MUSCULAR ACTIVITY IN THE STRETCH-SHORTENING CYCLE (SSC): NOT ONLY MAXIMIZATION BUT OPTIMIZATION IS NECESSARY
Keywords: drop jump, reactivity, stretch-shortening cycle (SSC), electromyogram (EMG), intermuscular coordination
AbstractINTRODUCTION: Drop jumping is a very complex skill with a stretch-shortening cycle (SSC) involving several phases – preparation for landing, landing itself, shock absorption and push-off. Only a very short span of time (£ 200ms) is available for the execution of a drop jump. For this reason much is required of the jumper’s neuromuscular system. Especially an EMG reduction of m. gastrocnemius activity just before and at the beginning of landing has been reported several times (Schmidtbleicher/Gollhofer, 1985). This reduction has been interpreted as a mechanism to protect the muscle from excessive stretch loads. Previously, only qualitative results have been reported. METHODS: Reactive capabilities were analyzed in two complex studies of 18 sport students. The following movements were studied: squat jumps, countermovement jumps and drop jumps from drop heights of 16, 24, 32, 40, 48 and 56 cm. EMG-activities of the m. tibialis anterior, m. soleus, m. gastrocnemius and m. rectus femoris were registered to describe muscle activity during vertical jumping. Simultaneously, ground reaction forces and changes in knee angle were registered. A new method was developed for measuring the EMG-reduction of m. gastrocnemius. RESULTS: In these studies it could be observed that EMG reduction of m. gastrocnemius occurs simultaneously with the maximal innervation of m. rectus femoris. Strong action in the knee joint is demonstrated by the activity of the leg extensor m. rectus femoris, which is needed to stop downward movement and to start the push-off. The m. gastrocnemius´ activity with its antagonistic effect on the knee joint was reduced when m. rectus femoris activity started, and reached maximal innervation significantly later than m. rectus femoris during ground contact. This is especially the case for good jumpers. After a training period of four weeks (only reactive movements) this program of innervation could be observed in almost all the participants in these studies. CONCLUSION: The results show that the EMG-reduction of m. gastrocnemius during drop jumping cannot be considered a reliable mechanism to protect the muscle from excessively large stretch loads, but that this EMG activity of the antagonistic muscles makes possible an optimal intermuscular coordination for drop jumping. REFERENCES: Schmidtbleicher, D.; Gollhofer, A. (1985). Einflußgrößen des reaktiven Bewegungsverhaltens und deren Bedeutung für die Sportpraxis. In: Bührle, M. (1985). Grundlagen des Maximal- und Schnellkrafttrainings, 271-281.
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