Changes in motor cortical excitability during human muscle fatigue

1. The excitability of the motor cortex was investigated during fatiguing contractions of the elbow flexors in human subjects. During sustained contractions at 30 and 100% maximal voluntary force (MVC), the short-latency electromyographic responses (EMG) evoked in biceps brachii and brachioradialis by transcranial magnetic stimulation increased in size. The reduced EMG in the elbow flexors following the evoked muscle potential (silent period), increased in duration during a sustained MVC but not during 30% MVCs nor during a sustained MVC of a remote muscle (adductor pollicis). 2. When the blood supply to brachioradialis was blocked with a sphygmomanometer cuff at the end of a sustained MVC, the changes in EMG responses to transcranial stimulation rapidly returned to control values. This suggests that changes in these responses during fatigue were not mediated by small-diameter muscle afferents. 3. Tendon vibration during sustained MVCs indicated that the changes in the responses to magnetic cortical stimulation were not mediated by reduced muscle spindle inputs. 4. Muscle action potentials evoked in brachioradialis by electrical stimulation at the cervicomedullary junction did not increase in size during sustained MVCs. Thus, growth of the cortically evoked responses during sustained MVCs reflects a change in cortical excitability. Although the silent period following cervicomedullary stimulation lengthened, it remained substantially shorter than the cortically evoked silent period. 5. The altered EMG responses to transcranial stimulation during fatigue suggest both increased excitation and increased inhibition in the motor cortex. As these changes were unaffected by manipulation of afferent input they presumably result from intrinsic cortical processes and/or altered voluntary drive to the motor cortex.