Also termed pulse width, phase duration indicates how long each phase lasts. It usually ranges between 5 and 400 ^s. Short durations minimize the stimulation discomfort, minimize skin-electrode interface impedance, and reduce the chance of electrochemical skin irritation (the latter in the case of monophasic waveforms only) (Rollman, 1975; Iggo, 1978; Friedli and Meyer, 1984; Baker et al., 1988; Frijns et al., 1994; Gold and Shorofsky, 1997; Mulcahey et al., 1999; Wesselink et al., 1999). As either pulse amplitude or duration is increased above threshold, spatial recruitment of additional motor units will occur. The product of the amplitude and pulse duration represents the net charge injected into the tissue. The charge injected and the force output of the muscle being stimulated is related by an S-shaped curve within which a portion of the curve has a very high gain. This relationship is important to be aware of when attempting gradual proportional recruitment of motor strength.
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