Civil Engineering Reference
In-Depth Information
moment capability obtained from the previously measured populations. This provides a prediction of the
percent of the population that is capable of producing the necessary strength moments at each joint.
The logic for computing the lumbar motion segment compression force is shown in Figure 48.3. Once
the lumbar moment is computed (as described in the preceding), torso muscle contraction, which stabil-
izes the column, is estimated. Two models of torso muscle contraction will be described. In the sagittal
plane low-back model by, Chaffin (1975), a single equivalent torso muscle contraction force is
implemented. When the necessary reactive torso muscle force is added to body segment weights and
hand forces (with a minor adjustment for abdominal pressure effects) a prediction of the compression
force on the L5
S1 disc results, as shown in Figure 48.4.
When an asymmetric exertion (e.g., one-handed force, or twisted or laterally bent torso) is being ana-
lyzed, many different torso muscle actions and passive supporting tissue reactions need to be considered
(Chaffin 1988b). The first step in such a procedure requires that the position, orientation, cross-sectional
size, and length of the various connective tissues be modeled at the lumbar spinal level. A geometric
torso model proposed by Nussbaum and Chaffin (1996) for this purpose is shown in Figure 48.5.
This model includes estimates of specific tissue geometry acquired from various computed tomography
(CT) scans (Tracy et al., 1989; Chaffin et al., 1990; Moga, et al., 1993), along with passive tissue reaction
forces estimated by McCully and Faulkner (1983), Nachemson et al. (1979), Miller et al. (1986), and
others.
The most important predictors of spinal column stress, however, are the muscle reaction forces
required to stabilize the spine to external load moments. In three-dimensional (3D) torso models
various approaches have been used to predict these required reactive muscle forces. Perhaps the most
commonly cited torso biomechanical model for 3D static analysis is that developed by Schultz and
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COMPUTE MOMENT AT L5/S1 DISC
DUE TO UPPER BODY WEIGHT AND
LOAD ON HANDS
PREDICT ABDOMINAL PRESSURE
FROM L5/S1 MOMENT AND POSTURE
COMPUTE MUSCLE FORCE
NECESSARY TO COUNTERACT
MOMENT AT L5/S1 DISC
SHEAR FORCES RESULTING FROM
1) UPPER BODY WEIGHT
2) ABDOMINAL PRESSURE
3) LOAD ON HANDS, AND
4) REACTIVE TORSO MUSCLE EXERTIONS
FIGURE 48.3 Logic for computing L5
S1 compression forces in 2D
3D Static Strength Prediction Program.
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