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From left to right, muscles

represented in histograms are 1. and r. latissimus dorsi, 1. and r. erectores spinae, 1. and r.

Normalized peak EMG for each experimental condition.
external obliques, and 1. and r. rectus abdominis.

Figure 1,


DISCUSSION

Compared to a previously reported study on
symmetric lifting in restricted postures
(Gallagher, et al., 1990), many of the internal
responses to asymmetric lifting were affected by
interactions of the independent variables. For
example, both compression and anterior shear
were significantly affected by the PxW
interaction. This phenomenon was not observed
in the symmetric lifting study. Several
similarities between the two studies can be
noted, however. For example, in both studies
compression was consistently higher in the
kneeling posture when compared to the stooped
posture. This is primarily the result of
increased erectores spinae activity when lifting
in the kneeling position.

In the study examining symmetric lifting
(Gallagher, et al., 1990), peak lateral shear
was found to be significantly greater in the
stooped posture. In contrast, the present study
showed a trend (non-significant) towards greater
peak lateral shear in the kneeling posture.

This reflects a difference in recruitment of the
latissimus dorsi muscles in these two postures
according to the symmetry of the lifting task.
As with the previous study, anterior shear was
generally greater in the stooped posture,
indicating greater activity of the abdominal
obliques in this position.

Another similarity of the two studies was
the complex recruitment pattern of the trunk
musculature for the treatments studied.

However, certain observations can be made with
regard to trunk muscle activity. For example,
the activity of the latissimus dorsi were higher
in the stooped posture than when kneeling.
However, increased erectores spinae activity was
consistently demonstrated in the kneeling
posture. The rectus abdominis were slightly
more active in the stooped posture.

While the current model appears to give a
good picture of the muscular loading on the
spine, it is worth noting that this model is as
yet unable to address other biomechanical
factors that may be responsible for production
of low back pain. For instance, in the stooped
posture investigated here, a large portion of
the restorative moment to maintain the position
of the trunk is provided through the muscle-
Sparing action of the posterior ligaments
(Gracovetsky and Farfan, 1986). The strain on
these ligaments is apt to be quite considerable,
but unfortunately remains difficult to quantify,
and is not addressed by the current model.

The data reported in this study will
assist in the development of recommendations for
manual lifting tasks in the underground mining
environment. The model estimates of compression
will be compared to known compression tolerance
limits of the spine, and recommended load limits
will be designed so that these tolerance limits
are not exceeded.

REFERENCES

Davis, P.R., and Troup, J.D.G., 1966. Effects
on the trunk of erecting pit props at
different working heights. Ergonomics, 9,6,
475-484,

Gallagher, S., Hamrick, C.H., and Love, A.C.,
1990. Biomechanical Modeling of Symmetric
Lifting Tasks in Constrained Lifting
Postures, Paper in Advances in Industrial
Ergonomics and Safety II (B. Das, ed.),
London, Taylor and Francis, pp. 559-566.

Gallagher, S., Marras, W.S., and Bobick, 1.G.,
1988. Lifting in stooped and kneeling
postures: effects on lifting capacity,
metabolic costs, and electromyography of
eight trunk muscles. International Journal
of Industrial Ergonomics, 3,1, 65-76.

Gallagher, S., and Unger, R.L., in press.
Lifting in four restricted lifting
conditions: Psychophysical, physiological,
and biomechanical effects of lifting in
stooped and kneeling postures. Applied
Ergonomics.

Gracovetsky, S., and Farfan, H.F., 1986. The
optimum spine. Spine, 11, 6, pp. 543-573.

Marras, W.S., 1987. Preparation, recording and
analysis of the EMG signal. Paper in Trends
in Ergonomics/Human Factors IV (8.8. Asfour,
ed.), Amsterdam, North-Holland, pp. 701-708.

Marras, W.S., and Sommerich, C.M., 1990. A
‘Three-Dimensional Motion Model of Loads on
the Lumbar Spine, Part 1: Model Structure,
unpublished manuscript.

Reid, J.G., and Costigan, P.A., 1987. Trunk
Muscle Balance and Muscular Force, Spine, v.
12, pp. 783-786.

Ridd, J.E., 1985. Spatial restraints and intra~
abdominal pressure. Ergonomics, 28, 1, 149-
166.

Schultz, A.B., and Andersson, G.B.J., 1981.
Analysis of Loads on the Lumbar Spine, Spine,
v. 6, Pp. 76-82.

Schultz, A. B., Andersson, G. B. J., Haderspeck,
K,, Ortengren, R., Nordin, M., and Bjérk, R.,
1982. Analysis and Measurement of Lumbar
Trunk Loads in Tasks Involving Bends and
Twists. Journal of Biomechanics, 15, 669-675.

