LONG-LASTING DEPRESSION OF MOTOR-EVOKED POTENTIALS TO TRANSCRANIAL MAGNETIC STIMULATION FOLLOWING EXERCISE

We used transcranial magnetic stimulation to study the modulation of m otor cortex excitability after rapid repetitive movements. Eleven heal thy subjects aged 24-32 years were evaluated. Serial motor-evoked pote ntial (MEP) recordings were performed from the right thenar eminence e very 5 min for a period of 20 min at rest and for a period of 35 min a fter repetitive abduction-adduction of the thumb at maximal frequency for 1 min. All subjects presented distinct changes in MEP amplitude af ter exercise with an approximately 55% mean maximal decrease compared with basal conditions and complete recovery 35 min after the end of th e exercise. The time course of MEP amplitude changes presented the fol lowing trend: (1) a rapid decrease phase within the first 5 min; (2) a maximal depression phase of 10 min duration (from the 5th to the 15th min); and (3) a slow recovery phase. No significant modifications in post-exercise MEP amplitude were found in ipsilateral non-exercised mu scles. In order to determine the level where these changes take place, we recorded the M and F waves induced by median nerve stimulation at the wrist (all subjects) and MEPs in response to transcranial electric al stimulation (five subjects) at rest and during the decrease and max imal depression phases. None of these tests were significantly affecte d by exercise, indicating that the motor cortex was the site of change . Evaluation of maps of cortical outputs to the target muscle, perform ed in four subjects, showed an approximately 40% spatial reduction in stimulation sites evoking a motor response during the maximal depressi on phase. These data prove that exercise induces a reversible, long-st anding depression of cortical excitability, probably related to intrac ortical presynaptic modulation, which transitorily reduces the motor r epresentation area.