Changes in excitability of tendon tap and Hoffmann reflexes following voluntary contractions.

The effects on the Hoffmann reflex and tendon tap responses in the human soleus and medial gastrocnemius muscles of a brief conditioning volitional contraction have been compared with the objective of distinguishing contributions from the central nervous system and the periphery. The H-wave on successive trials over a 50 sec period following contraction for the most part demonstrated a depression, especially upon the initial trial. The T-wave responses in the first few trials were greater than those in the control period. Using the assumption that responses to tendon taps had been reduced by the amounts reflected in the excitability curve for H-response, it was deducted that a marked enhancement in T-wave excitability had been present at the initial tendon tap and more moderate facilitation thereafter. This is consistent with the post-contraction discharge and increase in stretch sensitivity of spindle receptors seen after contraction of a muscle in experimental animals. It is concluded that such effects on spindles may occur in man, maybe of such force as to detectably influence dynamic movements.