## 2 Bioelectrical Impedance Analysis (BIA)  
BIA is an indirect technique for estimating body composition that determines the electrical impedance of a tissue to an alternating current with very low intensity and specific frequency. Electrical impedance (Z) consists of two components: resistance (R) and reactance (Xc). R is defined as the opposition to the flow of an alternating current through the intra- and extracellular ionic solutions. In the model, membranes behave like a capacitor that charges and discharges when current flows. Then, Xc (the inverse of the capacitance) is defined as the delay in the conduction of the current flow produced by cell membranes and tissue interfaces. In way, a lag is generated during the current flow that is expressed as a phase shift called the phase angle (AF) [16; 17]. When a low-frequency current (0-5 kHz) flows, it is distributed exclusively through the extracellular space and Z is the same as R. Medium-frequency currents (50 kHz, most commonly used in single-frequency BIA) pass through some membranes and tissue interfaces. In this way, the current flows through the extracellular space and partly through the intracellular space, such that Z is determined by its two components: R and Xc. Finally, when a high-frequency current (>100 kHz) flows, the capacitances of membranes and interfaces are exceeded, and the current is distributed through all water compartments (total body water) and Z is also the same as R [16]. The estimation of the volume and body compartments of the BIA is based on [PERSON]'s law, which states that the opposition to the flow of an electrical current (Z) through a uniform cylindrically shaped conductor (in this case, the human body or its segments) is directly proportional to the length of the conductor (L) and indirectly proportional to the cross-sectional area of the conductor (A): Z = \(\rho\) L/A (where \(\rho\) is the resistivity coefficient of the conductor). Multiplying the numerator and denominator by L, Z is directly proportional to the square of the length and inversely proportional to the volume of the conductor (multiplying the cross-sectional area and the length). From this ratio, the volume of the conductor can be calculated (V = \(\rho\) L\({}^{2}\)/Z). Obviously, for the human body, L is equivalent to height (H). The application of BIA to body composition analysis involves measuring the Z-value of a conductor (the human body) or its components (R and Xc). Transforming these electrical properties into body composition variables requires a two-compartment model. Total body water (TBW) is the largest component of the FFM and the best electrical current conductor of the human body. The TBW compartment is composed of water placed inside the cells (intracellular water (ICW)) and outside the cells (extracellular water (ECW)). Both are excellent current conductors, and they are separated by cell membranes that act as capacitors as described above. Estimation of body compartments by BIA requires the use of predictive equations. Because of the strong linear relationship that exists between the \"impedance index\" (T\({}^{2}\)/Z) and TBW, all predictive models include it as a variable. In fact, the \"resistance index\" (T\({}^{2}\)/R) is often used instead of the \"impedance index\" because when the current is flowing at 50 kHz, the main component of Z is R (Xc represents a very small percentage of the total Z), so R and Z can be equated. Predictive models typically include other variables such as weight, height, age, sex, Xc, etc. [16, 17, 18]. In habitual practice, the most commonly used BIA is the _single-frequency BIA_ (SF-BIA), an analysis characterized by a fixed low-current AC at 50 kHz flowing through the body. Based on the body part and electrode arrangement, _whole body-BIA_, through the use of a tetrapolar hand-to-foot electrode placement, is the most commonly selected analysis for SF-BIA, MF-BIA, BIVA, and BIS by technicians.