11 13 24 29
4 ll 12 21 25

6 10 11 18 21

8 10 11 15 17

46 2 10 12 18 22
4 9 ll 18 20

6 9 11 17 20

8 9 10 17 20

84


Table A.1: Action Limit Sensitivity Analysis

ISOMETRIC BACK
% CAPABLE COMPRESS.
LOWER |UPPER } LOWER

178


CHAPTER 6

ADMINISTRATIVE CONTROLS

As discussed in Chapters 3-5, a large variation in lifting capac-
ity exists in the working population. One must be conscious of
the fact that with any ergonomic criterion (e.g., strength, aero-
bic capacity, etc.) it is often true that the standard deviation
of any general population sampled will be between 30 and 40 per-
cent of the mean of the population attribute measured.

Such a large variation in the population's tolerance to physical
stress supports the need for administrative controls to assure
that both,

alo a worker who is either weak or unfit is not exposed
to the demands of lifting heavy loads, or,

2. a strong, fit worker is suitably trained to avoid
certain lifting posture and activities which are
believed to increase the hazard level.

This chapter develops guiddlines for selection and training of
such workers who are to be placed on jobs requiring the lifting
of materials.

SELECTION OF WORKERS

Present selection procedures vary widely. A large number of
smaller manufacturing, distribution and service industries have
neither medical nor nursing staffs, and no formal selection
system exists. The principal method has been self-selection by
the worker based on their initial tolerance for the demands of
the job. In larger industries, new employees are often asked to
complete a questionnaire on health and medical history; and are
submitted to routine tests of visual, auditory and pulmonary func-
tion, of blood pressure, mobility, etc., often with the addition
of a chest X-ray. A physician will only see those whose replies
and test results reveal abnormality or doubt on the part of the
test administrator. In a few large industries, every recruit is
examined by a physician but this usually depends on the existence
of recognized physical or environmental hazards.

The clinical examination is widely regarded as the first essential
step in a good selection procedure for physical labor (Magnussen

89


Table A.2 (cont.)

v H D F MAX ISOMETRIC BACK 7
(cm) (em) (cm)| {per JpERMIs | DYNAM %_ CAPABLE COMPRESS. MET. RATE
VERT | HORZ |DIST|min) {(mpL) |} CAP [LOWEP |CPPER | LOWER |UPPER | LOWER|UPDER
150 15 25] <42 84 --- -- 42 623 ad 1.9 1,9
1 78 = = 54 584 -- 2.4 2.4
10 28 oS =9 ad 249 = 3.9 3.9
150 15 50} <.2 | 71 al 68 539 aS 1.9 dof)
1 67 = 77 508 2.7 2.7
1o { 24 --- -~ -- 218 = 5.1 5.1
150 45 25} <.2 28 = -- 79 437 a 1.9 1.9
1 26 SS = 84 198 2.0 2.0
10 | 9 88 co = 198 a 2.8 2.8
|
150 45 SO{ <.2 24 os a 87 378 = Lok) 1.9
1 22 34 5 89 361 = Bo 2.2
10 8 95 - = 175 a 3.2 3.2
150 15 25 2 17 = 81 567 to 1.9
1 16 = 86 543 2.0 2.0
10 6 | oS -- -- 365 ad auf 2.6
ee
150 75 50 2 14 = 89 519 = 1.9 4.9
1 13 => 91 503 -— 2.1 doik
10 5) ad = 350 a 2.9 2.9
|
175 15 25] <.2 72 -- 13 544 op elo) 1.9
1 67 = 22 513 cS Boe 2.3
lof 24 | --- J -- | -- 223 | -- 3.7 | 3.7
175 45 25 | ..2 24 --- | 35 95 487 = 1.9 1.9
1 22 33 = 97 464 -- 2.0 2.0
10 8 cam —- as 286 — 2.8 2.8
| |
|
|
1
|

w U.S, GOVERNMENT PRINTING OFFICE: 1981—757-074/1080

183


container friction (assuming the material is not caustic or hot) thus
relieving the static forces in the arm and shoulder muscles.

There are no studies on the effect of hand position on this
carrying task, and therefore, no recommendations on handle posi-
tion are possible. It is possible to set limits on container

size for this mode of carriage from anthropometric considerations.
For the general population, if the forward edge is to be reached
the length in the fore- and aft direction should not exceed 71 cm
(males) or 65 cm (females). To prevent interference with forward
vision, the maximum height of the container above handle position
should be 82.5 cm (males) and 80 om (females). If handles are

not placed so that the whole container is above hip height with
elbows extended, then interference with the legs will result during
walking.

In general, handles should be placed above the container center
of gravity for ease of container control but this may not be
possible with bulky containers which would then interfere with
leg movement. Kellerman and Van Wely (1961) in evaluating differ-
ent shaped boxes to carry a constant (17.5 kg) weight of flower
bulbs found both subjective preference and physiological evidence
of the superiority of a wide, shallow container. Their optimum
100 x 30. x 12 cm deep container allowed the worker to carry it at
arm's length without. interfering with the legs during walking. A
compact container is recommended but if deep ones are used it is
suggested that handles be above the center of gravity for lifting
from floor to table height to reduce stooping, and at or below
the center of gravity for carrying or lifting above table height
to reduce leg interference.

