cult to lift weight in the kneeling posture
than stooped. This is probably attributable
to the size of the muscle mass used during the
lifting procedure. It seems clear that in the
kneeling posture, the muscular mass available
to accomplish a lifting task is a good deal
smaller than that available in the stooped
posture, thus less weight is subjectively
chosen in this posture by the subjects during
the lifting capacity tests. Although the leg
muscles have a limited utility in the stooped
posture, apparently they still are able to
contribute somewhat to the lifting process in
this position. It should be noted that some
subjects found the stooped posture to be more
uncomfortable than the kneeling posture,
although none of the subjects terminated their
participation due to discomfort.

One unanticipated finding of this study
is that the physiological measures of heart
rate, oxygen consumption, and ventilation
volume are all higher in the kneeling posture
despite the fact that significantly less
weight is lifted in this position. Although
this difference does not achieve statistical
significance, it is probably due to the
difference in workload handled in the two
postures. In other words, it may be that in
order for the subject to achieve the same
physiological workload in each posture, more
weight must be handled by the subjects in the
stooped position.

There are two possible physiological
principles that may explain why the kneeling
posture may lead to elevated heart rate,
oxygen consumption, and ventilation volume
values. The primary reason deals with the
size of the muscle mass available to perform a
given workload. The smaller the muscle mass
used to accomplish a workload, the higher the
heart rate and respiratory adjustment will be,
and the sooner the work will have to be
interrupted due to exhaustion (Stegemann,
1981). The second reason that kneeling may
contribute to higher physiological values is
that blood flow to the lower extremities may
be partially inhibited, Diminishing the blood
flow to the legs has also been shown to
increase the heart rate (Stegemann, 1981).

Analysis of the relationship between
back strength and lifting capacity shows that
two back strength tests (i.e. kneeling static
45°, and kneeling dynamic 60°/sec) correlate
well with HAWL in both stooped and kneeling
positions. None of the standing back strength
measurements are found to correlate signifi-
cantly with lifting capacity in either
posture, These data suggest that certain
kneeling back strength measurements may be
better predictors of lifting capacity in
restricted work postures than back strength
measurements taken in the fully erect posture.
Further research is necessary to assess this
method of predicting lifting capacity.

REFERENCES

Adams, M.A., and Hutton, W.C. The effect of
posture on the strength of the lumbar

spine, English Medicine, 1981, 10, 199-
202.

Astrand, P.0., and Rodahl, kK. Textbook of
work physiology. McGraw-Hill Book Co.,
New York, 1977,

Ayoub, M.M., Bethea, N.J., Bobo, M., Burford,
C.L., Caddel, K., Intaranont, K.,
Morrissey, S. and Selan, J. Mining in
Low Coal: Volume I - Biomechanics and
Work Physiology (Final Report -U.S.
Bureau of Mines contract H0387022),
1981.

Basmajian, J. V., and DeLuca, C.J. Muscles
alive. Williams and Wilkins, Baltimore,
1985.

Floyd, W.F., and Silver, P.H.S. The function
of the erectores spinae muscles in
certain movements and postures. Journal
of Physiology, 1955, 129, 184-203.

Gallagher, S. Back injury research at the
U.S. Bureau of Mines ergonomics labora-
tory. Paper in Proceedings, Fourth
Annual Meeting of the Collegiate
Association for Mining Education (Rolla,
MO, October 3-4, 1985), 170-191.

Marras, W.S., King, A.I., and Joynt, R.L.
Measurements of loads on the lumbar
spine under isometric and isokinetic
conditions. Spine, 1984, 9, 176-198.

Moreno, F., and Lyons, H.A. Effect of body
posture on Jung volumes. Journal of

Applied Physiology, 1961, 16, 27-29.

Peay, J.M. (comp.) Back injuries, U.S. Bureau
of Mines Information Circular 8948,
1983, 110 pp.

Poulsen, E. Back muscle strength and weight
lifting limits in lifting burdens.
Spine, 1981, 6, 73-75.

Stegemann, J. Exercise physiology. Yearbook
Medical Publishers, Chicago, 1981.

Stobbe, T.J., and Plummer, R.W. A test-retest
criterion for isometric strength
testing, Proceedings of the Human
Factors Soctety Annual HNeeting, 1984,
455-459.

Westgaard, R.H., and Aaras, A. Postural
muscle strain as a causal factor in the
development of musculo-skeletal ill-

nesses. Applied Ergonomics, 1984, 15,
162-174.


