Voluntary and electrically-induced muscle fatigue differently affect postural control mechanisms in unipedal stance.

The repetition of muscle contractions is likely to generate fatigue which can provoke alterations of postural control. Regulatory mechanisms can be triggered to counteract these alterations. However, these mechanisms would occur only when fatigue is induced through voluntary (VOL) contractions and not with electrically stimulated (ES) contractions. Hence the aim was to compare the effects of VOL and ES fatiguing contractions inducing a similar level of strength loss on unipedal postural control (assessed by means of force platform and EMG measurements), maximal voluntary contraction (MVC) and central activation ratio (CAR) to characterize the alterations induced by both modalities of fatigue and the associated regulatory mechanisms. Results showed that the VOL exercise induced a significant decrease of the CAR whereas the ES exercise did not, thus illustrating that central fatigue was present only after voluntary contractions. The VOL exercise also induced greater postural disturbances and larger regulatory mechanisms than the ES exercise, which also induced postural regulatory mechanisms. The present study reveals that postural control mechanisms are modulated according to the nature of the fatiguing contractions, likely due integration of specific fatigue signals according to the modality of the contraction. Because of a larger neurophysiological impact of VOL than ES fatiguing contractions due to greater central disturbances, VOL exercise-induced larger regulatory mechanisms. Nevertheless, the presence of regulatory mechanisms with ES contractions clearly underlines the ability of the central nervous system to display an accurate motor control following acute externally induced neuromuscular perturbations.