The effect of prolonged skeletal muscle stretch-shortening cycle on recoil of elastic energy and on energy expenditure.

The enhancement of performance in stretch-shortening exercises has been attributed to the recoil of elastic energy stored during the stretching phase. If the time between stretching and shortening (coupling time) is too long the stored elastic energy can be wasted. In the present study, coupling time was increased by asking ten male subjects to run on a treadmill at different speeds (2.2-5.2 m X s-1) using special soft shoes in addition to normal shoes. The results indicated that running with soft shoes required greater energy consumption than running with normal shoes except at slow speed (2.2 m X s-1). When the running speed was increased the extra energy consumed using soft shoes was parallelly enhanced (0.4 J X kg-1/step at 5.2 m X s-1). It was suggested that the effect of coupling time as limiting factor for recoil of elastic energy was relevant in fast twitch (FT) fibers, which were progressively recruited when the running speed was increased. This is consistent to the fact that cross-bridge life time in FT fibers is very short, and therefore more sensitive to coupling time. At slow running speed (2.2 m X s-1) only slow twitch (ST) fibers were recruited and the enhancement of coupling time was not long enough to provocate detachment of cross-bridges of ST fibers, which possess a long cross-bridge life time. It was concluded that the different recruitment of ST and FT fibers influenced the pattern of recoil of elastic energy which was dependent on the running speed.