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. 1994). When the exercise is exhaustive, depression can also be seen in motor evoked potentials (MEPs) of homonymous non-exercising muscles (Williams et al. 2003) and can therefore be attributed exclusively to central fatigue processes. We have now induced central fatigue in non-exercising muscles and measured its influence on force and speed of movement and reactions.

With local ethical approval and informed consent, six healthy male volunteers (aged 18-22 years) were seated with their arms relaxed on horizontal armrests and bilateral surface EMG recordings taken from the biceps brachii (BB) muscles. TMS was applied using a MagStim 200 stimulator connected to a 9-cm circular coil centred over the vertex. The stimulus intensity was set to 1.2 X threshold for evoking MEPs in relaxed BB muscles; MEP areas were measured bilaterally. Functional assessments were made bilaterally as follows: maximum voluntary contraction (MVC) force in elbow flexors, maximum hand grip (MHG) force, movement times (MTs) and simple reaction times (SRTs). Two assessment trials were completed before starting the exercise protocol and five more were made during the 35 min immediately following exercise. A 3.5kg weight was strapped to the wrist and exercise consisted of right-arm biceps curls, to a tone repeating at a frequency of 1 Hz, until exhaustion.

MEP areas in the exercised BB were depressed to 26.2 ± 6.6 % (mean ± S.E.M.) of the pre-exercise level (PEL) and MVC to 74 ± 8.6 % PEL in the nine min after exercise (P < 0.05; ANOVA on ranks). MEP areas in the non-exercised BB were depressed to 69.5 ± 8.8 % (PEL) but MVC showed a small insignificant (P > 0.05) rise (103.1 ± 5.2 % PEL) in the eleven min after exercise. MEPs remained depressed for up to 30 min (exercised 25.8 ± 3.8 % PEL; non-exercised 64.8 ± 8.2 % PEL); MVC recovered slightly in the exercised arm to 80.3 ± 4.6 % PEL and was unchanged in the non-exercised arm (102.8 ± 4.8 % PEL). Decrease in MEP area correlated with an increase in latency in the exercised (P < 0.05; r² = 0.13; linear regression) and non-exercised (r² = 0.13) arm but MEP area correlated with MVC only in the exercised BB (r² = 0.25). MHG showed drop to 96.5 ± 1.6 % PEL after ten min in the exercised arm and to 91.2 ± 2.6 % PEL in the non-exercised arm increasing to 88.0 ± 1.6 % PEL after 30 min. SRT decreased after 13 min in the non-exercised arm (95.4 ± 1.1 % PEL) and there was no change in MTs.We conclude that the central fatigue seen in the non-exercised arm has no effect on MVC in that arm. MHG was reduced in both arms after exercise but this could have been the result of peripheral fatigue induced by repeated testing procedures. The small improvement in SRT (faster reactions) seen after 13 min in the non-exercised arm may have been the result of practice. Although we saw depression of MEPs and increased latency in the non-exercised BB there appeared to be no measurable functional deficit in force or speed of movement or reaction.