Time course of neuro-mechanical changes underlying stretch–shortening cycle during intermittent exhaustive rebound exercise

This study analysed the time course of neuro- mechanical changes underlying stretch-shortening cycle during intermittent exhaustive rebound exercise. On a sledge apparatus, ten subjects repeated until exhaustion a series of 30 unilateral submaximal rebounds, with interme- diate 3-min rest periods. Rebound height, ground reaction force, 3D tibial acceleration and electromyographic activity of major lower limb muscles were recorded. A maximal drop jump test performed before and after the exhaustive exercise revealed a 10% drop in maximal stretch-shorten- ing cycle (SSC) performance. SpeciWc investigation of the neuro-mechanical changes along the exhaustive exercise included classical comparison of the Wrst (BEG) and last (END) rebound series. From the initial accommodation phase, an optimized (OPTIM) series was individually deter- mined as the Wrst of at least two subsequent series with sig- niWcantly shorter contact time than in the BEG series. The OPTIM series was reached after 3 § 1 series, with associ- ated increased lower limb stiVness during the braking phase and decreased muscle activities during the push-oV. The major result was that the early (BEG-OPTIM) changes explained most of the BEG-END ones whereas the actual (OPTIM-END) fatigue eVects remained quite limited. This conWrmed our expectation that erroneous quantiWcation of the SSC fatigue eVects might be drawn when using the early beginning of rebound exercise on the sledge as a ref- erence. Actual fatigue eVects included medio-lateral insta- bility as suggested by increased peroneus longus preactivation and medio-lateral tibial acceleration. The present methodology is thus considered as improving the distinction between SSC optimization and its deterioration with fatigue.