Supraspinal fatigue during intermittent maximal voluntary contractions of the human elbow flexors

Response to transcranial magnetic stimulation in human subjects (n = 9) wer e studied during series of intermittent isometric maximal voluntary contrac tions (MVCs) of the elbow. Stimuli were given during MVCs in four fatigue p rotocols with different duty cycles. As maximal voluntary torque fell durin g each protocol, the torque increment evoked by cortical stimulation increa sed from similar to 1.5 to 7% of ongoing torque. Thus "supraspinal" fatigue developed in each protocol. The motor evoked potential (MEP) and silent pe riod in the elbow flexor muscles also changed. The silent period lengthened by 20-75 ms (lowest to highest duty cycle protocol) and recovered signific antly with a 5-s rest. The MEP increased in area by >50%, in all protocols and recovered significantly with 10 s, but not 5 s, of rest. These changes are similar to those during sustained MVC. The central fatigue demonstrated by the torque increments evoked by the stimuli did not parallel the change s in the electromyogram responses. This suggests that part of the fatigue d eveloped during intermittent exercise is "upstream" of the motor cortex.