Facilitation of motor evoked potentials in the anterior tibial muscle by repetitive subthreshold electrical stimulation.

Subthreshold electrical stimulation with an intensity less than the threshold for evoking M-waves is applied repetitively to the common peroneal nerve via surface electrodes. The stimulation intensity is varied by adjusting the pulse width from 0 to 240 micros, while the pulse interval (40 ms) and current amplitude are kept constant. Single magnetic stimuli are applied to the motor cortex using a circular coil. Motor evoked potentials are recorded from the anterior tibial muscle in six normal subjects for various subthreshold stimulation intensities. Signal processing (filtering in the time and frequency domains) removes the artifact caused by the subthreshold electrical stimulation from the motor evoked potential. Statistically significant motor evoked potential facilitation (p < 0.05) is observed for pulse widths ranging from 72 to 240 micros in all the tested subjects. A pulse width corresponding to 90% of the electrical threshold facilitated the motor evoked potential in five of the six subjects.