Early experiments with application of electrical current (McNeal, 1977) demonstrated various effects in humans. Some present applications of externally controlled electrical current are known as functional electrical stimulation (FES). The use of functional electrical stimulation has recently expanded substantially (heart pacing devices, cochlear prostheses, etc). In various motor disorders, it is possible to activate dysfunctional muscles with FES, known also as functional neuromuscular stimulation (FNS). It is now nearly three decades since Lieberson et al. (1961) first proposed usage of one channel surface electrical stimulation to assist the gait of stroke patients. However, the progress in FES for restoration of movements is rather slow. The application of FES to the restoration of gait was first investigated systematically in Ljubljana, Yugoslavia (Gracanin et al., 1966; Vodovnik et al., 1967; Kralj et al., 1980; 1987; Bajd et al., 1982). Currently, FES for gait rehabilitation is used in a clinical setting in several rehabilitation centers (Watters, 1977; Brindley et al., 1978; Merletti et al., 1979; Kawamura, 1982; Marsolais & Kobetic, 1983; 1987b; Petrofsky & Phillips, 1983; 1985; Mizrahi et al., 1985; Hermens et al., 1986; Thoma et al., 1987; Vossius et al., 1987; Andrews et al., 1988; Jaeger et al., 1989; Solomonow et al., 1989b; Stein et al., 1990) and there is a limited, but growing trend for the design of devices for home use. FES systems developed for the upper extremities can restore limited grasp functions in some quadriplegic patients, and these devices are at a similar transitional stage from the laboratory to the home environment (Peckham et al., 1980a; 1980b; 1983; Smith et al., 1987; Hoshimiya et al., 1989; Kilgore et al., 1989).

Functional movements are externally elicited either by direct stimulation of efferent nerve fibres, or by the stimulation of afferent nerve fibres, leading to plysynaptically-mediated, synergistic reflexes (Kralj & Bajd, 1989). For effective design of FES systems it is important to understand the biomechanics of the muscles, the neurophysiological mechanisms involved in normal motor control and the functional status of the injured neuromuscular structures (Vodovnik et al, 1985).