[Biomechanical aspects of muscle-tendon functions]

Moment arms of muscle forces with respect to joints are useful parameters in muscle-function analysis. In general, moments are defined with respect to a (functional) axis, but in the case of a complex motion, as in the case of foot and hand motion, they can be defined with respect to the instantaneous (clinical) rotation axis of this motion. Since in this case the exact rotation axis is not known, the moment arm cannot be determined by measuring the distance between the rotation axis and action line of the muscle. The purpose of this work is to show how the moment arm can be determined with respect to an unknown clinical rotation axis. As is the case for the triceps surae, the moment arm about the rotational axis is not constant thorough the whole range of motion. Muscle function also markedly depends on the insertion pattern of its tendon. The tibialis posterior, for instance, has five major insertions to the medial foot, and its main function is therefore more a stabilizing function than a rotator function of the foot.