Activation of Human Stretch Reflex by Experimental Muscle Pain

Objective: This study was performed to investigate the effect of experimental muscle pain on the stretch reflex through electrophysiologic measuring of H-reflex and ankle tendon reflex. Method: Muscle pain was produced by the continuous infusion of 5% hypertonic saline into the soleus and tibialis anterior muscles respectively in the fourteen healthy, male volunteers. Control was made with infusion of 0.9% isotonic saline. H-reflex and ankle tendon reflex were recorded at the soleus before, during and 30 minutes after infusion of saline. Results: The amplitude of ankle tendon reflex increased significantly during soleus and tibialis anterior muscle pain by the infusion of hypertonic saline as compared with those of before and after injection. But the H-reflex showed no significant changes during the infusion in either muscles. Conclusion: These results have demonstrated a muscle pain increased the amplitude of the stretch reflex without a corresponding increase of the H-reflex amplitude. One explanation could be an increased dynamic sensitivity of the muscle spindles during muscle pain caused by an increased firing of the dynamic V-motor neurons.