Human muscle fatigue investigated by transcranial magnetic stimulation

Corticospinal motor output was studied during a 1.5 min maximum isometric voluntary contraction of the first dorsal interosseous muscle in five healthy subjects. Compound muscle action potentials (CMAPs) evoked by suprathreshold magnetic cortical stimuli and by supramaximal ulnar nerve stimuli were compared. On average, force fell to 25% of its initial value, CMAPs from nerve stimuli declined to 58% of initial, but CMAPs from cortical stimuli showed no significant change in amplitude after the first 5 s. It is concluded that there is no marked net change in corticospinal output during fatigue. It is postulated that cortical stimuli engage a subgroup of low threshold, fatigue-resistant motor units which remain active throughout the contraction.