Effect of antagonistic contractions on the reflex response of a bifunctional muscle.

With reference to the concept that contraction of a muscle can be inhibited by contraction of its antagonists, the excitability of a bifunctional muscle, the soleus (plantarflexor and adductor) was studied during voluntary isometric contractions of the abductors in man. For this purpose, the soleus monosynaptic stretch reflex (MSR) was solicited by means of quick-release dorsiflexion movements which also induce inactivation of the tibialis anterior (unloading reflex). When different initial torques of dorsiflexion were developed (reference conditions) the amplitude of the soleus MSR was found to be related linearly to the maximal value of angular acceleration. When an abduction torque was superimposed (combined efforts), the soleus response was always depressed, but no significant reinforcement of this inhibition was observed when the abduction torque increased. From present results, and from classical data concerning reciprocal inhibition between soleus and tibialis anterior, it is concluded that the MSR of a bifunctional muscle is depressed when the antagonists of either one of its functions are voluntarily activated.