ABSTRACT
Electrical stimulation of femoral nerve modulates vonluntary tonic activity of ipsilateral soleus muscle. Stimulus time-locked inhibitory and facilitatory phases can be distinguished. EMG temporal analysis suggests that early perturbations are correlated with spinal effects of centripetal electrical activity. The inhibitory effects which momentarily abolish voluntary soleus activity are thought to result from quadriceps lb fibres recruitment. While no heteronymous activity is induced at rest, femoral nerve Ia fibres activation can produce soleus muscle reflex when soleus motor nucleus excitability is increased by voluntary command. Recurrent discharge resulting from soleus reflex response enhances inhibition initially due to quadriceps lb volley. Secondary effects of isometric quadriceps contraction [and soleus contraction when the femoral stimulus elicits a reflex in this muscle] have their own effects later. These findings suggest that proprioceptive relationships of the two muscular groups are efficient during tonic isometric voluntary command