Human/Container Coupling Design

The handle or hand-hold is the coupling between the container and
the worker and should be designed with the worker's hand in mind.
The importance of proper hand/container couplings cannot be over-
stressed. They have a large effect on both the maximum force

a worker can exert on a container and on the energy expenditure

in manual materials handling tasks. Aside from manual materials
handling injuries, Rigby (1973) shows that lack of handles is a
prime reason for people dropping products, with resultant product
damage. Handle design is usually rather poor in practice. Wood-
son (1971) notes that off-the-shelf handles appear to be "designed
as decorative appointments" rather then “designed to fit the hand".
The major problems Woodson reports are insufficient hand clearance,
sharp edges which can cut into a worker's hand and too small a
handle diameter.

Postures of the hand with respect to grasped objects have been
classified by Napier (1956) into

do A_hook grip in which the fingers are flexed around the
object and the thumb is not used for gripping.

105


Assuming that a physically fit 35-year-old man (16 Kcal
capacity) should be capable of working this job, then it is
predicted (from Figure 4.2) that the working time will need
to be reduced from a 480 minute day to a 340 minute day, with
140 minutes of rest.

>
=
°o
25
=
4
oO
x 20
= AVERAGE PHYSICAL WORK CAPACITY
zs 75 kg Healthy 35 Year-Old Man
a 15
S$
B83
= = 10
a
Ww
gos
e
Ww
z ° pe tl howe
2 4680 50 100 500 1000 5000 10,000

WORKING TIME (minutes)

Figure 4.2: Physical Work Capacity and Working Time (Bink, 1962

and Bonjer, 1962).

Estimating Task Energy Requirements

Given the ability to assess an individual's metabolic work
capacity, adjusted for work duration, the question remains:
"How can this be related to a job?" At present, the three
commonly used methods for determining the metabolic demands
of a particular job are:

1) measurement of oxygen consumption on incumbent
workers
2) estimating using tabulated survey values

3) estimating using mathematical models.

52


WwW ike) -

a

=)

F

Se

Ww

x

Wi = 6

a>

ao

Ww 2 4p KEY

Wi @ 4.5 kg Cartons

Ww 4 6.8 kg Cartons

g ar 4 9.1 kg Cartons

= © 114kg Cartons

Ww e Ave. for All Wts

Zz ol 1 l l
6 9 \2 15

WORK PACE (cartons/minute)

Figure 4.5: Energy Expenditure versus Work Pace

(Hamilton, 1969).

Vertical Travel Distance of Lift. Since mechanical work
is also directly proportional to vertical travel, meta-
bolic energy expenditure should increase with an increase
in vertical distance of lift. The results of Aquilano
(1968) shown in Figure 4.6 also illustrate this point.
When an 11 kg load was lifted from the floor to waist
height (92 cm) and head height (168 cm) at a pace of 11
and 10 lifts per minute respectively, the corresponding
energy expenditures were 5.21 and 6.58 Kcal/min. The
exact relation between metabolic rate and vertical travel
distance in this case depends on body posture. Tasks
which require raising and lowering the body as well as
the load affect efficiency and the total weight moved,
consequently the metabolic cost of lifting.

58


Table 1.3: Arkansas 1976, Supplemental Data Systems
Report (BLS, 1978)

% of All Injuries % of All Injuries
Industry Reported as Reported as
Overexertion Strain/Sprain

General Manufacturing of

Durable Goods: 24% 29%
Household Furniture 358 34%
Primary Metals 29% 34%
Fabricated Metal Products 35% 37%
Fabricated Structural Metals 383 40%
Metalworking Machinery 43% 54%
Electric & Electronic Equip. 31% 383
Ship Building & Repair 34% 40%

General Manufacturing of

Non-Durable Goods: 308 36%
Beverages 42% 40%
Knitting Mills 50% 60%
Chemicals & Allied Products 41% 54%
Inorganic Chemicals 45% 62%
Drugs 44% 38%
Rubber & Misc. Plastics 35% 45%
Tires and Inner Tubes 43% 55%

Wholesale Trade & Distribution: 32% 37%

General Merchandising Stores: 35% 38%

Service Industries: 36% 49%
Health Services 48% 62%

Clearly, human suffering and high economic burdens are greatest
in industries which require manual handling of objects. Whether
such acts are the primary cause or simply aggravate a pre-exist-
ing susceptibility to injury of the musculoskeletal system is
often debated. Further, how many of the medical problems are
falsely reported in the worker's compensation statistics is an
open question. From a labor relations standpoint, the question
is irrelevant. The fact is that the physical act of manual mate-
rials handling in industry is regarded as hazardous by many work-
ers who claim,.with regular success, that such caused harm and
resulting disability. Be the harm of organic or psychological

3


Labeling can be used to indicate handhold positions, cautions
against single person lifting if the load is heavy, or a note
of caution if its center of gravity is not near the center or if
it is likely to shift.

