Patterns of muscle activation in human hopping.

In the present study, we examined the electromyogram (EMG) patterns of the soleus and medial gastrocnemius (MG) muscles during rhythmical, two-legged hopping to investigate the contributions of the monosynaptic short- and long-latency stretch reflexes during such a natural movement in human. During rhythmical hopping, soleus muscle is activated reflexly at near-monosynaptic latency by stretch resulting from passive ankle flexion upon landing. Soleus muscle also contracts voluntarily in order to launch the body into the next hop. This is part of the rhythmical bursts of activity producing the hops. Depending on the hopping interval, this phase of activation can follow the short-latency phase or precede landing at very short hopping intervals. In MG, there is an initial phase of activity that stiffens the muscle in preparation for landing, and continues through the contact phase. The monosynaptic reflex response to landing is usually superimposed on this activity. Depending on the hopping interval, both of these responses may be overlaid with activity that is time-locked to the take-off into the next hop, and serves to launch the body into the next hop. However, no evidence for a long-latency stretch reflex was found. In addition, the preferred hopping frequency for all subjects was about 2 Hz. This frequency is associated with a pattern of EMG activity the timing of which indicates that it balances the requirement for a comfortable landing from a hop with the optimal muscle activation required for launching the following hop.