Differential changes in corticospinal and Ia input to tibialis anterior and soleus motor neurones during voluntary contraction in man

Motor-evoked potentials (MEPs) were recorded in the tibialis anterior and s oleus muscles following transcranial magnetic stimulation (TMS) of the moto r cortex. In the soleus, the H-reflex amplitude increased with the contract ion level to the same extent as that of MEPs, whereas in the tibialis anter ior, the H-reflex amplitude increased significantly less than that of MEPs. The latency of the MEPs decreased with contraction, whereas this was not t he case of the H-reflexes. in the tibialis anterior, the response probabili ty of single-motor units (SMU) to TMS increased more substantially during v oluntary contraction than following stimulation of the peroneal nerve. In t he tibialis anterior, the response probability of SMU increased more substa ntially during Voluntary contraction than following stimulation of the pero neal nerve. The short-latency facilitation, presumably monosynaptic of orig in, of the soleus H-reflex evoked by subthreshold TMS increased as a functi on of the plantarflexion force. This was not the case for the heteronymous ia facilitation of the soleus H-reflex following stimulation of the femoral nerve. It is concluded that the corticospinal input to lower limb motor ne urones generated by TMS increases with the level of voluntary contraction, whereas this is true only to a limited extent for the synaptic input from l a afferents. it is suggested that this reflects changes in the susceptibili ty of corticospinal cells to TMS during voluntary contraction.