Effect of elastic energy and myoelectrical potentiation of triceps surae during stretch-shortening cycle exercise.

Stretching of an activated muscle prior to its shortening enhances its performance during the concentric contraction. This phenomenon has been interpreted to be primarily due to the utilization of elastic energy stored in the series elastic elements of the muscle during the stretch. However, reflex potential has also been suggested as an additional cause for performance increase. To examine this hypothesis five subjects performed maximal vertical jumps on a force platform from two different starting positions: (a) maximal plantar flexion from a static position (SJ) (pure concentric contraction), (b) a toe-standing position with preliminary counter-movement (CMJ). In this condition the calf muscles were actively stretched before concentric work. In both conditions the knee and hip joints were fixed. The results indicated that the utilization of the stretch-shortening cycle enhanced the performance over that of the pure concentric contraction (P less than 0.001). Similarly, the EMG recording demonstrated that in all five subjects the myoelectrical activity of the calf muscles (mm. gastrocnemius and soleus) was potentiated during the concentric phase in CMJ when compared to SJ (P less than 0.001). Therefore, the increase in performance was attributed to a combination of utilization of elastic energy and myoelectrical potentiation of muscle activation.