Yasuhiro KAGAMIHARA, Yoshihisa MASAKADO

Department of Neurology, Tokyo Metropolitan Neurological Hospital

(Received 25 June 2004; accepted 29 September 2004)

Abstract: Cocontraction of pairs of antagonistic muscles is not always pathological; it may be a type of normal voluntary movement. It functions to keep the position of the joint constant and markedly increases its stiffness. It is reported that there is selective activation of the medial gastrocnemius muscle during cocontraction at the ankle joint. Activity in the Ib inhibitory pathway, which leads to the soleus motoneuron from the gastrocnemius medialis nerve (GM), is important in the cocontraction mechanism. We examined the activity of this circuit during cocontraction and compared it with activity in the ankle at rest and in tonic dorsiflexion and tonic plantarflexion. Ib inhibition did not significantly differ in the four situations. Although reciprocal Ia inhibition was markedly depressed during cocontraction, Ib inhibition to the soleus from the medial gastrocnemius muscle was maintained throughout. This constant activity in the Ib inhibitory pathway might be attributable to the fact that the excitability of the medial gastrocnemius muscle is relatively larger than that of soleus muscle during cocontraction.

Key words: Ib inhibition, cocontraction, gastrocnemius medialis muscle, H-reflex, joint stiffness