Influence of training background on the relationships between plantarflexor intrinsic stiffness and overall musculoskeletal stiffness during hopping

The aim of this study was to characterize the influence of intrinsic musculotendinous and musculoarticular stiffness of plantarflexor muscles on (1) the overall musculoskeletal stiffness and (2) the performance during stretch-shortening cycles-type exercise. The influence of plyometric training background on these relationships was also analyzed. Musculotendinous (SIMT), passive (KP) and active (SIMA) musculoarticular stiffnesses were quantified, using quick-release and sinusoidal perturbation tests, on nine French elite long or triple jumpers (athlete group, AG) and nine control subjects (CG). These ergometric parameters were related with the lower-limb stiffness (Kleg) and the maximal performance (Hmax) measured from a force platform during vertical hopping. AG showed a significantly higher SIMT (2.76 rad−1), KP (55.6 N m rad−1), Kleg (30.3 kN m−1) and Hmax (0.48 m) compared to CG (1.83 rad−1, 37.8 N m rad−1, 19.6 kN m−1 and 0.38 m, respectively). Kleg was not significantly correlated with any of the intrinsic stiffness parameters (SIMT, SIMA or KP). For AG, a strong and negative correlation was observed between Hmax and Kleg. These data indicate that, while elite jumpers presented higher stiffness of both musculotendinous and passive musculoarticular structures, a high compliance of musculoskeletal system was beneficial to optimize the performance in vertical hopping for these athletes. We suggested that neuromuscular strategies were designed to counterbalance this higher intrinsic stiffness to solve the problem of the conflicting requirement of the musculotendinous elements: increase in compliance to enhance the elastic recoil and increase in stiffness for a better force transmission to the periphery.