Neural and muscular mechanisms of electrically induced fatigue in patients with spinal cord injury

Study design: Intervention study. Objectives: The present study aimed at examining whether spinal and/or peripheral alterations are in the origin of neuromuscular fatigue development induced by intermittent neuromuscular electrical stimulation (NMES) in subjects with complete spinal cord injury (SCI). Setting: Neurological Rehabilitation Center CMN Propara, Montpellier, France. Methods: Thirteen volunteers with complete SCI participated in the study. The right triceps surae muscle was fatigued using a 30-Hz NMES protocol (2 s ON-2 s OFF) composed of three series of five trains. Spinal excitability (assessed by the H-reflex), muscle excitability (assessed by the M-wave), muscle contractile properties (assessed by mechanical response parameters) and torque evoked by NMES were tested before and after each five-train series. Results: NMES-evoked torque significantly decreased throughout the protocol (P<0.001). This decrease was accompanied by a significant increase in M-wave amplitude (P<0.001), whereas H-reflex and the H-max/M-max ratio were not significantly modified. The amplitude of the mechanical response was significantly decreased at the end of the protocol (P<0.05). Conclusion: The results indicate significant fatigue development, which was attributed to impaired cross-bridge force-generating capacity, without modification of spinal excitability nor muscle excitability.