Characterization of postexercise facilitation and depression of motor evoked potentials to transcranial magnetic stimulation.

We studied the effects of exercise on motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES). Subjects performed 30-second periods of isometric exercise of the extensor carpi radialis until fatigue, which was defined as the inability to maintain half maximum force. The amplitude of MEPs to TMS recorded from the resting muscle after each exercise period was on average more than twice the pre-exercise value (postexercise MEP facilitation). After fatigue occurred, the MEP amplitudes were approximately 60% of the pre-exercise value (postexercise MEP depression). There was a gradual recovery of the depressed MEPs to pre-exercise values over several minutes of rest. Postexercise MEP facilitation was constant when exercise intensity ranged from 10 to 50% of maximum voluntary contraction and it decayed to baseline over several minutes after the end of exercise. There was no postexercise MEP facilitation to TES. We hypothesize that both postexercise MEP facilitation and MEP depression are due to intracortical mechanisms.