Impedance is the frequency-dependent opposition of a conductor to the flow of an alternating electric current. A measure of impedance (Z) is composed of the sum of two vectors, resistance (R) and reactance (Xc), measured at a particular frequency and is described mathematically by the equation Z² = R² + Xc². Resistance is the opposition of a conductor to the alternating current, and as the electric current travels through the body, resistance is basically the same as in nonbiological conductors (Kay et al., 1954; Nyboer, 1959). Reactance is produced by the additional opposition to the current from the capacitant (storage) effects of cell membranes, tissue interfaces, and structural features (Baker, 1989; Barnett and Bagno, 1936; Schwan and Kay, 1956). The occurrence of these capacitant effects produced by the bilipid cell membrane reaches a peak and then declines as the current changes from low to high frequency. The phase angle (see Figure 7-1), which has been found to be important for describing relationships between bioelectrical impedance and the body and for measuring physiological variables (Baumgartner et al., 1988; Lukaski and Bolonchuk, 1987; Subramanyan et al., 1980), is expressed in degrees as the arctangent of the ratio of Xc/R and changes with changes in the frequency of the current.

Early studies of bioelectrical impedance focused on the meaning of impedance measures in relation to the water and electrolyte content of the body and to physiological variables such as thyroid function, basal metabolic rate, estrogenic activity, and blood flow in human and animal tissues (Barnett, 1937; Lukaski, 1987; Spence et al., 1979). These explorations developed into some of the

FIGURE 7-1 Impedance plot curve of resistance and reactance with frequency. Z, impedance; R, resistance; Xc, reactance; Hz, hertz; kHz, kilohertz. SOURCE: Chumlea and Baumgartner (1990), used with permission.