Sims, M.T., and Graveling, R.A., 1988. Manual
handling of supplies in free and restricted
headroom. Applied Ergonomics, 19,4, 289-292.


BIOMECHANICAL MODELING OF ASYMMETRIC LIFTING
TASKS IN CONSTRAINED LIFTING POSTURES

Sean Gallagher, Christopher A. Hamrick, and Arnold C. Love
U.S. Bureau of Mines
P.O, Box 18070
Pittsburgh, PA 15236

Twelve subjects participated in an investigation of the biomechanical stresses
of asymmetric lifting in stooped and kneeling postures. Three factors were
manipulated in this study: Posture (stooped or kneeling), height of lift (35 or 70

cm), and weight of lift (15, 20, or 25 kg).

Subjects were required to lift or lower

a box every 10 seconds for a period of 2 minutes. Electromyography (EMG) of eight
trunk muscles was collected during a lift in this period. The EMG data, normalized
to maximum extension and flexion exertions in each posture, were input to a
biomechanical model and used to predict compression and shear forces at the L, level
of the lumbar spine. Results from the EMG-driven biomechanical model indicated that
compression was greater when lifting to a higher shelf (p< 0.001), and indicated a
significant interaction between posture and the weight of the lifting box (p < 0.01).
Peak lateral shear was not significantly affected by any main effects or interactions
(p< 0.05). Anterior shear was increased with increasing height of lift (p < 0.001),
and also by the posture x weight interaction (p< 0.01}. A multivariate analysis of
variance (MANOVA) indicated a complex relationship for recruitment of the eight trunk
muscles, with the triple interaction being significant (p< 0.001). The results of
this investigation will be used to evaluate safe loads for lifting in these

restricted postures.

INTRODUCTION

The height of an underground coal mine is
generally determined by the thickness of the
coal seam. In several cases, the coal seam of a
mine may be less than 48" high. Such mines are
often called “low-seam” coal mines. Workers in
these mines often have to lift heavy materials
in restricted postures (usually stooped or
ymeeling). There is reason to believe these
postures result in significant compressive and
shear loading on the lumbar spine.

Previous Bureau of Mines research has
described the psychophysical lifting capacity,
metabolic demands, and electromyography (EMG) of
trunk muscles when performing tasks in
restricted postures (Gallagher, et al., 1988;
Gallagher and Unger, in press). Other
investigators have researched the intraabdominal
pressure associated with lifting in these
postures (Davis and Troup, 1966; Ridd, 1981;
Sims and Graveling, 1988). However, little
research has been performed estimating the
internal forces on the lumbar spine due to trunk
muscle contraction in these postures. Marras
and Sommerich (1990) recently described a
biomechanical model driven by trunk muscle EMG
that allows estimates of forces acting on the
lumbar spine. A paper describing the results of
this model’s estimates of forces during
symmetric lifting in restricted postures can be
found elsewhere (Gallagher, et al., 1990). The
purpose of the current investigation was to
estimate the internal forces acting on the

lumbar spine during asymmetric lifting tasks in
stooped and kneeling postures.

METHOD

Subjects

Twelve healthy male subjects (M = 35.7
years of age + 6.8 S.D.) volunteered to
participate in a study examining the
biomechanics of asymmetric lifting in restricted
postures. Nine of the subjects were experienced
underground miners, while three of the subjects
were volunteers from the U.S. Bureau of Mines.
All participants operated under terms of
informed consent.

Experimental procedure

Three independent variables were
manipulated in this experiment -- posture (P)
for the lift (stooped or kneeling), height (H)
to which the box was lifted (35 or 70 cm), and
weight (W) of the lifting box (15, 20, or 25
kg). A within subjects repeated measures design
was employed. EMGs of eight trunk muscles (1.
and r. erectores spinae, latissimus dorsi,
external oblique, and rectus abdominis) were
collected during lifting tasks, and were later
digitized and input to a dynamic biomechanical
model (Marras and Sommerich, 1990). The model
output included estimates of compression,
anterior-posterior shear, and right lateral
shear (at the L, level of the lumbar spine), as
well as torques about the X, Y, and Z axes,


using the coordinate system described by Schultz
and Andersson (1981). In addition, the model
produced estimates of muscle forces for the
eight trunk muscles. The assumption was made
that the maximum stress that could be exerted by
a muscle was 50 N/em (Reid and Costigan, 1987).
Estimates of the cross-sectional areas of trunk
muscles were obtained through anthropometric
measurements, as described by Schultz, et al.
(1982). All dependent variables except muscle
forces were analyzed using 2 x 2 x 3 (P x H x W)
analyses of variance (ANOVA). The data on
muscle forces were subjected to a 2 x 2x 3 (P x
H x W) multivariate analysis of variance
(MANOVA). Critical alpha levels were .05 for
all statistical tests.

