The effect of a pre-hop on the muscle-tendon interaction during vertical jumps

Many movements use stretch-shortening cycles of a muscle-tendon unit (MTU) for storing and releasing elastic energy. The required stretching of medial gastrocnemius (MG) tendinous tissue during jumps, however, requires large length changes of the muscle fascicles, due to the lack of MTU length changes. This has a negative impact on the force generating capacity of the muscle fascicles. The purpose of this study was to induce a MG MTU stretch prior to shortening by adding a pre-hop to the squat jump. Eleven well-trained athletes specialized in jumping performed a pre-hop squat jump (PHSJ) and a standard squat jump (SSJ). Kinematic data was collected using a 3D-motion capture system and used in a musculoskeletal model to calculate MTU lengths. B-mode ultrasonography of the MG was used to measure fascicle length and pennation angle during the jumps. By combining the muscle-tendon unit lengths, fascicle lengths and pennation angles, the stretch and recoil of the series elastic element of MG was calculated using a simple geometric muscle-tendon model. Our results show less length changes of the muscle fascicles during the upward motion and lower maximal shortening velocities, increasing the moment generating capacity of the plantar flexors, reflected in the higher ankle joint moment in the PHSJ as compared to the SSJ. Although muscle-tendon interaction during the PHSJ was more optimal, athletes were not able to increase their jump height compared to the SSJ.

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