A muscle's action on a joint not only depends upon the muscle's size and activation level, but also upon the mechanical advantage (or moment arm) of the muscle upon the joint. This relationship is made more complex by the fact that the mechanical advantage can change drastically with skeletal configuration. Here, we describe a model of muscle geometry for several muscles involved in elbow and shoulder actuation. The model captures the gross changes in muscle moment arm while preserving reasonable computational efficiency which facilitates its use in simulation.
Fagg, A. H. (2000), "A Model of Muscle Geometry for a Two Degree-Of-Freedom Planar Arm," Technical Report #00-03, Department of Computer Science, University of Massachusetts, Amherst