TABLE 3. -
during lifting tasks.

significantly different at the 0.05 level.

Duncan range test significance for maximum EMG activity

Conditions with the same letter are not
Numbers in parentheses

represent the mean percentage of maximum EMG activity for the cell (N

= 7).
at 44 and 48 inches.

The subjects were kneeling at 36 and 40 inches, and were stooped

TLD RL

LES RES. LEO REO TRA RRA

36
inches

K A
(33.64)

A K
(34.86) | (36.63)
pi) Ly A

inches

K A A A
(72.83) | (77.21) | (56.75) | (46.91) | (45.05)
A ny K x K

(26.77) | (32.85)
tT 8B

a

‘K
(67.01) | (72.37) | (56.26) | (37.18) | (51.99) | (46.09)
B B B K LY Ly

inches | (62.51) | (60.69)
Be B B

inches | (47.98) | (58.22)

(40.69) | (39.39) (41,33) (41,41) (33.37)
B B A
(30.10) | (42.40) | (42.65) | (37.90) | (33.01) | (35.19)

(48.13)
K

NOTE: TLD = Left Latiss

mus Dorsi; RLD = Right Latissimus Dorsi; LES =

Left Latissimus Dorsi; RES = Right Latissimus Dorsi; LEO = Left External
Oblique; REO = Right External Oblique; LRA = Left Rectus Abdominis; RRA =

Right Rectus Abdominis.

the interspinous, and the supraspinous) are
primarily responsible for supporting the
vertebral column when the trunk is bent
forward. It is likely that the subjects in the
present study were dependent upon their
ligaments for support during the lifting tasks,
and were therefore placing those ligaments
under considerable stress. Unfortunately, it
is difficult to determine such stresses.

The results of the electromyography
indicate that, in the stooped posture, the
latissimus muscle is working at a higher
percentage of its* maximum capacity than the
erectores during the lift. The Bureau of Mines
is currently examining the biomechanical
stresses of lifting in these two postures using
a model by Schultz (Schultz and Andersson,
1981; Schultz et_al., 1982), in order to
determine the compression and shear forces
experienced by the spine when lifting in
restricted working conditions.

The data presented in this paper (along
with data collected in previous studies) will
be used by the Bureau of Mines to make
recommendations for lifting materials in the

low-seam coal mining environment. The results
of this study may also be valuable information
for other industries where workers must handle
materials in restricted work spaces.

REFERENCES

Gallagher, S. Back Strength and Lifting
Capacity of Underground Miners. Paper
in Proceedings: Bureau of Mines
Technology Transfer Symposia,
Pittsburgh, PA, July 7, 1987, St. Louis,
MO, July 15, 1987, and San Francisco,
CA, July 21, 1987, pp. 21-32.

Schultz, A. B., and Andersson, G6. B. J.
Analysis of Loads on the Lumbar Spine,
Spine, v. 6, n. 1, 1981, pp. 76-82.

Schultz, A. B., Andersson, G. B. d.,
Haderspeck, K., Ortengren, R., Nordin,
M., and Bjork, R. Analysis and Measure-
ment of Lumbar Trunk Loads in Tasks
Involving Bends and Twists, Journal of
Biomechanics, v. 15, n. 9, 1982, pp.
669-675.


TABLE 2. - Mean acceptable weights of lift and
metabolic costs of lifting in the four restricted

postures (N= 13)

Kneeling Stooped
36" " 48"
MAWL (Ibs) 54.2 53.5, 63.5 66.0
HR (bpm) 41 133 126 126
Ve (L/min) 34.0 31.8 29.6 28.8
VO. 15.8 16.3 13.7 13.8
(mt /kg/min)
R 0.92 0.90 0,89 0.87
another test. This data was later transferred DISCUSSION

to a mainframe computer for plotting and
statistical analyses.

RESULTS

Analysis of the data indicates that
restricting the posture of underground miners
has a significant effect on lifting capacity (F
3,36 = 9.000, p < .001). Table 2 lists the
psychophysical lifting capacity for each of the
experimental conditions along with data on the
metabolic cost of performing at each roof
height. The greatest amount of weight lifted
by these miners was at the 48" roof height
(mean = 66.0 pounds), and the least at the 4u"
roof height (mean = 53.5 pounds). Heart rate
(E 3,36 = 9-543, p < .001), oxygen consumption
(F 333 = 8.05, p < .001), and ventilation
volume (F 3,33 = 4.880, p < .01) were all
significantly affected by roof height with the
higher physiological values occurring in the
kneeling postures. The respiratory exchange
ratio (F 3,33 = 3.825, p < .05) was also
significantly affected by posture.

The results of the MANOVA for maximum EMG
indicated that seam height has a significant
effect on muscle activity (F = 5,115, <
+001). Similarly, the MANOVA for mean EMG
during the lifting tasks demonstrated a
significant effect due to seam height (F =
6.395, p < .001). Neither initial box weight
(light or heavy) nor time of the data
collection (minute 2 or minute 18 of the test)
demonstrated a significant main effect on
muscle activity in the miltivariate analysis (p_
> .05), Table 3 presents a summary of
significant results using the Duncan Multiple
Range Test describing the effects on percentage
of maximum EMG activity for the eight trunk
muscles due to lifting in the four seam
heights.

