Leg and joint stiffness in human hopping.

The present study investigated the regulation of leg and joint stiffness in hopping at different intensity levels. Eight male subjects performed bilateral hopping at various intensity levels that were determined by peak vertical ground reaction force (GRF). In addition to the GRF, the measurements included hopping kinematics and electromyography (EMG) of selected leg muscles. While the leg and ankle joint stiffness remained invariant, the knee joint stiffness increased significantly (P<0.01) with the hopping intensity. EMG analysis revealed a significant increase in averaged EMG for all the measured muscles before and during the early phase of ground contact (P<0.05-0.001) with increasing hopping intensity. However, only the vastus lateralis muscle showed significant increase in stretch reflex EMG with increasing hopping intensity (P<0.01). The present study indicates that in hopping with short contact time the leg stiffness modulation is sensitive to changes in ankle joint stiffness and the role of knee joint stiffness is to regulate the jumping performance (height). Furthermore, our results suggest that leg and joint stiffness in hopping is mainly adjusted by centrally programmed motor commands and the contribution of stretch reflexes to muscle force output is muscle-dependent.