Strength and Fatigue: Experiments and Modeling - Human Motion Simulation: Predictive Dynamics - page 126

CHAPTER
6

Strength and Fatigue:
Experiments and Modeling

The majority of this chapter was contributed by Laura Frey-Law, PhD, MS, PT

The difference between a successful person and others is not a lack of
strength, not a lack of knowledge, but rather a lack of will.
Vince Lombardi

6.1 Joint space
In order to determine whether a digital human model can accomplish a task, it is
essential to define strength limits. The simple concept of measuring strength is
indeed simple to understand, but difficult to attain. The intuitive approach is to
consider muscle strength and forces as an assessment of strength for various
regions of the anatomy. However, one quickly realizes that this is a difficult task
as the concept of muscle recruitment and activation is difficult to ascertain.
This makes a seemingly simple modeling process somewhat more complex than
one might initially expect. Clearly an individual who is “strong” can be readily dif-
ferentiated from one who is “weak”. However, there are several physiological
aspects of muscle strength that may be relevant to digital humans, as well as the
decay in strength (i.e., muscle fatigue) that occurs naturally with physical activity.
Reasonable representations of normative human strength and endurance are
needed for digital human models (DHM) to behave in expected and meaningful
ways. Without accurate models of strength (and fatigue) DHM cannot model typi-
cal or realistic human behavior reliably.
Models of muscle strength typically fall into two basic approaches: modeling
individual muscle forces or modeling net torque produced about a joint, due to
multiple synergistic muscles. Both modeling approaches have their advantages
and disadvantages, particularly for whole-body digital human modeling.
However, we will focus our attention on strength modeling at the joint level as
that is the level at which we can assess strength in humans, and thus for valida-
tion. This is also called the joint space, as all calculations, predictions, and experi-
mental measurements are done at the joint level.
Muscle model approaches require assumptions of load sharing and activation
levels that are also pertinent to the issues discussed in this chapter. However, non-
invasive direct muscle force measurements are not possible in humans; only net