Stimulation of the motor cortex and corticospinal tract to assess human muscle fatigue

This review aims to characterize fatigue-related changes in corticospinal excitability and inhibition in healthy subjects. Transcranial magnetic stimulation (TMS) has been extensively used in recent years to investigate modifications within the brain during and after fatiguing exercise. Single-pulse TMS reveals reduction in motor-evoked potentials (MEP) when measured in relaxed muscle following sustained fatiguing contractions. This modulation of corticospinal excitability observed in relaxed muscle is probably not specific to the fatigue induced by the motor task. During maximal and submaximal fatiguing contractions, voluntary activation measured by TMS decreases, suggesting the presence of supraspinal fatigue. The demonstration of supraspinal fatigue does not eliminate the possibility of spinal contribution to central fatigue. Concomitant measurement of TMS-induced MEP and cervicomedullary MEP in the contracting muscle, appropriately normalized to maximal muscle compound action potential, is necessary to determine the relative contribution of cortical and spinal mechanisms in the development of central fatigue. Recent studies comparing electromyographic (EMG) responses to paired-pulse stimuli at the cortical and subcortical levels suggest that impaired motoneuron responsiveness rather than intracortical inhibition may contribute to the development of central fatigue. This review examines the mechanical and EMG responses elicited by TMS (single- and paired-pulse) and cervicomedullary stimulation both during and after a fatiguing exercise. Particular attention is given to the muscle state and the type of fatiguing exercise when assessing and interpreting fatigue-induced changes in these parameters. Methodological concerns and future research interests are also considered.