Muscle fatigue in interrupted stimulation: Effect of partial recovery on force and EMG dynamics

Muscle fatigue is a major problem in functional electrical stimulation (FES); the understanding of fatigue and recovery processes is thus of great interest. In interrupted stimulation, fatigue and recovery occur in sequence, and the history-dependency of the muscle's response to FES becomes significant. In this work, the force and electromyographical (EMG) fatigue characteristics of FES-activated paralysed muscles were studied, both in the initially unfatigued state (primary fatigue) and in the reactivated state, after rest periods of prescribed durations (post-recovery fatigue). Because the data were collected over weeks, longitudinal studies were also made to account for long-term training effects of the muscle. Mechanical and myoelectric profiles, the latter derived from the M-wave, were obtained from the right quadriceps of two paraplegic subjects under isometric stimulation. Force was found to correlate highly with peak-to-peak amplitude of the EMG M-wave. Training did not affect this correlation, but as the recovery duration increased, the force-EMG curves became less concave. Training was found to increase the muscle force and EMG peak-to-peak amplitude, as well as the residual force achieved, but it had no noticeable effects on the M-wave duration parameters. Both the force and EMG parameters demonstrated substantial recovery within the first 3 min of rest, and exhibited a consistent tendency to level off for higher periods of rest. After comparing this finding to those expected from previous metabolic models, it was concluded from the subjects studied and model developed that, in addition to metabolic factors, electrolytic factors may be significant in governing the dynamics of fatigue and recovery.