One intriguing phenomenon in human motor control is the very long-term effects of fatiguing muscle contractions described recently. After fatiguing contractions, responses to TMS are reduced for many minutes. Since H-reflex responses and responses to stimulation of corticospinal fibres in the medulla recover to control values much more rapidly than TMS responses, a major origin of the depression of TMS responses appears to be cortical (Gandevia et al. 1999). It is not known whether this long-lasting depression is specific to corticospinal neurones or represents a more global phenomenon.
In this study we have examined the after-effects of fatiguing contraction on both the excitability of the cerebral hemisphere that generated the contractions, by measuring motor-evoked potentials (MEPs) responses in contralateral hand muscles, as well as on transcallosal inhibition evoked from that hemisphere (Ferbert et al. 1992). Experiments were performed on six subjects with local ethical committee approval and informed consent. Two Magstim 200 stimulators were used to deliver TMS pulses (55 mm figure-of-eight coils). Stimulation of the left cortex evoked test MEPs in right hand muscles (1st dorsal interosseous and thenar). Stimuli delivered to the right hemisphere 12-15 ms before these evoked MEPs in left hand muscles (which were used to measure fatigue induced depression) and reduced the MEPs evoked by stimulation of the left hemisphere (used to measure transcallosal inhibition). Stimuli to the left hemisphere alone (test responses) were interleaved with stimulation of both hemispheres (conditioned responses). After recording responses during an initial control period of at least 5 min, a repetitive task was performed with the left hand until the muscles were exhausted. Conditioned and test MEPs were collected for at least 15 min after the contraction.
In all six experiments MEPs evoked in the left hand (stimulation of the right cortex) were significantly reduced after the fatiguing task (P < 0.05, Mann-Whitney rank sum tests). In only one experiment was there a significant reduction in the size of the transcallosal inhibition in the right hand (left cortex). In the majority of cases the transcallosal inhibition was as prominent or more prominent after the fatiguing contraction. These results suggest that the depression of MEPs evoked by TMS after fatigue is specific to the corticospinal system and is not accompanied by a reduction of transcallosal inhibition.