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

Subthreshold electrical stimulation with an intensity less than the thresho ld for evoking M-waves is applied repetitively to the common peroneal nerve via surface electrodes. The stimulation intensity is varied by adjusting t he pulse width from 0, to 240 mus, while the pulse interval (40ms) and curr ent amplitude are kept constant. Single magnetic stimuli are applied to the motor cortex using a circular coil. Motor evoked potentials are recorded f rom the anterior tibial muscle in six normal subjects for various subthresh old stimulation intensities. Signal processing (filtering in the time and f requency domains) removes the artifact caused by the subthreshold electrica l stimulation from the motor evoked potential. Statistically significant mo tor evoked potential facilitation (p <0.05) is observed for pulse widths ra nging from 72 to 240 mus in all the tested subjects. A pulse width correspo nding to 90% of the electrical threshold facilitated the motor evoked poten tial in five of the six subjects.