Biomechanical Basis for Ergonomics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 264

INOR EXOR

b

d

RCAR

LATR

IAP

RCAL

ERSR

C

S

a

γ

ERSL
LATL

INOL

EXOL

FIGURE 11.31 Cross-sectional view of the human trunk at the lumbrosacral junction. (Adapted from Schultz, A.B.
and Andersson, G.B.J., Spine, 6, pp. 76-82, 1981. With permission.)

during work. Because of the variability in muscle recruitment patterns it has been virtually impossible
to predict the instantaneous coactivation and resultant loading on the spine during dynamic trunk
exertions. One of the few means to accurately account for the effect of the trunk muscle system
coactivation upon spine loading is through the use of biologically assisted models. The most
common of these models are electromyographic or EMG-assisted models. These models take into
account the individual recruitment patterns of the muscles during a specific lift for a specific individ-
ual. By directly monitoring muscle activity the EMG-assisted model can determine individual muscle
force and the subsequent spine loading. These models have been developed and tested under bending
and twisting dynamic motion conditions and have been validated (McGill and Norman, 1985, 1986;
Marras and Reilly, 1988; Reilly and Marras, 1989; Marras and Sommerich, 1991a, b; Granata and
Marras, 1993, 1995b; Marras and Granata, 1995, 1997a, b; Marras et al., 2001). Figure 11.32 shows
how such models can assess the effects of lifting dynamics upon spine loading. These models are
the only ones that can predict the multi-dimensional loads on the lumbar spine under many 3D
complex dynamic lifting conditions. The limitation of such models is that they require significant
instrumentation of the worker.

11.4.5.2 Stability-Based Models
Efforts have also been attempted to use stability as criteria to govern detailed biologically assisted biome-
chanical models of the torso (Panjabi, 1992a, b; Cholewicki and McGill, 1996; Solomonow et al., 1999;
Cholewicki et al., 2000; Granata and Marras, 2000; Granata and Orishimo, 2001; Granata and Wilson,
2001; Cholewicki and VanVliet, 2002). One potential injury pathway for LBDs suggests that the unna-
tural rotation of a single spine segment may create loads on passive tissues or other muscle tissues
that result in spine injury (McGill, 2002a). Most of the work performed in this area to date has been