Muscle and tendon force-length properties and their interactions in vivo.

Many attempts have been made to model muscle and/or muscle-tendon (MT) behavior for the purpose of predicting muscle forces in vivo. One important parameter often considered in such models is muscle length. This study was conducted to evaluate (1) the force-length properties of a MT complex and the range of these properties over which the muscle operates in vivo, and (2) the effect that tendon compliance has on a muscle's force-length behavior. The rat tibialis anterior (TA) MT complex was used as the experimental model. Muscle and tendon lengths as they occurred in the body during ankle joint motion ranging from 20 degrees to 90 degrees of flexion were determined for both passive and active muscle. Force-length (FL) properties for the tendon, passive muscle, and active muscle were determined from a partially isolated MT preparation. Results suggest that during movement involving a normal range of joint motion, the TA muscle operates within an optimal region of its FL relationship, generating minimal passive force and nearly constant active force. However, the passive force increases rapidly for extreme foot extension while the active force decreases for both extreme foot flexion and extension. For the rat TA muscle, the effect of tendon compliance does not alter the active force generated by the muscle over a normal joint range of motion. However, tendon compliance does effect the muscle's ability to generate force at the extremes of joint motion.