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relatively light 18 kg case, without handles,
resulted in over 70% of the lifts exceeding the
3400 N spine compression tolerance limit.
This effort has also indicated that this goal of
elevating the load can sometimes be partially
accomplished by including handles in the case to
be lifted. Handles interact strongly with load
location to mediate spine loading and risk. In
order to appreciate how spine compression
loading changed as a function of these features as
well as the region factor, Table 50.2 has been
included to indicate the percentage of observations
that exceed the two compression force “bench-
marks.” This table indicates how the inclusion of handles and cases of different weights interact with
pallet region to define loading, and therefore, risk of LBD. Similar analyses for the shear forces indicated
that, for the most part, spine compression was more problematic than the shear forces for the weight
factor alone. Table 50.1 indicates that the practical benefit of handles is realized for the heavier cases
located at the lower levels of the pallet. Specifically, regardless of case weight, the inclusion of handles
can reduce the percentage of observations that exceed the 6400 N limit by approximately 50%. The
effect of handles also interacts with the region factor. Compression forces often exceed the tolerance
limit for all case weights at the lowest layer of the pallet. The decrease in compression force by the use
of handles most likely resulted from the decrease in the external moments. This would be expected
since the handles were located at the top of the cases and subjects were not required to bend the torso
to the same extent. This would reduce the external moment generated by the torso mass. Further evalu-
ation of the model results confirmed that significant differences in maximum L
5
FIGURE 50.1 Load locations when lifting from a
pallet. Refers to region locations identified in Table 50.2.
S
1
compression force
were associated with differences in three of the four maximum moment measurements (i.e., sagittal,
twisting, and resultant moment). The external moment associated with cases containing handles was sig-
nificantly less than the moment for cases without handles. The most pronounced reduction of maximum
moment was for the sagittal moment, which reduced to 151.8 N m from 176.4 N m (a 13.9% decrease)
by the inclusion of handles on the cases. Examination of the results also shows that the effect of including
handles is to raise the lift point of the case. This affects the length-strength relationship of the trunk
muscles moving the load position to a location where the length-strength relationship of the muscle
results the most power. Thus, including handles is analogous to slightly changing the region of the case.
A large portion of the A
/
P shear distribution also exceeded the 1000 N tolerance at regions E and F
when no handles were present on the case. Specifically, A
/
P shear was found to be problematic for
the 27.3 kg (60 lb) cases in regions E and F. Handles were also found to reduce this average peak
shear to well below the 1000 N tolerance. Observations of the subjects indicated that this reduction in
shear associated with handles was due to the fact that subjects could grab the handle and slide the
load toward them as opposed to reaching to the far end of the box to lift as was observed without handles.
These results based upon spine loading were also compared to an analysis of the same features eval-
uated with a LBD risk model (Marras et al., 1993). This analysis yielded very similar results. Thus, the
spinal loading profiles were for the most part similar to trunk motion and workplace factor measures
that match trunk motions and workplace factors historically associated with jobs placing workers at
an increased risk of LBD.
These studies form the basis for the limits of lift in the various lifting zones defined in Table 50.1. Col-
lectively, these biomechanical studies clearly indicated the need for lower weights in lifts at the greatest
distance from the body and at the lower lifting heights. The selected weights in cells E and F, and G are
based in part on the implications of the biomechanical studies.
The epidemiological literature provides additional scientific justification that lifting below knuckle
height, or at a greater horizontal distance from the trunk are more hazardous than lifting done
between knuckle height and mid-chest height close to the body. Kerr et al. (2001) had reported that
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