Stiffness regulation provided by short-latency reflexes in human triceps surae muscles.

The triceps surae muscles of normal human subjects were rapidly stretched and released by rotating the foot about the ankle joint with a torque motor. Following the initial intrinsic resistance, the yielding observed in incremental force records was more rapid for stretch than for release responses. Short-latency EMG responses elicited by stretch recruit force, to compensate for the yielding and to maintain the total (intrinsic plus reflex) resistance constant as the prior force level changes.