The Thermal Environment

Deviations from the comfort zone (18°C (64.5°F) to 21°C (70°F)
have been shown to influence injuries in many industries. It is
thus particularly important to ensure that thermal stress does
not contribute to manual materials handling safety problems.

Cold environments are not usually a problem in manual materials
handling because the strenuous nature of the work provides suffi-
cient heat production, and the thermal clothing may provide some
personal injury protection. The cold, however, does reduce dex-
terity which could lead to human error in lifting.

If a heat stress problem is suspected, it should be assessed by
the Wet Bulb Globe Temperature (WBGT) index. Récommendations

for this index can be found in the ACGIH (1980). Engineering
control measures such as radiant heat shielding, forced air
movement, clothing design and work-rest schedules can be used to
prevent body temperatures from increasing over the 38°C (100.4°F)
recommendation.

Materials Handling System Alternatives

Jobs which exceed the MPL should be performed with the aid of some
mechanical device or redesigned so that they can be performed
safely. Some examples of simple lifting aids found at the work-
place are:

a. Hooks: The worker should be trained in the use of
handling packing hooks so that they will not glance
off hard objects.

b. Bars: The major hazard of a crow bar is that it may
slip. The edge should have good "bite".

Go Rollers: These are effective in moving heavy and bulky
objects.

d. Jacks: They should be marked to indicate the safe load
which can be raised.

e. Platforms: These provide the maintenance of a conven-
zent height for lifting and handling.

£35 Trestles: These can be used to maneuver long loads on

the point of balance, or for readjusting the grip or
carrying posture,

151


Ayoub, et al., (1978) and Snook (1978). The horizontal location
of the hands is assumed to be 1/2 the horizontal box size plus
15 centimeters for body clearance.

A similar illustration of the effects of object size (horizontal
and vertical location of the load) on the isometric strength
capabilities of these two population percentiles is presented

in Figures 5.6 and 5.7. These figures are based on the biomechani-
cal.model of Chaffin, et al., (1978) discussed earlier in this

chapter. They apply for occasional lifts (less than 1 lift per
5 minutes).

Depending on the specific task (weight handled, vertical and hori-
zontal hand location, vertical lift distance, and frequency of
lifting) different guidelines would be suggested based on iso-
metric versus dynamic strength. Both are required for safe lift-
ing. The guideline presented in Chapter 8 is evaluated in terms
of both isometric and dynamic strength ‘capacities in the Appendix.

120

25% OF MALE
POPULAT/ON CAPABLE

100 (75%ile)

F
(=)
w
FE
5
a
b
x
°Q
Ww
=
20k ~Nis0 em,
Vertical Location
of Load
re) rT 1 1 1
15 30 4s 60 75

“HORIZONTAL LOCATION OF LOAD (cm)

Figure 5.6: Maximum Recommended Weights Based on Male Isometric
Strength

86


lifting of heavy loads. Weaker workers in terms of muscle strength
(a psychophysical measure) are more susceptible to injury on these
jobs than their stronger cohorts.

The large number of studies cited in this Guide indicate that a
complex set of limitations exist in the working population's
capability to safely handle loads of varying weights and sizes,
and for various frequencies and durations during the work day.

In essence this Guide summarizes a wealth of research and presents
recommendations to control the various types of hazards associ-
ated with the unaided act of symmetric (two-handed) lifting of

an object of known weight and size. Quantitative recommendations
regarding the safe load weight, size, location and frequency of
handling are presented (Chapter 8). Factors which mitigate these
recommendations are also discussed (e.g., worker training, physi-
cal fitness, strength, workplace layout, load handles, etc.).

The Guide includes recommendations regarding both the selection
and training of workers who must perform manual materials handling
activities (Chapter 6) as well as presenting some engineering
guidelines for the design of workplaces where manual materials
handling is performed (Chapter 7).


WEIGHT (kg)

175 cm,
Vertical Location
of Load

fol 1 1 1 1
15 30 45 60 75
HORIZONTAL LOCATION OF LOAD (cm)

Figure 3.11: Task Variables Producing 650 kg Male Back Compression.

Is/S] disc of the average male. In each case, the most advanta-
geous posture (that producing minimal compression) is assumed for
these illustrations.

These figures suggest a simple inverse linear relation between
the maximum weight lifted (W) and the horizontal location of the
load (H). This is,

W = K(1/H)

where K is a constant which depends on gender and vertical loca-
tion of the load.

