The exploration of muscle mechanics during natural human locomotion is a continuous challenge. Ultrasonography is a technique, which can be used to study the fascicle and tendon length changes during movements. However, many of the ealier studies were unable to precisely identify how the fascicles behave in different muscles. This may have caused misleading generalizations. In addition, because the muscle spindles are located inside the contractile tissue, it is important to determine the role that stretch reflexes may play during rapid and intensive dynamic movements. Brightness mode ultrasonography has revealed that, in human locomotion, the nature of fascicle and tendon length changes varies depending on the task, contraction intensity, and the muscles involved. The findings emphasize that the active fascicles of the gestrocnemius muscle are suddenly stretched, although they are shortening. This short-lasting stretch triggers the stretch reflex, timing of which is dependent on running speed.
