Relationship between the motoneuron pool excitability in the resting state and its amount of change during the voiuntary movement in man

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We examined the relationship between the amount of natural reciprocal inhibition during dorsiflexion exerted at 10%, 30%, and 50% of maximal voluntary contraction (MVC) on the soleus motoneuron (MN) pool excitability of several normal subjects and their MN pool excitability in the resting state. During dorsiflexion at 10% and 30% of MVC,the inhibitory effects were decreased with increasing MN pool excitability. On the other hand, amount of presynaptic inhibition of the soleus MNs in the resting state, as assessed by vibration of the Achilles tendon, was greater in subjects with lower MN pool excitability in the resting state. These observations suggest that the decrease in MN pool excitability induced by natural reciprocal inhibition during dorsiflexion is predominantly dependent on the differences in level of presynaptic inhibition which affect MN pool excitability in the resting state.