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epidemiological studies the highest risks are seen in highly repetitive lifting tasks (Marras et al., 1993) or
when the spine is flexed [(Punnett, Fine, Keyserling, Herrin & Chaffin, 1991; Vingard et al., 2000).]
Lifting tasks not only generate compressive forces on the spine, but also generate three-dimensional
loading of the spine. As a result damage can occur not only from compressive forces but also from
shear loading. Recently tolerances have been estimated for shear loading of the spine. These are expected
to occur between 750 and 1000 N (McGill, 1997). Quantitative workplace measures by Marras et al.
(1993, 1995) and Norman et al. (1998) have evaluated the biomechanical factors associated with jobs
that put the worker at a high risk of LBD. Both of these studies have evaluated the three-dimensional
factors that are associated with risk. The results of these studies agree well with the load-tolerance
model as well as the increase risk with substantial spinal flexion.
50.2.3 Studies of Lift Location
Several studies have employed EMG-assisted models to assess spine load in compression, lateral shear,
and anterior-posterior shear relative to spine tolerance limits (Marras, Granata, Davis, Allread & Jorgen-
sen, 1997; Marras & Davis, 1998; W. S. Marras et al., 1999). In these studies, workers were asked to lift
from various horizontal and vertical locations on a pallet. Spine compression and shear as well as back
injury risk (Marras et al., 1993) were evaluated as a function of load weight magnitude and location of the
load on the pallet. This analysis indicated that the location of the load in space had a far more dramatic
effect on spinal loading than did the weight of the object lifted. Table 50.2 summarizes the spine com-
pression relative to the spine tolerances for the load lifted from various locations (Figure 50.1) as well
as for loads of various weights and with different handle conditions. This analysis has shown quantitat-
ively that the greatest influence on spine loading is the location fromwhich the case is lifted from a pallet.
Spine loading measures have shown that the lower regions of the pallet (regions E and F) contribute the
most to spine loading. These regions also represent the conditions where most of the spine compression
distribution and a large portion of the A
P shear forces exceed the spine tolerance limits (S. McGill, 2002;
S. M. McGill, 1997; NIOSH, 1981). The compressive forces exceeded the tolerance limits of the spine
more often (a greater portion of the distribution) than did the A
/
P shear forces. In fact, even the
/
TABLE 50.2
Summary of the Percentage of Data within the Benchmark Zones for Spine Compression
Case Weight
40 lbs (18.2 kg)
50 lbs (22.7 kg)
60 lbs (27.3 kg)
Region
on the Pallet
Benchmarks
Handles
No Handles
Handles
No Handles
Handles
No Handles
A
3400 N
100.0
100.0
100.0
99.2
99.2
100.0
,
3400-6400 N
0.0
0.0
0.0
0.8
0.8
0.0
6400 N
0.0
0.0
0.0
0.0
0.0
0.0
.
B
3400 N
98.2
89.1
84.5
76.4
83.6
67.3
,
3400-6400 N
1.8
10.9
15.5
23.6
16.4
32.7
6400 N
0.0
0.0
0.0
0.0
0.0
0.0
.
C
3400 N
98.7
91.3
94.7
82.7
92.6
76.0
,
3400-6400 N
1.3
8.7
5.3
17.3
7.4
23.3
.
6400 N
0.0
0.0
0.0
0.0
0.0
0.7
D
,
3400 N
88.7
82.0
80.7
75.3
76.7
64.7
3400-6400 N
11.3
18.0
19.3
24.7
23.3
34.6
.
6400 N
0.0
0.0
0.0
0.0
0.0
0.7
E
,
3400 N
45.3
30.0
29.3
14.0
16.0
3.3
3400-6400 N
52.0
62.0
62.7
65.3
72.0
66.0
.
6400 N
2.7
8.0
8.0
20.7
12.0
30.7
F
,
3400 N
35.3
24.0
30.0
10.7
9.3
2.0
3400-6400 N
60.7
67.3
56.7
65.3
71.3
62.0
.
6400 N
4.0
8.7
13.3
24.0
19.3
36.0
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