Regulation of muscle stiffness in human locomotion.

The tension development of the triceps surae muscle was analyzed during the stance phase of locomotion at different speeds with respect to its electrical activation and its lengthening and shortening behavior. The main point was the coincidence of maximal dorsiflexion, maximal Achilles tendon tension, transition from amortization to acceleration phase, and projection of the center of gravity to the supporting leg. It is concluded that the stiffness regulation needed for load compensation after impact and pushing off at the end of stance phase depends on modulation and enhancement of a centrally programmed muscle activation by a segmental stretch reflex activity.