A STABLE LATE SOLEUS EMG RESPONSE ELICITED BY CORTICAL STIMULATION DURING VOLUNTARY ANKLE DORSIFLEXION

Transcranial electrical or magnetic stimulation was performed in 47 ex periments on 42 normal adult subjects. Surface compound muscle action potentials were recorded from the antagonistic tibialis anterior (TA) and soleus (SOL) muscles. A stable late response recorded from SOL dur ing voluntary ankle dorsiflexion is described and called soleus MEP-80 . Its origin has been studied with different experimental procedures. (1) It is only obtained during active voluntary dorsiflexion of the an kle within 70-100 msec (mean 86.9 +/- 6.4 msec) whereas passive dorsif lexion does not promote such a response. The size of SOL MEP-80 was al ways bigger than the primary SOL response. (2) A similar constant resp onse has not been obtained during voluntary plantar flexion from SOL a nd TA muscles. (3) SOL MEP-80 is not changed significantly by the Jend rassik manoeuvre. (4) It is not obtained or is suppressed when the sub ject is sitting with the feet free in space. On the other hand, it is obtained when the subject is standing, especially when standing is mad e difficult. (5) SOL MEP-80 has not been obtained by brain-stem, spina l cord or root stimulation. (6) A later response of longer than 100 ms ec latency was sometimes obtained at rest but such an S100 soleus resp onse was abolished or transformed to the SOL MEP-80 immediately during ankle dorsiflexion. (7) It is concluded that SOL MEP-80 is a polysyna ptic extensor response related to postural mechanisms and originating through convergence of descending motor commands and peripheral sensor y feedback.