A Dynamic Biomechanical Lifting model developed by El-Bassoussi
(1974) and further reported by Ayoub and El-Bassoussi (1978) also
appears in the literature. This model calculates the compressive
and shear forces on the Ls5/S; disc during the time course of a
lifting movement for lifts made in the sagittal plane from the
floor to a height of 30 inches. The output forces of this model
include a static component resulting from the respective weights
of the load and body segments plus a dynamic component due to the
acceleration of the same during a lift. Figure 3.12 shows the
compressive and shear forces for both a leg lift and a back lift
when lifting a 4.5 kg (10 1b.) load with a horizontal location of
50 cm (20 in.). It is important to note that for either type of

37


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Attitude

The characteristics of a worker's personality which may increase
susceptibility to the hazards of manual materials handling are
not easily measured or interpreted. Studies by Blow and Jackson
(1971), Magora (1969, 1970) and White (1966) conclude that the
psychosomatic aspects of low-back pain cases need further research
study. Unfortunately, personality characteristics are often con-
founded with other demographic or anthropometric variables such

as age, training, or experience. Clear evidence of how worker
values and job satisfaction contribute to risk does not exist.

Training

The importance of training and work experience in reducing hazard
is generally accepted in the literature. The lacking ingredient

is largely a definition of what the training should be and how
this early experience can be given to the naive worker without
harm. Lacking strong epidemiological support at the present time,
this topic is deferred to Chapter 6 where the criteria for improved
training are addressed in detail.

Strength

Over the last few decades a large amount of strength data has

been gathered on various populations from Olympic athletes to in-
fant toddlers. There has also developed an awareness that strength
assessments could be useful in determining personal risk of injury
to a person assigned to materials handling activities. Kraus
(1967) believed that strength tests should be an essential part of
pre-employment examinations. Such a policy has also been advocated
by Hanman (1958), Koyl and Hanson (1969), Kelly (1975) and Chaffin,
et al., (1978).

Epidemiological support for strength testing as means of matching
worker capabilities and job demands is provided from several
studies.

Rowe (1971) found that abdominal weakness correlated with in-
creased incidence of low-back pain. From the biomechanical point
of view (Chapter 3) abdominal strength is a major factor in reduc-
ing the compressive forces acting on the lumbar spine during lift-
ing (Davis, 1959; Bartelink, 1957; Alston, et al., 1966; and
Morris, et al., 1961). Further, Troup and Chapman (1969) and Poul-
sen and Jorgenson (1971) believe the strength of the back extensors
are of primary importance in protecting the back during manual
materials handling.

A study by Chaffin and Park (1973) monitored 400 newly hired em-
ployees to determine whether medical incidence rates were related
to the relative match of job requirements to strength ability.
Volunteer new hires participated (anonymously) in a battery of

20


Chaffin, D.B. "Ergonomics Guide for the Assessment of Human Static
Strength." Am. Ind. Hyg. Assoc. J. 36:505-510,1975.

Chaffin, D.B. and Baker, W.H. "A Biomechanical Model for Anal-
ysis of Symmetric Sagittal Plane Lifting." AIIE Transactions.

II(1), 1970.

Chaffin, D.B. and Moulis, E.J. "An Empirical Investigation of
Low Back Strains and Vertebrae Geometry." J. Biomech. 2:89-
97, 1970.

Chaffin, D.B.; Herrin, G.D.; Keyserling, W.M. and Foulke, J.A.
Pre-employment Strength Testing, NIOSH Technical Report,
DHEW(NIOSH) Publication No. 77-163, 1977.

Chaffin, D.B.; Herrin, G.D.; Keyserling, W.M. and Garg, A. "A
Method for Evaluating the Biomechanical Stresses Resulting

from Manual Materials Handling Jobs." Am. Ind. Hyg. Assoc.
J. 38:662-675, 1977.

Chaffin, D.B.; Herrin, G.D. and Keyserling, W.M "Pre-employment

Strength Testing - An Updated Position." J. Occup. Med. 20:
403-408, 1978.

Chaffin, D.B. and Park, K.S. "A Longitudinal Study of Low Back
Pain as Associated with Occupational Lifting Factors." Am.

Ind. Hyg. Assoc. J. 34:513-525, 1973.

Christensen, E.H. "Physical Working Capacity of Old Workers and
Physiological Background for Work Tests and Work Evaluations."
In: Bulletin of the World Health Organization, Geneva, 1955.

Clark, A.B. and Russek, A.S. "Back Injuries." In: Rehabilitation
in Industry, D.A. Covalt, Ed. Grune and Stratton, New York,
58.
Coetsier, P. "A Psychological Study of Assemblyline Work ina
Motor Vehicle Assembly Plant." Etude de Travail. 164:11-29,
1965.

Colcher, A.E. and Hursch, A.M.W. "Pre-employment Low Back X-Ray

Survey: A Review of 1,500 Cases." Indust. Med. Surg. 31:
319-321, 1952.

Coleman, P.J.; Gottlief, M.S.M.; Kaplan, M.C.; Knutson, S.J.;
McPeek, J.S.; Smith, K.U. and Robinson, G.H. "Human Factors
Analysis of Materials Handling Equipment." Final Report,
DHEW(NIOSH) Contract 210-76-0115, 1978.

Corlett, E.N. and Bishop, R.P. "A Technique for Assessing Pos-
tural Discomfort." Ergonomics. 19:175-182, 1976.

