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RESEARCH PRODUCT

Effets aigus et chroniques de l’électrostimulation appliquée au niveau du nerf moteur : importance du retour afférent

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The aim of this thesis was to investigate the effects of electrical stimulation protocols favouring an indirect motor units’ (MU) recruitment via sensory axons activation and giving rise to extra force development, on the neuromuscular system. These protocols use wide pulse duration, low stimulation intensity, low and high stimulation frequencies and are applied over the motor nerve. The aim of the first study was to examine the effects of these protocols on the extent and origin of neuromuscular fatigue during an acute application. Results showed that for a similar impact on maximal force generating capacity, low stimulation frequencies limit force decreases during the stimulation trains as compared to high stimulation frequencies. The aim of the second study was to investigate the effects of chronic application of these protocols. Results showed important torque gains after the training period despite the low stimulation intensity used, while the induced neural adaptations were frequency-dependent. Results of these two studies also highlighted the importance of the phenomenon of extra torque on induced adaptations. Thus, the aim of the third study was to determine the conditions permitting the occurrence of extra torque, by modulating the frequency and intensity of stimulation. Main results showed that when the initial MU recruitment was mostly indirect, the developed torque was higher than the one expected for the given stimulation parameters, independently of the stimulation frequency, suggesting that the indirect MU recruitment plays a preponderant role in the occurrence of extra torque. Moreover, a frequency-dependent impact on spinal excitability was observed, resulting in a decrease after the low stimulation frequency and an increase after the high frequency. Consequently, the last study investigated the mechanisms responsible for the distinct modulation of spinal excitability. Results showed that the decrease in spinal excitability observed after the low stimulation frequency could be attributed to increased homosynaptic post-activation depression, while this latter mechanism could have been compensated by an enhanced motoneuron excitability as a result of persistent inward currents after the high stimulation frequency. All these results underline the importance of the afferent volley to the induced neuromuscular adaptations after acute and chronic electrical stimulation application.