Effect of ankle joint stiffness during eccentric phase in rebound jumps on ankle joint torque at midpoint

The purposes of this study were to investigate firstly, the ankle joint stiffness during the eccentric phase in rebound jumps, and secondly, the effect of ankle joint stiffness during the eccentric phase on ankle joint torque at midpoint. Nine active males executed rebound drop jumps from a height of 30 cm (RDJ30) and 50 cm (RDJ50), and 5-repetition rebound jumps (5RJ). Force plate data, limb position and electromyogram (EMG) of the medial head of gastrocnemius (GAS), soleus (SOL), and tibialis anterior (TA) muscles were recorded simultaneously during all jumps. When compared with RDJ30 and RDJ50, 5RJ displayed significantly higher jumping height, ankle joint torque at midpoint, ankle joint stiffness during the eccentric phase, and integrated EMG (IEMG) of GAS and SOL. Furthermore, 5RJ displayed significantly lower contact time and IEMG of TA than RDJ30 and RDJ50 did. The ankle joint torque at midpoint showed a significant positive correlation with jumping height but a significant negative correlation with contact time in rebound jumps. There was a significant positive correlation between the ankle joint stiffness during the eccentric phase and ankle joint torque at midpoint in rebound jumps. These results suggested that for the rebound jumps to be effective, it may be important to enhance the ankle joint torque at midpoint by greater ankle joint stiffness during the eccentric phase. These results also suggested that compared to 5RJ, RDJ30 and RDJ50 may reduce ankle joint stiffness in order to protect the tendomuscular system from the stretch load. However, ankle joint stiffness is possibly increased in 5RJ because the restraint to defend the tendomuscular system is reduced by prediction or adaptation to the stretch load because it is a continuous jump.