The lifting tasks were performed under a
1.2 m roof that restricted the subject's

Table 1.-- Summary of significant main effects and

posture. An aluminum lifting box (50.8 x 33.0 x
17.8 cm) with two covered compartments was used
to perform the lifting tasks. The subject
performed a series of twelve asymmetric lifting
and lowering tasks in a counterbalanced order.
The subject was required to lift or lower the
box to the appropriate height every 10 seconds
for a period of 2 minutes. EMG data were
collected during the third lift of this
sequence.

RESULTS

Table 1 summarizes significant main
effects and interactions for the dependent
variables (outputs from the biomechanical model)
in this investigation. Table 2 contains average
model estimates of compression, shear, and
torque for each of the experimental conditions.

interactions for all dependent variables.

Pos

PxH | PxW HxW PxHxwW

Peak Compression *

od

Peak Lat. Shear +067

-080

Peak Ant. Shear +100

+075

Peak X Torque

Peak Y Torque

Peak Z Torque

Peak Mus. Force

Ave. Compression

Ave. Lat. Shear

Ave. Ant. Shear

Ave. X Torque

Ave. Y Torque

Ave. Z Torque

Ave. Mus. Force

* p< 0.050
** p< 0.010
“** p< 0.001


Table 2.-- Model estimates of compression, shear, and torque for all experimental conditions. Values
represent the mean for all 12 subjects. Numbers in parentheses represent the standard deviation.

Compression | A-P Shear | R-L Shear | X Torque | Y Torque | Z Torque
Kneeling 1599.6 187.3 21.9 56.5 22.0 5.9
35 om, 15 kg (383.4) (86.0) (16.0) (18.5) (11.2) (5.2)
Kneeling 1788.8 207.5 20.0 62.9 24.7 6.2
35 cm, 20 kg (345.7) (88.0) (16.7) (20.0) (21.2) (4.3)
Kneeling 2114.2 278.2 22.3 75.6 25.4 9.4
35 om, 25 kg (374.2) (144.8) (16.4) (21,1) (12.9) (4.8)
Kneeling 1942.1 244.5 24.0 69.4 22.3 8.0
70 cm, 15 kg (378.6) (124.2) (17.2) (18.0) (10.1) (5.5)
Kneeling 2188.2 262.5 24.4 78.2 24.9 8.9
70 cm, 20 kg (322.8) (98.3) (16.0) (17.0) (11.1) (4.9)
Kneeling 2560.2 360.1 23.6 89.4 22.1 1d
70 cm, 25 kg (343.5) (150.9) (18.5) (17.7) (9.8) (6.3)
Stooped 1375.0 159.8 24.8 44.8 13.6 6.6
35 cm, 15 kg (450.9) (75.8) (47.1) (23.0) (9.3) (4.3)
Stooped 1521.6 185.9 27.1 49.2 13.5 9.4
35 cm, 20 kg (502.0) (75.0) (14.4) (25.2) (8.8) (5.3)
Stooped 1719.3 194.5 23.4 58.2 15.9 9.4
35_cm, 25 kg (433.3) (66.4) (11.5) (21.0) (9.7) (4.8)
Stooped 1593.5 208.5 27.7 49.7 15.5 12.8
70 cm, 15 kg (507.6) (63.7) (16.6) (22.9) (10.4) (9.0)
Stooped 1810.1 244.3 25.7 57.3 17.5 13.0
70 cm, 20 kg (549.6) (99.6) (13.9) (27.3) (12.7) (7.7)
Stooped 1974.3 279.6 31.4 64.3 20.2 17.1
70 cm, 25 kg (600.5) (97.0) (21.2) (31.6) (10.8) (9.0)

Compression

As can be seen in this Table 1, there was
a significant PxW interaction on compression.
Table 2 shows that compression increased at a
higher rate for kneeling compared to stooping;
however, kneeling compression was always higher
than that observed when stooped. The height of
lift had the expected effect on compression
(i.e., lifting to a higher shelf increased
compression).

Shear Forces

Lateral shear was not significantly
affected by any independent variables. However,
peak anterior shear was significantly affected
by lifting height, as well as by the PxW
interaction. Examination of Table 2 shows that
anterior shear was increased with increasing
height of lift, while anterior shear increased

at a higher rate in the kneeling posture,
compared to stooped.

Muscle Forces

The MANOVA on muscle forces for the eight
trunk muscles studied indicated a complicated
recruitment pattern due to the changing
conditions. The P x H x W interaction achieved
significance at the 0.001 level. Figure 1
illustrates the average over the 12 subjects of
peak EMG activity during lifts for the eight
trunk muscles studied for all experimental
conditions. These EMG data were normalized to
maximum exertions for each of the subjects,
using procedures described by Marras (1987).