159


LIFTING TASK VARIABLES

The primary task variables identified earlier in this Guide on the
basis of epidemiology (Chapter 2), biomechanics (Chapter 3), phys-
iology (Chapter 4) and psychophysics (Chapter 5) of lifting include:

al Object weight (L) - measured in kilograms (pounds)

Ao Horizontal location (H) - of the hands at origin of
lift measured forward of the body centerline or mid-
point between ankles (in centimeters or inches)

So Vertical location (V) - of the hands at origin of lift
measured from floor level in centimeters (inches)

4. Vertical travel distance (D) - from origin to destina-
tion of lift in ¢entimeters -(inches)

Bo Frequency of lifting (F) - average number of lifts per
minute

6. Duration or period - assumed to be occasional (less
than one hour) or continuous (8 hours)

The latter two variables (lift frequency and period) are the most
difficult to define and consequently measure and provide guidance.
For the purpose of this Guide jobs will be grossly classified in
three categories:

1. Infrequent - either occasional or continuous lifting
less than once per 3 minutes

Oo Occasional high frequency - lifting one or more times
per 3 minutes for a period up to 1 hour

So Continuous high frequency - lifting one or more times
per 3 minutes continuously for 8 hours.
5

Evidence presented in earlier chapters shows that for infre uent
lifting a person's musculoskeletal strength (Chapter 5) and

potential high stress to the back (Chapter 3) are the primary
limitations to ability. As such, biomechanical variables are
predominant in determining hazard. Occasional high frequenc:
lifting results in psychophysical stress (Chapter 5) and possible
muscle fatigue as the primary limitations. For continuous, high
frequency lifting the primary limitations are based on cardio-
vascular capacity and metabolic endurance (Chapter 4).

Table 8.1 summarizes which task variables are emphasized by the
four approaches examined in Chapters 2-5. A careful review of
these approachés shows two important points. First, all of the
task variables are highly interactive. In other words, the

122


nature, the facts presented support the need to reduce the ex-
posure to such acts as a matter of protecting worker health and
well-being. Evidence of the epidemiological, biomechanical,
physiological and psychophysical bases for such claims are pre-
sented in this guide to further define the exact conditions which
seem to be of most concern to both scientists studying the problem
and exposed workers.

EXPOSED POPULATION

A large and diverse group of industries appear to have significant
overexertion injury and illness claims, as reported in Table 1.3.
How many workers in these industries are actually exposed to
hazardous manual materials handling is difficult to estimate.
Assuming for the moment that lifting a 20 kilogram (44 pound)
object once a day could be hazardous (see Table 1.1) for the
occupational groups shown in Table 1.4, it is estimated that over
30 percent of the total workforce is exposed. If this mix of
occupations were representative of the U.S. workforce then over

30 million workers are exposed daily. It should be noted that,
with the exception of farm workers, all occupational groups listed
in Table 1.4 are expected to increase in employment between 1970
and 1980 from 5% to 40% (Wisconsin Bureau of Research and Statistics,
1974).

Table 1.4: Estimated work force performing manual
materials handling activities in Wisconsin
(Wisconsin Bureau of Research and Statistics,
1974)

Craft Workers: 137,000 workers or 60% of total

(especially carpenters, brick layers, plumb-
ers, structural iron & steel, tin smiths,
tool & die, mechanics, & telephone installers)

Operatives: 100,000 workers or 30% of total

(especially press operators, packagers,
sanders & buffers, welders, truck drivers,
bus drivers, railroad brakemen & switchmen,
delivery person)

Service Workers: 132,000 workers or 60% of total

(especially janitors, waiters & waitresses,
nurse assistants, porters, firefighters,
police, housekeepers, groundskeepers)

Laborers, Except Farm: 65,000 workers or 90% of total

(especially construction, freight handling,
garBage collection, lumbering, stock handling)

Farm Owners & Laborers: 131,000 workers or 100% of total
Total Work Force in Wisconsin (1970): 1,703,810 workers

Estimated Workers Performing Manual Materials Handling: 565,000
or 32% of total



3) When boxes are lifted down from 60 cm (24 inches to
the floor, the VOz2 is lower than when lifting upwards
over the same distance.

4) Lifting and turning to place the weight at 90° to the
point of origin results in the same regression
equations for lifting in the sagittal plane. (Aquilano,
1968; Hamilton, 1969; Miller, et al., 1977; Petrofsky
and Lind, 1978a; Snook (1978; and Lind, et al., 1979).

The results from laboratory experiments suggest that lifting exer-
cises in industry will seldom reach or exceed the average male
worker's 50% Voz max, which is taken to be the physiological limit
beyond which muscular fatigue (and its physiological sequelae) will
inevitably occur. Indeed, it seems that there will be few practical
situations where the average energy cost of lifting in industry is
likely to exceed 35% Woo max, a level of work which is considered
by most research workers to be compatible with daily levels of

work in "heavy" industries irrespective of age, sex, and other
factors which are known or suspected to limit the permissible

daily energy expenditure in industry.

