'Direct' and 'crossed' modulation of human motor cortex excitability following exercise.

Rapid repetitive movements of the thumb (1 min duration) produce a reversible decrease in the activated primary motor cortex (MI) excitability to transcranial magnetic stimulation (TMS) with recovery within 35-40 min. In the present study we investigated (1) the role of peripheral sensory feedback in inducing such decrease and (2) possible effects of exercise on the non-activated MI. Stimulation of peripheral Ia afferent fibres, induced by 1 min vibration of thenar muscles and 2 Hz electrical stimulation of the median nerve at the wrist, have no effect on motor evoked potential (MEP) amplitude to TMS suggesting no role for sensory feedback to produce MI excitability modulation. Exercise produces a significant (P < 0.01) decrease of MEPs for homologous non-exercised muscles with concomitant contraction of corresponding motor cortical output maps, suggesting that changes in MI excitability also occur in the nonactivated hemisphere. This 'crossed' effect might relate to an interhemispheric transfer of information, via homotopic connections of the corpus callosum.