Table 5. Pearson correlation coefficient and statistical significance
between stooped NAWL and kneeling static and dynamic back strength measure-
ments (N=4).
Stooped Kneeling Kneeling Kneeling Kneeling Kneeling Kneeling
MAWL Static Static Static Dynamic Dynamic Dynamic
225° 45° 67.5° 30°/sec 60°/sec =: 90° /sec
Correlation
Coefficient 86 97 -.15 83 +96 58
Statistical
Significance
(p=) 14 <.05 85 7 <,05 42
Table 6. Pearson correlation coefficient and statistical significance

Between kneeling MAWL and standing static and dynamic back strength

measurements (N=4),

Kneeling Standing Standing Standing Standing Standing Standing
MAWL Static Static Static Dynamic Dynamic Dynamic
225° 45° 67.5° 30°¢/sec  60°/sec  — 90°/sec
Correlation
Coefficient .52 75 74 81 80 42
Statistical
Significance
(pe) +48 225 25 19 20 58
Table 7. Pearson correlation coefficient and statistical significance

between kneeling MAWL and kneeling static and dynamic back strength

measurements (N=4).

Kneeling
‘MAW

Kneeling Kneeling Kneeling Kneeling

Kneeling Kneeling

Static Static Static Oynamic Dynamic Dynamic
22.5° 45° 67.5° 30°/sec  60°/sec 90° /sec
Correlation
Coefficient .79 98 -.38 89 299 78
Statistical
Significance
(p= 21 <,05 62 wll <.01 222
Four subjects performed both back this case, the back muscles may be better
strength and psychophysical lifting tests. In isolated. In the present study, it is likely
an effort to determine whether any of the back that there is still a considerable contribu-

strenyth measurements related to the psycho-
physically determined maximum acceptable
weight of lift, a post hoc correlation
analysis was performed. Results of this
analysis are given in Tables 4 through 7.

DISCUSSION

The results of the back strength tests
supported the hypothesis that a significant
amount of muscular force is generated by the
posterior ley muscles when back strength is
measured in the standing posture. Measurement
of back strength in the kneeling posture
apparently reduces the contribution of the leg
muscles to the back strength measurements. In

tion from muscles other than those directly
supporting the vertebral column (especially
the glutei and hamstrings) during the kneeling
back strength measurements. However, the
input from these muscles is somewhat decreased
and the contribution from the gastrocnemius ©
and soleus is probably diminished to a yreater
extent. The findings of the present investi-
gation suggest that further research may be
necessary ‘to determine the best posture for
examination of back musculature function.

The data from the psychophysical lifting
study indicate that subjects unaccustomed to
performing materials-handling tasks in
restricted work postures find it more diffi-


EFFECTS OF POSTURE ON RACK STRENGTH AND LIFTING CAPACITY

Sean Gallagher and Thomas G. Bobick

U.S. Department of the Interior - Rureau of Mines
Pittsburgh Research Center
Pittsburgh, Pennsylvania

ARSTRACT

The Rureau of Mines performed a pilot study examining the effects of posture on
back strength and Maximum Acceptable Weight of Lift (MAWL) on six healthy male subjects
(M = 32 years + 4 SD), Six back strength measurements (3 static and 3 dynamic) were
made while the subjects were kneeling and standing. In addition, these subjects (who
were unaccustomed to lifting in these postures) volunteered to participate in a study
of psychophysically determined MAWL in both postures, Results of the back strength
tests showed a significantly lower peak torque per body weight output in kneeling
versus standing back strength measurements for five out of six test comparisons (p <
.05). Subjective estimates of lifting capacity in the kneeling posture were signifi-
cantly lower than those for the stooped posture (p < .05). The results of tests of
back strength and lifting capacity in these two postures provide useful information to
consider in determining the physiological and psychophysical stresses imposed by these

work postures.
INTRODUCTION

Miners who work in low-seam coal mines
(roof height < 48 inches) often must handle
materials in severely constrained postures
(Peay, 1983). Supplies that must be manually
lifted in low coal are typically lifted while
kneeling or stooped (Gallagher, 1985).
Despite the abundance of manual materials~
handling literature, relatively little is
known about the physiological and psycho-
physical responses to materials handling in
these positions. Body posture has been shown
to affect muscular strength capabilities
(Ayoub, et_al., 1981). However, a review of
the literature provided no information on the
back strength capabilities of persons while
kneeling.

