Studies on the effects of strenuous exercise have clearly identified both peripheral and central components of fatigue (Bigland-Ritchie & Woods, 1984), which can limit voluntary muscle performance. Stimulation of the motor cortex using transcranial magnetic stimulation (TMS) and electromyographic (EMG) recordings have shown corticospinal excitability to be depressed following exercise (Brasil-Neto et al. 1993). We have further investigated central fatigue processes in a leg muscle and whether this fatigue spreads to the homonymous muscle in the contralateral leg.

With local ethical approval and informed consent seven healthy individuals (aged 21Ð42 years, 5 males) were recruited and seated in a reclining armchair. Surface electromyographic (EMG) recordings were made bilaterally from tibialis anterior (TA). Subjects maximally contracted their dominant TA muscle isometrically for as long as they could sustain (typically for 3Ð4 min) while keeping the other leg relaxed. TMS was applied using a MagStim 200 stimulator and an angled double-cone stimulating coil with its cross-over located over the vertex. Trials of six stimuli were conducted while relaxed (stimulus strength 1.2 X relaxed motor threshold) and during a 20 % maximum voluntary contraction (MVC) in the dominant leg (stimulus strength 1.0 X relaxed motor threshold). Trials were conducted 5 and 10 min before exercise and at regular intervals up to 2 h post-exercise.

While relaxed, the mean (± S.D.) motor-evoked potential (MEP) areas in exercised leg were significantly (ANOVA, P < 0.05) depressed, relative to pre-exercise levels, in trials conducted 5 min (63.5 ± 23.7 %), 10 min (58.9 ± 32.7 %) and 15 min (61.4 ± 23.8 %) post-exercise. MEP areas had returned to pre-exercise levels by the trial conducted 20 min post-exercise. No changes in MEP areas were seen in the non-exercised leg. MEPs recorded during voluntary activation showed no depression in either leg at any time after exercise.

Central fatigue processes appear to be lateralised to the exercised leg. When the exercised muscle underwent voluntary activation, post-exercise depression was not observed. Presumably the voluntary drive to produce a 20 % MVC contraction countered any effect of post-exercise depression. This confirms that the post-exercise depression was a central rather than peripheral phenomenon.

All procedures accord with current local guidelines and the Declaration of Helsinki.