RESPONSES OF THE SOLEUS MUSCLE TO TRANSCRANIAL MAGNETIC STIMULATION

Soleus muscle responses are difficult to elicit by cortical stimulatio n in normal humans at rest. We have studied in normal volunteers the b ehavior of the soleus and tibialis anterior muscle responses to maxima l intensity transcranial magnetic stimulation (TMS) in the following e xperimental conditions: lying in supine position, active ankle dorsal flexion, active plantar flexion, standing on the soles, standing on th e toes, and standing on the heels. At rest, consistent responses were recorded in the soleus to 61% of the stimuli, only. Maximal facilitati on of the response in the soleus occurred when standing an the toes. I n this condition, responses were recorded to 100% of the stimuli, at a latency that was, on average, 5.2 msec shorter than the latency of th e responses at rest, and similar to the latency of the responses recor ded in the tibialis anterior muscle when standing on the heels. Centra l motor conduction time, calculated in conditions of maximal facilitat ion, was not different for soleus or tibialis anterior muscles. We con clude that the soleus muscle receives short latency excitatory inputs from cortico-spinal axons activated by TMS, with a conduction time sim ilar to that for the tibialis anterior. Such short latency cortico-spi nal connections to the soleus muscle may become functionally effective only during maximum enhancement of motoneuronal excitability by muscl e contraction.