Electromechanical behaviour of human muscles in vertical jumps.

The relationships of muscle structure to the potentiation of myoelectrical activity and to the use of prestretching in five lower limb muscles were studied in different vertical jumping conditions. The subjects for the study were six male students, divided according to the muscle fiber distribution in m. vastus lateralis into "fast" and "slow" groups. The subjects performed vertical jumps (1) from a static squatting position (DJ) from five different heights. Myoelectrical (EMG) activity was recorded from mm. gluteus maximus, vastus lateralis, vastus medialis, rectus femoris and gastrocnemius in each jumping condition and integrated (IEMG) for the eccentric and concentric phases of contact. EMG activity showed potentiation during the eccentric phase of movement when compared to the concentric phase. The "fast" and "slow" groups did not differ significantly in this respect, whereas in DJ conditions the relative (% from SJ) height of rise of the center of gravity was greater in the "slow" than in the "fast" group. The result indicated that the utilization of elastic energy during jumping was possible better in subjects having a high percentage of slow twitch muscle fibres in their vastus lateralis muscles.