Review on fatigue in muscles by functional electrical stimulation

This review deals with fatigue of electrically activated muscle. Special reference is made to paralyzed muscles activated by functional electrical stimulation (FES). These muscles differ from normal muscles in the following main respects: (I) activation is by the simultaneous firing of all motor units; (2) easily fatiguable fibers are recruited at low threshold; (3) fatigue is of peripheral nature; and (4) sensory feedback is lacking. The consequence is that fatigue in FES develops more rapidly and the force decline is steeper when compared with muscles of healthy individuals. Several factors affect fatiguability in FES, including stimulation parameters, particularly frequency, pattern of stimulation, methods and electrodes of stimulation, and training status of the muscle and task to be performed. Feedback approach alone is not sufficient to adequately control the muscle force without information on the muscle dynamics and on the progressively developing fatigue. Measured changes in phosphometabolites and in myoelectric activity have been attributed to the failures of the contractile machinery and of the excitation-contraction coupling mechanism, respectively. A complete description of the dynamics of muscle fatigue in FES should thus combine muscle properties together with the myoelectric and metabolic status of the muscle.