Kinematic assessment of a functional role for recurrent inhibition and selective recruitment

It has been proposed that recurrent inhibition, which is presumed to act more powerfully on type S than on FR or FF motoneurons, and a selective recruitment order of synergistic motoneurons may function to prevent slowly contracting muscles from impeding rapid contractions. The possibility that such mechanical interactions do occur was investigated kinematically by analysis of plantar flexion in the rat hind limb under loaded and unloaded muscle conditions. The left limb was denervated to isolate the lateral gastrocnemius-soleus nerve and muscles. Plantar flexion was induced before and after soleus denervation by posterior tibial-nerve stimulation with the ankle in one of three positions: (i) resting (unloaded), (ii) dorsiflexed (held by a restraint bar), or (iii) with ankle position set by a load applied in series to the intact Achilles tendon. With the initial ankle position near maximal dorsiflexion, peak velocities of foot movements associated with passive elastic muscle properties approached one-half or more of the values achieved by active contractile mechanisms. Under unloaded and loaded conditions, no evidence was found in support of mechanical interference by the slower contracting soleus muscle of maximal lateral gastrocnemius shortening responses. Contributions of soleus tension to foot velocity and acceleration began to emerge with loads greater than 3 N. These findings are in agreement with previous reports that the slow-contracting soleus muscle, though fully activated, cannot contribute effectively to plantar flexion movements at speeds above a critical level.