The neurophysiology of central and peripheral fatigue during sub-maximal lower limb isometric contractions
Fatigue is one of the most common psychophysiological symptoms that interact with the control mechanisms regulating task behaviour. The cortical processes involved in preparation and feedback control of voluntary movement are associated with EEG activity time-locked to movement onset: a pre-movement Movement-Related Cortical Potential (MRCP) is followed by a post-movement potential (PMP). The aim of this study was to determine whether changes in subjective fatigue which arise in the course of a simple repetitive motor task affect cortical information processing as measured by MRCPs or PMPs. MRCPs/PMPs were recorded in 33 healthy subjects while they made 100 self-paced unilateral button presses with their left or right index finger, and then continued with the other index finger for another 100 movements. Before and after the motor tasks, subjective fatigue was assessed via questionnaire. (1) Subjects who reported a higher increase of fatigue when they had finished the motor tasks showed smaller (more negative) amplitudes of the PMP. (2) This increase of negativity was strongest during the initial part of the tasks. (3) Physical aspects of perceived fatigue had a stronger effect on PMP amplitude than cognitive aspects. Smaller amplitudes of the PMP in more fatigued subjects might be explained by reduced attention to somatosensory feedback. Adaptation of this effect may result from more automatic performance at later stages of the task when all subjects required a lower degree of attentional control. In conjunction with previous studies, effects of fatigue could be separated from habituation.