Conditioning hops increase triceps surae muscle force and Achilles tendon strain energy in the stretch-shortening cycle

Postactivation potentiation can improve athletic performance, but the underlying mechanisms are poorly understood. This study investigated the effect of conditioning hops on triceps surae muscle force and tendon strain and its contribution to potentiated stretch-shortening cycle (SSC) performance. Thirty-two subjects participated in two experiments. In both experiments, subjects performed three drop jumps (DJs) after prior conditioning with 10 maximal hops, three unconditioned DJs served as control. Ground reaction forces, kinematics, and triceps surae electromyographic activity were recorded. Ultrasound imaging was used to determine fascicle lengths (FASC) of the gastrocnemius (GM) and soleus muscles (experiment 1) and the length of the Achilles tendon (experiment 2) during the DJs. DJ height after the conditioning hops was significantly higher compared to control DJs (experiment 1: +12% and experiment 2: +19%). A significantly shorter GM FASC during the DJs performed after the conditioning hops coincided with an increased force acting on the triceps surae muscle. Moreover, the triceps surae muscle-tendon unit (MTU) showed increased energy absorption during the eccentric phase of the DJs and increased energy release during the concentric phase. The second experiment revealed a higher Achilles tendon strain in DJs performed after the conditioning hops compared to control DJs. No significant differences in muscle activities were observed. The shorter FASC in GM and the larger Achilles tendon strain facilitated MTU energy transfer from the eccentric to the concentric phase during the DJ. Thereby, conditioning hops improved SSC efficacy and DJ performance.peerReviewe