When a sufficiently strong external electric field is applied to a nerve via a pair of electrodes, depolarization of the axon will occur. If depolarization occurs with sufficient intensity and speed, the membrane will reach threshold and an action potential will fire and propagate bidirectionally. The number of nerve fibers activated during applied stimulation will be related to the amount of phase charge delivered with each pulse (Adams et al., 1993).
Motor units are activated electrically by depolarization of motor axons, or terminal motor nerve branches. Electrical current can directly depolarize muscle fibers, but the amount of current necessary for this to occur is considerably greater than that for depolarization of nerve axons (Crago et al., 1974). Therefore for practical purposes, FES systems stimulate nerves, not muscles. Unlike the normal physiological recruitment order of motor units that follows the size principle, electrically induced recruitment order is neither reversed nor predictable. The number and type of motor units activated depends on motor unit size, the distance of each motor unit from the stimulating electrode, and the relative impedance of intervening tissues (Knaflitz et al., 1990). There has been some literature on electrical stimulation of denervated muscle, but clinical applicability remains controversial. Tetanic contraction can be achieved in denervated muscle so long as long duration and high amplitude stimulation is used via very large surface electrodes (Kern et al., 1999, 2002).
Similar to muscles undergoing voluntary exercise, FES-stimulated muscles will change morphologically and physiologically. Type II glycolytic fibers will convert to Type I oxidative fibers over weeks to months, depending on the intensity and frequency of stimulation. This phenomenon is associated with changes in vascular supply and increased fatigue resistance (Munsat et al., 1976). It is still not clear how much FES "exercise" is adequate to achieve fiber changes and allow for functional use.
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