Stimulation électrique fonctionnelle et fatigue musculaire du muscle paralysé chez le blessé médullaire paraplégique: effet de la vitesse d'activation et du niveau d'activation des unités motrices

Abstract

Introduction The aim of this study was to determine how a reduction in the rate and the level of motor unit activation influences the mechanical and electrophysiological characteristics of muscle fatigue when quadricep muscles are paralyzed by lesions to the main motor neuron. Material and methods Knee torque output and surface action potential (SAP) of three muscular heads of the quadriceps were recorded bilaterally in eleven paraplegic patients during a sustained isometric elicited contraction. The fatiguing muscle contraction was elicited on the one hand by supramaximal intermittent and continuous electrical stimulation, and on the other hand by continuous electrical stimulation with three different stimulation intensities. Knee torque output and SAP variables (amplitude, latency, duration, surface, and root mean square) were measured in each case and compared. Results The amount and rate of torque output decrease were higher at high activation rates (continuous stimulation, P < 0.01), but not related to the level of motor unit activation. Considering the SAP variables, their evolution was extremely affected by a high activation rate ( P < 0.001). However, it was not related to the level of activation. We observed a greater alteration of the muscle fiber conduction velocity with respect to the nerve conduction velocity and neuromuscular junction transmission. No significant difference in the evolution of the EMG signal was noticed between the three muscular heads studied. Conclusion We suggest that these data can be explained by considering the velocity of motor unit activation i.e., the total number of pulses per minute, as one of the main keys of the fatigue phenomenon during functional electrical stimulation, the reduction of the level of motor unit activation, has poor effects in the control of fatigue.