A transcranial magnetic stimulation (TMS) study on central fatigue

The influence of a 30-second maximal voluntary contraction on cortical excitability was investigated using single and paired TMS in 13 healthy volunteers. In experiment 1 we studied the course of cortical excitability up to 6 minutes after the contraction. A significant reduction of amplitude with a maximum 4 minutes after the end of the contraction was observed. In experiment 2 we studied facilitatory and inhibitory effects 4 minutes after the contraction using paired magnetic stimuli with interstimulus intervals (ISI) fixed at 3 and 10 ms. As in the first experiment the amplitudes were significantly reduced after the contraction. However, neither the degree of inhibition at 3 ms ISI nor the degree of facilitation at 10 ms ISI were altered. Experiment 3 was supposed to detect short-term effects immediately after the contraction using single and paired stimuli with an ISI of 3 ms. No significant changes could be seen. The significant reduction of amplitude observed in experiments 1 and 2 was interpreted as a sign of central fatigue. Summing up, paired TMS is a useful method to analyse central fatigue after voluntary muscle contraction. It has been shown that central fatigue has a differential influence on cortical inhibitory processes.