This study aimed to determine the effect of external load on the contribution of tendon lengthening during eccentric phase and tendon shortening velocity during concentric phase in stretch-shortening cycle exercises. Fifteen men performed no-countermovement jump (noCMJ) and countermovement jump (CMJ) using only ankle joint with three different loads (0, 30, and 70% of 1 repetition maximum (RM)). Mean torque, angular velocity, power, and fascicle length of the medial gastrocnemius muscle were measured during jumping. In addition, the relative differences in the measured variables of CMJ compared to noCMJ were defined as pre-stretch augmentation. During concentric phase, the pre-stretch augmentation in angular velocity and power was significantly correlated with that in tendon shortening velocity (except for 70%1RM) but not in fascicle shortening velocity. The increases in tendon length during eccentric phase of CMJ were highly correlated with mean power during concentric phase for all load conditions. These results suggest that the tendon shortening velocity during concentric phase and the amount of tendon lengthening during eccentric phase (i.e., tendon dynamics) strongly contribute to increased performance in stretch-shortening exercises.
Kosaka et al. (Sun,) studied this question.