Back strength measurements have been
shown to be positively correlated with the
ability to lift (Poulsen, 1981). Typically,
back strength has been measured while the
subject is standing (Ayoub, et al., 1981;
Poulsen, 1981; Marras, et al., 1984). This
posture may involve the measurement of
muscular forces other than those of the low
back region, especially the strong muscles of
the posterior leg region (i.e. biceps femoris,
Semimembranosus, semitendinosus, gastroc-
nemius, and soleus). While these measurements
may be an indication of the total muscular
force that workers may utilize when lifting in
a standing posture, they may not accurately
reflect the muscular strength available when
they must perform lifting tasks while kneel-
ing. In addition, it is possible that back
Strength measurements taken in the kneeling
Position may correlate better with the ability
to lift during kneeling materials-handling
activities. One aim of the present study was
to examine the hypothesis that back strength
Measurements (both isometric and isokinetic)

taken in a kneeling posture may be less than
those obtained when standing.

The position or bearing of the body has
been shown to have an important effect on
biomechanical, physiological, and psycho-
physical parameters during the performance of
work tasks (Astrand and Rodahl, 1974; Adams
and Hutton, 1981; Westgaard and Aaras, 1984).
A stooped posture causes the weight of the
torso to be added to the stress imposed on the
Yow back during a lifting task. Forward
flexion of the vertebral column causes an
increase in electrical activity of the back
muscles until flexion is extreme (Floyd and
Silver, 1955). In the extremely flexed
posture, electrical discharge from the back
muscles ceases and the load is assumed by the
ligamentary structure of the back (Basmajian
and DeLuca, 1985). This may lead to increased
incidence of muscle strain or sprain.
Deviation from the erect posture also causes a
decrease in total lung volume and oxygen
consumption and results in higher ventilation
rates (Moreno and Lyons, 1961). Finally,
changes in posture have been demonstrated to
increase loadings on the small muscle groups
of the upper limbs and torso, as well as
causing circulatory changes such as higher
blood pressures and heart rates, and re-
distribution (pooling) of the blood supply
(Ayoub, et_al., 1981). All of these changes
increase the physiological responses of
workers that have to assume these postures,
thus increasing fatigue and the corresponding
risk of injury they may experience. The
second purpose of the present investigation
was to examine the effects of posture on
psychophysically determined lifting capacity.
The results of the present study and future
Bureau studies will be used to develop lifting
guidelines for low coal mines.


METHOD AND PROCEQURE

Six healthy males (M = 32 * 4 SD)
participated in a study examining the effects
of posture on back extensor strength and
lifting capacity. Subjects were volunteers
from the U.S. Bureau of Mines Research Center
in Pittsburgh, PA, and had no prior experience
in handling materials in restricted work
postures. Each subject was required to
undergo a thorough physical examination and
graded exercise tolerance test prior to their
participation in the experiment, to ensure
that no health problems were present that
would put the subjects at an increased risk of
injury. Informed consent was obtained from
all participants in the study. Prior to the
start of testing, each subject warmed-up by
exercising for five minutes on a bicycle
ergometer and then performed a series of five
back and trunk stretching exercises prior to
testing.

Back strength was measured in standing
and kneeling postures using a CYBEX Isokinetic
Dynamometer (LUMEX. Inc.)+. A total of 12
conditions were studied in this experiment:
six kneeling and six standing. In each
posture, three back strength measurements were
taken using an isometric contraction (22.5°,
45.0°, and 67.5° from the vertical) and three
measurements were made using a dynamic
contraction (30°/sec, 60°/sec, and 90°/sec).
Figure 1 shows the device used to measure back
strength during tests in the standing and
kneeling postures. The subject was secured by
two pelvic stabilization straps in each
posture. All back strength test conditions
were conducted in a randomized order.

The Naximum Voluntary Contraction (MVC)
for each test condition was obtained using a
test-retest procedure whereby peak torque
measurements (kilogram-meters) of two maximal
exertions were required to be within 10% of
one another. The higher of these two values
was taken as the HVC for that test condition
(Stobbe and Plummer, 1984). Two minutes rest
was given between exertions, and consistent
verbal encouragement was given to the subject
in order to facilitate maximal exertions from
the participants. One subject did not
complete the back strength testing portion of
the experiment due to equipment problems and
was excluded from the analysis of these data.

In the study of lifting capacity, each
subject was asked to adjust the weight in a
50.8- by 33.0- by 17.8-cm (20- by 13- by 7-in)
lifting box eccording to his estimate of
lifting capacity for each posture (stooped or
kneeling). The lifting tasks were performed

lReference to specific brands, equipment, or
trade names in this report is made to facili-
tate understanding and does not imply endorse-
ment by the Bureau of Mines.

