The influence of fatigue on the stretch reflex evoked in ankle extensor muscles by hopping was investigated in six healthy men. The men hopped on a force platform, at spontaneous frequency and amplitude, until they were unable to maintain the initial frequency or amplitude of the jumps. This task was done with the knees flexing normally during ground contact or under instructions to straighten the knees. Surface electromyograms (EMG) of soleus (SO), gastrocnemius medialis (GM), and tibialis anterior (TA) muscles were recorded simultaneously with the vertical component of the ground reaction force. Spectrum analysis of the EMG recorded during isometric tests performed immediately before and after the fatiguing hopping task demonstrated the existence of fatigue in SO and GM and often in TA. The stretch reflex was studied during the first and last ten jumps of every hopping series. The long-latency components of the reflex were too variable to be analysed. Whatever the hopping condition, latency and amplitude of the short latency component were not significantly modified by fatigue. Fatigue enhanced the occurrence of this reflex component in SO only. These data suggest that in fatiguing submaximal hopping, the neuromuscular system does not fundamentally change its stiffness regulation before the endurance time has been reached.