One factor that remains to be satisfactorily defined and catego-
rized as a limiting factor in lifting is the static isometric
component and its associated physiological responses.

DETERMINATION OF WORK CAPACITY LIMITS
BASED UPON ENERGY EXPENDITURE

1. Populatidén Capacity Estimates

Adjusting for age, sex, and body weight variation in the
working population Chaffin, (1972) predicted the variation in
aerobic capacities illustrated in Figure 4.1. The lower 95
percent prediction intervals are also plotted.

Unfortunately, a large scale evaluation of the aerobic
capacities of the American working population has not been
undertaken so that values presented in this figure are only
rough estimates. Cross-sectional population distributions
summarized by Cumming (1967) conform to these data. A study
by Rodahl and Issekutz (1962) of American policemen, however,
indicates that persons having relatively sedentary jobs
would have lower aerobic capacities than expected from
Figure 4.1. As a result Chaffin estimated that

"probably 80 percent or more of American men are not

physically fit, as judged by their aerobic capacities
being below a reasonable value of 16 Kcal/min."

48


3. rollover,

4, collision between the truck and other object,

Se the operator getting pinned between the truck and
another object, and

6. the truck running up the heels of the walking operator.

Many of these hazards also apply to dollies and wheelbarrows. They
can be reduced or eliminated by using the following guidelines.

abo All exposed handles should be equipped with knuckle
guards so that the operator's hands cannot be jammed
against external obstructions.

Bo All wheels should be recessed or otherwise guarded.

So All trucks should have brakes that apply automatically
when the handle is either fully raised or fully lowered,
or released.

4, Major features should be standardized, e.g., brake
activation on powered trucks.

Fox (1971) found many instances of negative transfer between
different vehicles with different positions and directions of
controls. He also found critical incidents caused by errors in
blind reaching for controls while the driver's vision was directed
to the load rather than the controls.

Bo All operators must be trained to follow safe practices,
such as

a. proper loading (i.e., so that the vehicle is bal-
anced, that the load will not fall off, that
vision is not obscured, etc.)

b. leaving the truck facing the traffic

c. maintaining safe speed

Glo backing loads down inclines to avoid front end
tipping

e. lowering the fork to floor level when not in use

fo chocking wheels if parked on an incline.

Pertaining OSHA regulations are detailed under Part 1910, Subpart

N. particularly section 1910.178. Two anonymous articles, "powered
Hand Trucks," (July 1975 issue, National Safety News, 53-57), and
"Industrial Trucks - Their Operations and Maintenance," (September
1975 issue, Industrial Engineering, 32-37) discuss many of the
safety features and lists applicable safety standards and recommend-
ations.

Fork-lift trucks have the dubious distinction of violating many
human engineering principles and recommendations, and of having

RIS)


6. Handling Skill: The actual handling tasks chosen for
teaching purposes should cover a range of materials but
the emphasis should be on the actual materials handled
by the organization concerned. There are a number of
points of general application which should be taught.
Skill depends on:

a. preparation to avoid being caught unaware,

b. being able to recognize what can be handled
comfortably without help,

c. keeping the center of mass of the load close
to the body when lifting,

d. not twisting or bending sideways,

e. using the legs to get close to the load and to
make use of the body weight and the kinetic
energy of the body and load,

£. timing for smooth MMH.

7. Handling Aids: Handling aids are available for most
material handling tasks. Often, they can be improvised
if they do not exist. One problem here is encouraging
their use, especially with unpaced or incentive jobs.

It is not enough to teach the theoretical aspects of safety in MMH
and demonstrate the practical points by slides or films. Every
trainee must be practically involved from the start: in the
theoretical teaching of basic physics and functional anatomy; in
learning how to recognize the loads that can be handled without
undue sense of effort, and where in relation to the body MMH is
easiest; and in acquiring the skills of safer and easier MMH.
Classes cannot be large or the element of personal involvement is
missing.

It is not enough to teach only in a classroom because the lessons
must be applied on site. There is another danger in teaching only

in a classroom and that is that many of the older (or longer em-
ployed) workers and supervisors have not been taught safe MMH
techniques. If the instructors are never seen on site, the trainees
quickly forget and go back to their old habits.

Many of the instructors of the past (and much of the published
teaching material) have tended to rely on a dogmatic style of in-
struction; and sets of 'rules' for safe lifting are common. The
drawback is that literal application of some of these rules has

led to quite unsafe lifting practices. For example, to insist on
the rule "Lift with the knees" is not well founded. In many people
the quadriceps femoris muscle is just not strong enough. Moreover,
this could lead to lifting at arm's length in front of the knees,
thus creating more stress on the spine than the straight-legged
stoop posture. Any rules used as memory aids should at least teach
a basic aim or principle. What matters is that the trainee is led
to a proper understanding of the problem and not merely expected to
remember a set of catch-phrases.