This study confirms the findings of an
earlier investigation by the Bureau of Mines
which demonstrated that underground miners
have a reduced lifting capacity in the
kneeling posture, while the metabolic costs of
lifting in the kneeling position are greater.
These findings indicate that muscular fatigue
due to materials handling may occur more
quickly in this posture, and that lifting
heavier weights in low-seam coal mines might
be better accomplished in the stooped posture
(due to the higher lifting capacity in this
position).

The analysis of the electromyographic data
indicates that the function of the back
muscles studied are quite different in the
kneeling posture than when stooped. The
erector spinae muscles are much more active
when lifting in the kneeling posture. It
seems reasonable to assume that these muscles
must bear much more responsibility for the
lift in this posture due to the fact that
several muscle groups typically called upon
for lifting are not available for use in this
position. As a result, there may be an
increased compressive load on the lumbar
region of the spine when lifting in this
position, The greater metabolic demand of
lifting in this posture is likely due to an
increased demand for oxygen by the back
muscles.

In the stooped posture, the latissimus
dorsi muscles were significantly more active
during the lifting tasks than when kneeling;
however, the erectores spinae were more
quiescent when lifting in this posture. Many
studies have shown that the erectores spinae
demonstrate much less activity when the trunk
is flexed, and it is assumed that the
posterior group of ligaments (i.e., the
posterior longitudinal, the ligamentum flavum,


EFFECTS OF LIFTING IN FOUR RESTRICTED WORK POSTURES

Sean Gallagher, Richard L. Unger, and E. William Rossi
U. S. Department of the Interior - Bureau of Mines
Pittsburgh Research Center
Pittsburgh, Pennsylvania

ABSTRACT

The purpose of this study was to examine the lifting capacity of low-seam coal
miners in four restricted work postures (roof heights of 36", 40", 44", and 48"),
investigate the associated metabolic costs, and to examine electromyographic (EMG) data
from eight trunk muscles during the lifting procedure. Subjects were thirteen
underground miners accustomed to handling materials in restricted work postures. Each
subject performed two twenty-minute periods of asymmetric lifting in each of four
postures during the day of testing. The: frequency of lifting was 10 lifts per minute.
A specially designed lifting box incorporated microswitches in one handle of the box
and another in the bottom of the box, in order to examine the trunk muscle function at

specific points during the lfting cycle.

The data collected will be used by the Bureau

of Mines to make recommendations for lifting materials in low-seam coal mines,

INTRODUCTION

Statistics obtained from the Mine Safety
and Health Administration's Health and Safety
Analysis Center (HSAC) files for the years 1981
through 1986 have shown back injuries as being
a significant and continuing problem in the
underground mining industry. During this six-
year period there have been a total of over
12,500 back injuries in underground coal mines.
HSAC statistics also indicate that the rate of
back injuries per 200,000 man-hours worked is
higher the lower the seam of the coal mine (see
Table 1). One reason for the increased
incidence in low seams is that underground
miners must perform lifting tasks in restricted
working positions.

Previous Bureau research has indicated
that underground miners have a reduced lifting
capacity when lifting in an unrestricted
kneeling position (i.e., a roof height of 48
inches), as compared to lifting in the stooped
Position at the same roof height (Gallagher,
1987). This research also demonstrate that
myoelectric activity of the trunk muscles is
significantly affected by the posture assumed
when lifting (i.e., stooped or kneeling).
Furthermore, the metabolic cost of lifting was
found to be significantly elevated in the
kneeling posture compared to stooped. The
Purpose of the present investigation was to
examine the effects of lifting using
restricted kneeling postures (i.e., roof
heights of 36" and 40"), and stooped postures

TABLE 1.- Incidence of Back Injuries in Underground

Coal Mines, 1981-1986,

Incidence rates are expressed

in terms of the number of back injuries per man-year

(200,000 hours).

Low-seam coal mines are defined as

those with a roof height of < 48 inches, while medium
and high seam mines have a roof height > 48 inches.

ATT underground Tow-seam Medium/hi gh-seam
coal mines coal _mines coal mines

1981 3.96 5.76 3.62

1982 3.74 5.08 3.51

1983 3.28 4.54 3.00

1984 3.25 4.44 2.99

1985 1.30 2.01 1.18

1986 3.13 5.67 2.71



(at roof heights of 44" and 48") on
psychophysical lifting capacity,
electromyography of eight trunk muscles, and
the metabolic cost of performing the lifting
activity.

