Store and recoil of elastic energy in slow and fast types of human skeletal muscles

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Abstract
Stretch-shortening cycle refers to the mechanical condition in which store and recoil of elastic energy occur in the skeletal muscle. This leads to a greater work output when compared to a simple shortening contraction. The subjects performed vertical jumps with and without preliminary counter-movement and with small and large knee angular displacement. Those subjects who had more fast twitch (FT) fibers benefited more from the stretching phase performed with high speed and short angular displacement. The amounts of elastic energy stored in this phase were 30 and 26 N .times. kgBW[body weight]-1, respectively, for FT and slow twitch (ST) type subjects. The recoil of elastic energy was proportional to the amount of energy storage. In large amplitude jumps where transient period between stretch and shortening is long the both types of subjects demonstrated similar amount of storage of elastic energy (17 and 16 N .times. kgBW-1, respectively). The reuse of this elastic energy was greater in ST group (24%) as compared to the FT group (17%). The results can be interpreted through differences in sarcomere cross-bridge life times between fast and slow muscle fibers. The slow type muscle may be able to retain the cross-bridge attachment for a longer period of time, and therefore it may utilize elastic energy better in a slow type ballistic motion.