101


CHAPTER 7

ENGINEERING CONTROLS

There are more hazards in manual materials handling than overex-
ertion of the musculoskeletal or cardiovascular systems. Discrete
mechanical hazards such as slips, falls and dropping of objects
handled with subsequent damage or injury are important - so too
are high and low temperature environments which can modify overall
bodily response to the exertion of MMH. This chapter reviews
these "other" hazards and suggests control methods and design
criteria.

Manual materials handling is not a job done in isolation; rather
it is part of the overall system by which industry handles and
transports materials. Materials handling as a whole represents a
large part of industry's costs with an average of 50 tons of mate-
rials moved for each ton of product produced. Hence materials
handling systems design has emerged as a discipline with its own
expertise and procedures. In order to aid the human in his/her
role as materials handler it is important to realize the materials
handling alternatives available, evaluate the alternatives
available and choose the one which is optimum with respect to

all criteria. The criteria include performance, cost and mainten-
ance as well as safety. Many of the alternatives to manual mate-
rials handling have their own hazards so that this chapter classi-
fies and reviews the methods available to avoid the introduction
of substitute hazards into the workplace.

THE MECHANICAL ENVIRONMENT

The chief factors to be considered in designing the mechanical en=
vironment are the design of the container itself and the ‘couplings'
which transmit the container forces through the body to the floor
or other working surface. The container itself affects safety
directly through its weight and size (biomechanical stresses) and
indirectly through the limitations it imposes on methods of hold-
ing and carrying it. The human/container coupling affects the
ability to exert forces on the object, to maintain grip and to
exercise control - all vital to safe handling. The human/work-
surface coupling is important for maintaining stability and pre-
venting the initiation of trips, slips and falls.

103


Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

1.1:

1.2:

3.1:

3.2:

3.3:

3.4:

3.5:

3.6:

SoVB

3.8:

3.98

3.10:

3.11:

3.12:

3.13:

3.14:

LIST OF ILLUSTRATIONS

Employment in United States by Gender,
MOSSES Oelern tO) opoa0b00000000G0G0000000

Female Labor Force as Percent of Women
by Age (Women's Bureau, 1975)........-eeeee0e

Illustration of Muscle Mechanics
(reves, IEE) o00c000G000000000000G00G000000

Illustration of Leverage on Shoulder, Elbow,
and Lumbosacral Joints (Chaffin, 1975).......

Illustration of Lifting Task Requiring High
Arm and Shoulder Strengths.........-e.eeee-0ee

Forces and Moments Operating at Ls5/S) Disc
During Load Lifting...........seee ce eeeeeenee

Predicted Compressive

Forces Acting on Ls/S)

Disc (Adapted from Chaffin, 1975).........---

Mean and Range of Disc Compression Failures
by Age (Adapted from Evans, 1959 and Sonoda,

oF) 0000000 0000c000000000000000G000000000000

Displaced Degenerated

Disc Exerting Pressure

on Spinal Nerves (Chaffin, 1975).............

Low Back Compression Associated with Two
Listing Postures (Park and Chaffin, 1974)....

Relation Between LBP and Compressive Force
(Chaffin and Park, 1973).........-cceceeeeeee

Task Variables Producing 350 kg Female Back
Compressionweelesllleleeilterriiitieiecnieeiieiee

Task Variables Producing 650 kg Male Back
Compression... .cecccccssvccccnccccccccvesseen

Changes in Compressive and Shear Forces on
the Low Back for a Leg Lift and Back Lift
During the Lifting Action.................00-

Combination of Weight
of Loads Resulting in
Compressive force for

Combination of Weight
of Loads Resulting in
Compressive force for

vii

and Horizontal Location
from 400 to 700 kg
Leg Lifts...........008

and Horizontal Location
from 400 to 700 kg
Back Lifts.............

24

25

26

28

28

29

30

33

36

37

37

38

39

39


Table 5.5: Mean and standard deviation of maximum weights
(kg) of lift acceptable to male and female in-
dustrial workers (corrected for one lift/min.).

q q Standard
Height of Lift Sex Mean Deviation
Floor to knuckle Male 28.0 7.66
Female 16.9 3.06
Floor to shoulder Male 23.3 5.50
Female 14.1 2.97
Floor to reach Male 22.3 5.09
Female 12.8 2.46
Knuckle to shoulder Male 26.1 6.67
Female 14.5 2.98
Knuckle to reach Male 24.3 4.86
Female 11.9 2.21
Shoulder to reach Male 19.8 4.75
Female 11.7 1.90

7 7 "O" Gaw Reach

Figure 5.5: Classification of Lifting Height

80


Deltoid
Ant. Head

Brachialis

Figure 3.2: Illustration of Leverage on Shoulder, Elvow, and
Lumbosacral Joints (Chaffin, 1975).

