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 Hmax/Mmax 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.