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In addition to the utilization of muscle's elastic energy enhancement of performance in exercise involving stretch‐shortening cycle might be also due to simultaneous increase of myoelectrical activity. This hypothesis was tested by examining three athletes during jumping exercise on force‐platform. Vertical jumps were performed with and without preliminary counter‐movement, and the jumps were called counter‐movement jump (CMJ) and squatting jump (SJ), respectively. In both conditions several jumps were performed also with extra loads on the shoulders (15–220% of b. wt.). Additional droppingjumps (DJ) were executed from different heights (20–100 cm). During jumping exercise myoelectrical activity of selected muscles from the quadriceps femoris was monitored with surface electrodes. The results obtained were similar to those reported in isolated muscle and as expected, the prestretch in CMJ shifted the force‐velocity curve of concentric work to the right. In two cases enhancement of performance was attributed primarily to restitution of elastic energy because myoelectrical activity was similar to that observed in SJ. In one subject increased myoelectrical activity was observed during the concentric phase of CMJ. In DJ condition the EMG activity during eccentric phase was much higher than in SJ. Therefore the high performance in this condition was attributed to both elastic energy and reflex potentiation. In eccentric work of CMJ the average force decreased with the increase of stretching speed. This phenomenon was associated with a light increase of EMG activity. The observed results emphasize that both elastic energy and reflex potentiation may operate effectively during stretch‐shortening cycle activity.
Bosco et al. (Thu,) studied this question.