Static Biomechanical Modeling in Manual Lifting - Fundamentals and Assessment Tools for Occupational Ergonomics - page 936

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FIGURE 48.6 Schematic of 10-muscle model. (developed by Schultz and Andersson 1981. Analysis of loads on the
lumbar spine. Spine 6(1): 76-82. With permission.)

strength is the most limiting muscle group strength (only 86% of men have sufficient hip strength to lift
the reel). This value is below that recommended by NIOSH, which proposes that jobs should accommo-
date 99% of men's and 75% of women's strength (or 90% of a mixed gender population). The estimated
3D low-back compression force of 969 lbs also is above the 760 lbs recommended by NIOSH. Two
balance analyses are performed: possible static equilibrium of moments at the ankles and an estimated
center of pressure within the basis of support on the floor. The balance status indicates that the task
as depicted provides acceptable balance.
Analysis output windows for these and other calculated results are available, and two are shown in
Figure 48.8. These depict percent capable strength predictions (for 34 muscle functions) and lumbar
compression forces at the L4L5 disc; the latter includes 3D predictions of a large number of individual
muscle and spinal forces.

48.4 Validation of Strength and Back Force Prediction Models

The validation of the static strength predictions has been accomplished in three different studies. All three
validations required using the models to simulate whole-body exertions and compare the percent capable
predictions with the mean, 10, and 90 percentile strengths of a group of volunteers who performed the
same tasks.
In the first validation Garg and Chaffin (1975) had 71 male Air Force personnel perform 38 different
maximum arm exertions (i.e., lifts, pushes, pulls, etc.) in a variety of arm

torso postures while seated.
They found the predicted strengths were highly correlated with the group strengths when performing
the 38 upper-body tasks (r

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0.93-0.97). Chaffin et al. (1987) simulated 15 different whole-body exertions
in the sagittal plane, which were also performed by both men and women from a variety of industries.
In some of these tests over 1000 people performed the exertions, though on average about 200 people

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