This means that the 10 kg acting on each hand requires about 292
kg of force in the elbow flexor muscles (not inclusing the extra
load imposed by the weight of the forearm and hand). One might
think that this muscle force is acceptable because muscles
develop most tension when stretched. This is an important mechan-
ical fact. The muscles may, indeed, be capable of producing such
high forces, but what about the bones, joint cartilage and joint
connective tissues? For instance, when the muscles pull across

an extended joint, they compress the joint with about the same
Magnitude of force. This coupling of the muscle and bone com-
pression forces is an important concept when considering low-
back biomechanics. In addition, high muscle forces inhibit blood
flow, placing extra stress on the heart and leading to early muscle
fatigue.

25


Human/Container Coupling Design. Handles on containers
should be designed with the worker's hand in mind. They
have a large effect on both the maximum force a worker
can exert on a container and on the energy expenditure
in manual materials handling tasks. The major problems
with most handles are insufficient hand clearance, sharp
edges which can cut into the hand and too small of a
handle diameter.

The postures of the hand with respect to grasped objects
can be classified as follows:

a. Hook grip. One in which the fingers are flexed
around the object and the thumb is not used for
gripping.

b. Power grip. One in which the object is clamped
between the partly flexed fingers and palm with the
thumb opposing the grip and lying along the plane
of the palm.

c. Precision grip. One in which the object is pinched
between the flexor aspects of the fingers and op-
posing thumb.

Most handles force the worker to use the hook grip (the
least effective) or the power grip. The power grip gives
a good gripping force and allows a large surface of the
hand to be used but often is inefficient if accurate
control is needed. The weight of the object often pre-
vents the precision grip from being used.

There is little agreement in the literature as to what
the optimum handle diameter is, but from 25 mm (1 inch)
to 38 mm (1.5 in.) is the range into which most recommen-
dations fall. The elimination of sharp edges, seams,
rubbing, and corners appears to be of more importance
than actual handle diameter. The shape of the handle is
better cylindrical than molded to the contours of the
hand.

Handle width should be at least1l.5 cm (4.5 in.) with 50
mm (2 inches) clearance all around the handle to accommo-
date a 95th percentile hand. If gloves are used, at
least 25 mm (1 inch) should be added to these dimensions.

Worker/Floor Surface Coupling. Poor worker/floor coupl-
ing will most often result in accidents such as slips,
trips, or missteps. The major mediating variable in
these types of accidents is the coefficient of friction
between the shoe sole and the working surface. This can
be affected by:

149


35 7

490

S 30P 7
oe 985 ~—
| ol
- to)
8 | leo?
= 980
25 7

o——o 8 hr Continuous Work 2510
a----1 75% Work-25% Rest Each Hour 775
| e----© 50% Work-50%Rest Each Hour 4
o—O 25 % Work-75 %Rest Each Hour

i= it 1 _1 J
a 2 300 , 400 500
keal/hr |
800 1200, 1600 2000

BTU/hr
aoe moderate, heavy
work

aan ENERGY EXPENDITURE RATES

Figure 7.2: Lines of Permissible Exposure to Work in Hot
Environments (Dukes-Dobos, 1976)

These can be combined with detailed treatments of the physics of
heat transfer between human beings and their working environment
(Berenson and Robertson, 1973) so that a detailed analysis of
human thermal load can be made (Givoni and Goldman, 1971, 1973).
These allow the effects of changes in environment, clothing and
workload to be predicted and engineering controls to be installed
in advance.

MATERIALS HANDLING SYSTEM ALTERNATIVES

The materials handling function is ubiquitous throughout industry
occurring in at least seven areas.

112


100
r 75% OF FEMALE
POPULATION CAPABLE
(25%ile)
80
&
o KEY
a a KEY
w 60 ae
wo seseeee 50
a 75
= 40 aA i80
©
Py] : 175 cm, .
= Vertical Location
of Load
a
fe) L L 1 i
15 30 45 60 75

HORIZONTAL LOCATION OF LOAD (cm)

Figure 5.7: Maximum Recommended Weights Based on Female Isometric
Strength

It is also interesting to note that the psychophysical criteria
discussed in this chapter lead to slightly different conclusions
than the physiological criteria in Chapter 4. Figure 5.8 shows
the trade-off between weight lifted and frequency of lift observed
by Snook (1978) using the psychophysical approach compared with
the continuous work capacities predictions of Lind, et al., (1977)
using the physiological approach. Based on lifting from the floor
to knuckle height, Snook observed that workers were willing to
work above 35% of their aerobic capacity (Vg, max) when lifting
more frequently than 6 times per minute, but well below that level
with infrequent lifts.

87


/ C,G, OF BODY WEIGHT (BW)
ABOVE LUMBOSACRAL JOINT

¢
Fuusc iN
N \
~Y AN \ \
V
Foome wey by

FH
VERTICAL HAND FORCE

Figure 3.4: Forces and moments operating at Ls/S, disc during
load lifting.

AT Ls /Sq DISC

PREDICTED COMPRESSIVE FORCES

{e} 20 40 60
LOAD ON HANDS (kg)

Figure 3.5: Predicted compressive forces acting on Ls/S, disc.

(Adapted from Chaffin, 1975).
28