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output from many muscles to directly assess which muscles are active in response to an external load.
Current methods use the real time recording of electromyographic (EMG) activity from trunk
muscles and three dimensional geometric model of the trunk to predict the three-dimensional
loading of the spine under dynamic lifting conditions over time (Granata & Marras, 1993, 1995;
Marras & Granata, 1995, 1997a, b; Marras & Sommerich, 1991a, b; McGill & Norman, 1985, 1986;
van Dieen, Hoozemans, van der Beek & Mullender, 2002; van Dieen, JJ, Groen, Toussaint & Meijer,
2001). Applications of these models have demonstrated that spine loading varies as a function of rep-
etition (Granata, Marras & Davis, 1999), forward bending (Granata & Marras, 1995; Marras & Sommer-
ich, 1991b), and trunk moment (Granata & Marras, 1993, 1995; Marras & Sommerich, 1991b). Loading
can occur in compression, shear, or torsion. To date, these models are the most accurate models available
for the assessment of realistic work conditions. When the results of these studies are combined with
the epidemiological studies 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, a strong rationale is present for assessing LBD risk as a function of load location during lifting.
50.2.2 Load Tolerance
While the tolerance limits for spine damage is not completely understood, for the vertebral end plate, the
range of in vitro tolerances is known from laboratory studies of the relationship of compression forces. All
direct tolerance data has been derived from cadaveric tissue damage to the disc or the vertebral end plates.
Several possible mechanisms of injury are thought to exist. One of the more plausible mechanisms for LBDs
involves microfracture of the vertebral end plates. As healing occurs, scar tissue develops at the endplate
that interferes with nutrient delivery to the disc. This loss of nutrient results in atrophy of the disc
fibers, which may initiate chronic damage to the disc and may result in disc degeneration or herniation.
There is some scientific evidence that the in vivo and in vitro tolerance levels do not differ greatly
(Waters, Putz-Anderson, Garg & Fine, 1993; Yoganandan, 1986). Increasing levels of disc compression
initiate a number of other harmful disc responses at the cellular level, thus, providing further evidence
of a cumulative damage to the spine (Lotz & Chin, 2000; Lotz, Colliou, Chin, Duncan & Liebenberg, 1998).
Jager and coworkers (Jager & Luttmann, 1999; Jager, Luttmann & Laurig, 1991) have shown that
lumbar vertebra tolerances vary as a function of gender and age. Their data suggests that approximately
30% of lumbar segments have a tolerance of 3.4 kN or less (Waters et al., 1993). This value of 3.4 kN was
selected as a tolerance criterion in developing the NIOSH lifting equation. While there is uncertainty
about whether this is a reliable predictor of risk for low back disorders, there is epidemiological data
to suggest that it is a reasonable one (Chaffin & Park, 1973; Herrin, Jaraiedi & Anderson, 1986).
Chaffin and Park found that the incidence for jobs with less than 2.5 kN of spine compression was
less than 5%, while jobs with more than 4.5 kN of compression had an incidence of more than 10%.
Andersson, Svensson, and Oden (1983) reported that when males performed lifting tasks resulting in
spine compression forces greater than 3.4 kN, they had a 40% higher incidence rate of low back pain
than did males employed in jobs with lower predicted forces. Herrin et al. (1986) found that jobs
with compression forces between 4.5 and 6.8 kN had an incidence rate of 1.5 times greater than jobs
with compression forces that were less than 4.5 kN.
Tolerance to spine loading is reduced in highly repetitive tasks or when there is substantial flexion of
the spine. Vertebral strength is reduced by 30% with 10 loading cycles and by 50% with 5000 loading
cycles (Brinkmann, Biggermann & Hilweg, 1988). Solomonow, Zhou, Baratta, Lu, and Harris (1999)
demonstrated that cyclical loading induces creep into the viscoelastic tissues of the spinal tissues
which desensitizes the mechanoreceptors, possibly increasing exposure of the tissues to instability and
risk of injury even before the muscles fatigue. In addition, the posture of the spine at which point the
load is applied appears to be of great significance to the tolerance of the spine as well as to the ability
of the spine to receive nutrients. A flexed spine may be as much as 40% weaker than during an
upright posture (Gunning, Callaghan & McGill, 2001). Such observations may explain why in some
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