The effect of fatigue on store and re-use of elastic energy in slow and fast types of human skeletal muscle.

Abstract

Stretch-shortening exercises are characterized by enhancement of performance when compared to the work output performed in shortening conditions. There is evidence that fast subjects are unable to re-use great amounts of elastic energy during stretch-shortening cycles performed with slow stretching speed and large stretching length. In the present study, 14 subjects possessing different fibre types in m. vastus lateralis performed vertical jumps with and without preliminary countermovement and with large angular displacement and slow stretching speed The jumping tests were executed before and immediately after fatigue induced by short intense exercises (60 s of continuous rebound jumping). The results indicated that the percentage of re-use of elastic energy was more pronounced in slow subjects compared to fast ones during the test performed before fatigue (28.3% vs. 22.8%). In contrast fast subjects demonstrated a greater percentage re-use of elastic energy than slow ones after fatigue (32% vs. 22.5%). Similarly, the negative relationship observed before fatigue, between the percentage of re-use of elastic energy and percentage of fast twitch fibres (r = 0.50, n = 14, P less than 0.05), was reversed after fatigue (r = 0.55, n = 14, P less than 0.05). The results can be interpreted through differences in sarcomere cross-bridges life-times between fast and slow twitch muscle fibres. The slow twitch-type muscle fibre may be able to retain the cross-bridge attachment for a longer period of time during no fatigued conditions, and therefore it may utilize elastic energy better in slow type ballistic motion. On the other hand, fast twitch type muscle fibres are more affected by fatigue, which might have induced a remarkable decrease of the cross-bridge attachment detachment cycle. Decrease of the cross-bridge rate cycle might allow fast twitch-type muscle fibres to retain longer the elastic energy stored during the stretching phase and then re-use it during positive phase.