EFFECT OF STRETCH REFLEX BLOCKAGE ON THE KINETICS AND KINEMATICS OF THREE JUMPERS PERFORMANCES

Abstract

It has been postulated that the stretch reflex enhances force production in walking (Capady & Stein, 1986), running (Deitz, 1981), hopping (Melvill-Jones & Watt, 1971) and jumping (Bosco, et a1., 1982). In addition, it is widely accepted that the elastic component of muscles plays an important role in dynamic activities such as jumping. For example, during eccentric contraction ofthe jump, energy may be stored in the series elastic components which can be re-used during the subsequent concentric contraction. In fact, evidence from isolated muscle, as well as from intact muscle in humans, has been presented to show that the elastic energy storage contributed to enhanced work, force, velocity, and power of the positive phase of muscle contraction (e.g., Cavagna, 1965, 1971 a,b). All of the enhanced performances during the positive phase was attributed to fast stretching of a muscle prior to the shortening. It is well known that in stretching a muscle the muscle spindle will be stretched during which a volley of discharge is carried by an afferent path to evoke the stretch reflex. It was concluded that EMG activity in the extensor muscle is much higher in a countermovement jump (CMJ) than in a squat jump. Thus, the enhanced performance produced by a CMJ is related to a combination of elastic energy storage and the stretch reflex (Bosco, et a1., 1982).