METHOD

Subjects were 13 underground miners
accustomed to handling materials in restricted
work postures. Informed consent was obtained
from each participant, and the subject was then
prepared for the lifting tests. The eight
trunk muscles investigated were identified and
the skin above the muscle was prepared by
shaving and cleaning the skin thoroughly with
alcohol. Bipolar surface electrodes filled
with an electrolyte gel were attached to the
skin above the muscle (3 cm apart center to
center), and a ground electrode was attached.
Surface electrocardiographic electrodes were
also placed for determination of heart rate
during the lifting tests. Each subject
performed two twenty-minute periods of lifting
in each of four postures during the day of
testing. In each posture, one period of
lifting started with heavy box (approximately
95 pounds), and one started with a light box
(approximately 25 pounds). The frequency of
lifting was 10 lifts per minute.

A specially designed lifting box was used
for this study. This box incorporated
microswitches in one handle of the box and
another in the bottom of the box (see figure
1). These switches allowed researchers to
determine the following events during the
lifting cycle: a) when the subject grasped the
box, b) when the subject lifted the box, c)
when the subject placed the box back down, and
d) when the subject released his grip on the
box. In addition, the collection of integrated

50.8 CU
3 CHAMBER) OPEN CHAMBER]
. WITH CHAMBER WITH
= Lio LID /|
°
\
MICROSWITCH UICROSWITCH
IN BOTTOM IN HANOLE
PLATE

FIGURE 1. Schematic of the lifting box used in
this experiment.

EMG data (at minutes 2 and 18 during the test)
was triggered when the subject grasped the
handle of the box. However, EMG data from all
eight trunk muscles was obtained starting two
seconds prior to the time the handle trigger
was activated (by means of the memory buffer
in the ISAAC! 5000 high speed data acquisition
system) to assure that all EMG data associated
with the lift was collected. Metabolic data
was obtained during the last five minutes of
every test using a Beckman Metabolic
Measurement Cart 1. Heart rates were taken
every minute during the lifting tests. EMG
data for each muscle was expressed as a
percentage of the overall maximum observed for
that muscle during all of the lifting tests.
The MAWL data, average heart rate, oxygen
consumption, ventilation volume, and
respiratory exchange ratio data were analyzed
using an analysis of variance (ANOVA) on
repeated measures statistical package. Both
mean and maximum EMG data were analyzed using
a multivariate analysis of variance (MANOVA)
on all muscles followed by separate ANOVAs and
post hoc Duncan Range Tests performed on each
separate muscle.

Lifting Box and EMG Instrumentation

EMG data during the MAWL experiments were
collected using a system of surface electrodes
coupled with and EMG amplifier and integrator,
an analog/digital (A/D) converter, and micro-
computer (figure 2). Collection of EMG data
was triggered by the lifting box, which
contained microswitches built into one handle
and the bottom of the box to allow the marking
of specific events during the course of a
lift.

The lifting box was made using a welded
aluminum construction and weighed
approximately 19.5 ib. when empty. The box
was divided into several compartments where
weights could be added or removed according to
the purposes of the test. Two compartments
had hinged covers in which the experimenters
randomly varied weight prior to the start of
the test. A microswitch was built into one of
the box handles, which was activated when the
subject grasped the box at the start of the
lift. This switch was armed by a second
switch at the microcomputer to prevent false
triggering due to vibration or incidental
contact. When the switch was closed a +10
volt direct current (VDC) signal was sent from
the lifting box to the A/D converter,
triggering the collection process.

A second microswitch was attached to the
bottom of the lifting box and was activated as

T Reference to specific brands, equipment or
trade names in this report is made to
facilitate understanding and does not imply
endorsement by the Bureau of Mines.


U.

S. BUREAU OF MINES

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FIGURE 2. U. S. Bureau of Mines Electromyographic Data Acquisition System.

long as the box was resting on its base. When
the box was lifted, the switch opened and the
signal dropped from +10 VOC to zero.

A micro-computer was used to control the
entire data collection process. Custom
software was developed to activate the A/D
converter and store the data collected. When
the researcher wanted to collect EMG data the
microcomputer sent the proper commands to the
A/D converter, which then would collect data
output from the EMG amplifier/integrator and
the lifting box. The data are allowed to pass
through the converter's buffer until a +10 VOC
Signal was recieved from the box handle
microswitch, indicating that the handle has
been grasped in preparation for a lift. At
this point, the A/D converter stored a preset
number of data points prior to the +10 VOC
signal and began to store data points at the
rate and duration specified by the researcher

through the software. In the present study,
integrated EMG data was collected at a rate of
100 Hertz (Hz) for a seven second period of
time; two seconds of data were stored prior to
the lifting box handle signal being received
by the A/D converter, and five seconds of data
were collected after the signal was received.

The box microswitches enabled the
investigators to analyze EMG data at the
following points: when the box handle was
grasped, when the box was lifted, when the box
was set down, and when the handle was
released. The EMG data described in this
paper were analyzed from the time that the
subject grasped the handle of the box until
the handle of the box was released. After the
data had been stored in the buffer of the A/D
converter, the microcomputer stored the
information on disk, cleared the buffer of the
A/D converter, and reset the system for