Central changes in muscle fatigue during sustained submaximal isometric voluntary contraction as revealed by transcranial magnetic stimulation

Changes in responses to transcranial magnetic stimulation (TMS) during submaximal isometric voluntary contraction (60% of maximal voluntary contraction (MVC)) of the adductor pollicis muscle and the subsequent recovery period have been studied in healthy volunteers. TMS at twice the motor threshold was applied during the sustained contraction, as well as at rest and during short-lasting (2 s) 60% MVCs before and immediately after the sustained contraction, and at 5 min intervals during the recovery period. Both motor evoked potential (MEP) magnitude (peak and area) and silent period (SP) duration in electromyographic activity (EMG) of the adductor pollicis muscle showed a gradual decrease up to the endurance point and an increase thereafter. MEPs elicited at rest immediately after the fatiguing contraction were larger, whereas those elicited later on during the recovery period did not differ significantly from the controls. It is suggested that the changes in responses to TMS, divergent from those in ongoing voluntary EMG during the sustained 60% MVC, indicate complex processes at levels preceding the motor cortex output cells in an attempt to maintain a submaximal contraction of the fatigued muscle. The increase in MEP magnitude after the sustained 60% MVC may indicate residual changes in cortical activity after fatiguing contraction.