The nervous system which control muscle group in lower extremity might also play an important role during jumping. We investigated the relationship between jump performance and neuromuscular function to find the determining factors of jump performance. In the present study, two male subjects of good jumpers (high ratio of jump performance and isokinetic strength in leg extensor muscles) and two male subjects of poor jumpers (low ratio of jump performance and isokinetic strength in leg extensor muscles) performed countermovement jumping on force-platform.

  Vertical component of ground reaction force, knee and ankle angular displacement and electromyographic activities from Soleus (SOL), gastrocnemius (GAS), vastus medialis (VM) and vastus lateralis (VL) were recorded during jumping.

  The peak force and Jark of good jumpers were significantly higher rather than poor jumpers. The time of eccentric and concentric phase of good jumpers were short compared with poor jumpers. In eccentric phase, EMGs of poor jumpers indicated the tonic discharge patterns in SOL, GAS and VM, while these of good jumpers were phasic discharge patterns. Compared with poor jumpers, root mean square and mean power frequency of EMG from VM in good jumpers were significantly high (0.375 vs 0.289mV, 123.5HZ vs 107.8HZ) in concentric phase.

  Therefore. these results suggest that higher jump performance in good jumpers were attributed to increased firing rate and recruitment of motor units due to higher excitation level of motoneurons. Thereby, large deceleration could be occurred in the eccentric phase. And also, it is suggested that nervous stimulation by eccentric contraction of muscle fider is very important factor, which contribute to improve jump performance.