Figure 1. Subject performing back strength
measurement in a) standing, and b) kneeling
posture.

under an adjustable-height mine simulator that
restricted the subject's posture. The height
of the simulator was set at 121.9 cm (48 in)
for this study. The test set-up is shown in
figure 2. Lifting instructions were given to
the subject before the experiment started. In
this study, the subjects were told to adjust
the weight in the box so the load could be
handled for a 20-minute period (the actual
lifting period) and to assume that this 20
minutes of lifting would have to be performed
four times during a workday.

Figure 2.
test.

Subject performing stooped MAWL

The subject lifted the box at a fre-
quency of 10 lifts/min for two 20-minute
periods in each posture. One period started
with a heavy box, weighing approximately 38.6
kg (85 1b) and the other with a light box,
weighing approximately 6.8 kg (15 Ib) in order
to control for bias due to initial starting
weight of the box. A ten-minute rest break
was provided between tests so that subjects
could rest and/or attend to personal needs.
The average subjectively determined weight
chosen for the two test conditions in a


posture was taken as the maximum acceptable
weight of lift (MAWL) for that posture.

The primary dependent measures for the
lifting study were the MAWL for the kneeling
(KMAWL) and stooped (SNAWL) postures.
Secondary dependent measures included heart
rate (HR), oxygen utilization (V0), and
ventilation volume (Ve). Heart rate was
obtained during the last ten seconds of every
minute using a Beckman Dynograph Recorder,
Model S511-A., The average heart rate for each
condition was taken as the average of the
final 15 values obtained. 02 and Ve values
were obtained approximately every 30 seconds
during the final five minutes of lifting using
a Beckman Metabolic Measurement Cart. The
data were averaged by the number of values
acquired during this five-minute period. One
subject was not able to finish the lifting
portion of the experiment due to other work
commitments, and was excluded from the data
analysis.

RESULTS

Tables 1 and 2 show the results of the
static and dynamic back strength tests,
respectively. The results showed a signifi-
cantly lower peak torque/body weight output in
kneeling versus standing back strength
measurements for five out of six test compari-
sons (p < .05). The only posture where
statistical significance was not achieved was
the comparison between standing and kneeling
static back strength in the fully flexed
(67.5° from the vertical) position (p = .08).

Table 3 gives the results for the
psychophysical lifting study. Subjective
estimates of lifting capacity were signifi-
cantly lower for the kneeling posture than for
the stooped posture (p < .05). However,
despite the fact that less weight was lifted
in the kneeling posture, the secondary depen-
dent measures of HR, VOg, and Ve were all
higher than those in the stooped posture.
These differences, however, were not statisti-
cally significant (p > .05).

Table 4.

ments (N=4).

Table 1. Static back strength (N=5) expressed
Yn torque produced (kg-m)/body weight (kg).

Standing Kneeling Significance

22.5° 0.34 0.23 p< 01
(+ 0.08) (+ 0.05)

45.09 0.40 0.21 p< 01
(+ 0.07) (# 0.04)

67.5° 0.26 0.17

ns.
+09) (# 0.03) (p = 08)

ns. - not significant

Table 2. Dynamic back strength (N=5) ex-
pressed in torque produced (kg-m)/body weight
(kg).

Standing Kneeling Significance

30°/sec 0.28 0.22 p< .05
(+008) (+ 0.07)

60°/sec 0.26 0.20 p< .01
(4.0.04) (# 0.03)

90°/sec 0.19 0.13 p< .05
(£0.06) (+ 0.02)

Table 3. Results of maximum acceptable weight
of Tift test (N=5).

Stooped kneeling Significance

WAWL (kg) 27.2 22.5 p< .05
(# 9.3) (# 6.9)

HR (bpm) 133 139 ns.
(+ 18) (= 27)

Vop 15.9 Wl nes

(ml/kg/min) (# 2.2)  ( 4.0)

Ve (V/min) 38.7 40.1 nes.
(£6.1) 9 (#9.1)

ns. - not significant

Pearson correlation coefficient and statistical significance
between stooped MAWL and standing static

and dynamic back strength measure-

Stooped Standing Standing Standing Standing Standing Standing

HAWL Static | Static Dynamic Dynamic Dynamic
22.5° 45° 30°/sec 60°/sec 90°/sec
Correlation
Coefficient 60 88 79 91 35
Statistical
Significance  .40 12 .2l 09 265

(e )