The purpose of this study was to quantify the effect of the length and stiffness variation in the series elastic element (SEE) to the work output during countermovement and the subsequent effects of enhanced work output. A Hill-type model of the muscle tendon complex was constructed. A range of SEE lengths (between 1 and 8 times the optimal length of the contractile element (CE)) and a range of strain rates in the SEE (between 2% and 8%) were investigated. Forward dynamics simulations were performed to evaluate the causal factors for the gain in height during concentric (CO) and countermovement (CM) conditions. Regarding the shorter SEE condition, the magnitude of work output enhancement associated with countermovement was negligible for all of strain rates. In contrast to the longer SEE condition (SEE length was 8 times of that of CE) the magnitude of enhancement was consistent for the moderate strain rates (4% and 8%). These results quantitatively demonstrate the effect of length and stiffness variation in the SEE on the work output during countermovement. The findings would be useful when considering the effect of the compliance in the SEE to the work output during countermovement.

The purpose of the study is to investigate the influence of contraction and relaxation times on the mechanical properties of human tendon. Ten healthy males participated in this study. They exerted ramp isometric plantar flexion torque with four conditions (1 s, 3 s, 5 s, 10 s) as each time of contraction (from rest to maximum) and relaxation (from maximum to rest) (exp. 1). In addition, eight of the subjects performed contractions and relaxations with 5 s and quick (contraction and relaxation as fast as possible) conditions (exp. 2). During the tasks, the elongation (L) of the tendon of medial gastrocnemius muscle (MG) was directly measured by ultrasonography. The mechanical properties of tendon (maximal L, stiffness, and hysteresis) were calculated from the relationship between the estimated muscle force (Fm) of MG and L. There were no differences in the L values at all the force production levels among the four conditions in exp. 1. On the other hand, the L values of quick condition were lower than 5 s condition at force development above 108 N. However, the mechanical properties of tendon (maximal L, stiffness, and hysteresis) did not differ among the five conditions. The present result indicated that the mechanical properties of human tendon were not influenced by the contraction and relaxation times.