Surface electrodes vary in composition, size, and configuration. Electrode materials that dominate in clinical practice are carbon-silicon and stainless steel or silver fibers-fabric mesh. Conduction through the entire electrode surface and the contact pressure should be as uniform as possible. Non-uniformity of the conductive medium can generate loci of high current density that can cause discomfort, skin irritation, or even burns.
Electrode size should be considered a major factor that can help minimize electrode-skin interface impedance and current density. The relationship between electrode size and impedance is non-linear. Larger electrode size enables stronger muscle force generation and less perceived stimulation discomfort (Alon, 1985; Kantor et al., 1994; Alon et al., 1996). However, the increased size is relative to the contractile mass of the target muscle. Small muscles require considerably smaller electrode sizes than larger muscles. Actual effective size of surface electrodes is not determined by simply measuring the diameter of the electrode because the effective size is influenced by the uniformity of conduction and pressure. Both are likely to change over time and with repeated electrode reuse. Clinicians must be aware of the need to continually ascertain the adequacy of the surface electrode-skin interface. Failure to do so may jeopardize treatment effectiveness, increase skin irritation and possibly cause skin burns (Hasan et al., 1996; Burridge et al., 1997b).
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