Civil Engineering Reference
In-Depth Information
LOW BACK
MUSCLE FORCE
L
5
/S
1
DISC
MOMENT
DISC
COMPRESSION
FORCE
BW
L
H
FIGURE 48.4 Simple low-back model of lifting for static coplanar lifting analyses. The load on the hands L
H
and
torso and arm weights BWact to create moments at the L5
S1 disc of the spine. The moments are resisted primarily
by the back muscles. The high muscle forces required in such a task cause high disc compression forces.
/
Andersson (1981). It is depicted in Figure 48.6. Bean et al. (1988) developed a revised version of this
model that provides a more efficient computational method for solving the linear programs used to sim-
ultaneously minimize the torso muscle contraction intensities and motion segment compression forces.
The present 3D low-back model included in the computerized version described in the following text
predicts the minimum muscle force contractile intensities required to meet the moment equilibrium
requirements about the three orthogonal axes of rotation of the motion segment. Given a set of
optimal forces so computed, the model further seeks to minimize the disc compression force. Because
such an approach attempts to minimize both muscle intensity requirements and disc compression
forces simultaneously, it is referred to as a “double linear optimization” approach.
Hughes and Chaffin (1995) proposed that a nonlinear objective function be used as the basis for select-
ing the various muscle reaction forces during a given exertion. They referred to this as the sum of the
cubed muscle intensity objective. Nussbaum et al. (1996) also have proposed a neural network model
to predict torso muscle actions. Raschke and Chaffin (1996) have proposed that the external moment
is normally distributed about the torso, and activates several muscles simultaneously depending on
the direction and magnitude of the external moment.
48.3 Computerization of Strength Prediction and Back Force
Prediction Models
It should be clear from the preceding descriptions that the biomechanical models used for population
strength and spinal motion segment force prediction are computationally intense, especially in the 3D
form. For this reason a number of faculty, staff, and students associated with the Center for Ergonomics
at the University of Michigan have worked to provide user-friendly, computer programs of the models.
Search WWH ::
Custom Search