## 2 Bioelectrical Impedance Analysis (BIA)
Since R is inversely proportional to ICW and ECW, and Xc is directly related to BCM, the vector length shows the tissue hydration status (vector displacements parallel to the major axis of tolerance ellipses indicate changes in hydration status: dehydration with long vectors, hyperhydration with short vectors), while the vector direction (phase angle) shows the BCM (vector displacements parallel to the short axis indicate changes in BCM: malnutrition, cachexia, or anorexia with smaller phase angles; obesity and high muscle mass subjects with larger phase angles) [15; 24]. (Figure 2). The BIVA model does not presume anything about body compartments, weight, geometry, or hydration status. Moreover, since it does not quantify body compartments, it is not subject to the sources of error associated with conventional BIA (errors due to regression models associated with gold standard techniques, cylindrical and homogeneous body assumptions, etc.). Figure 2: Graphical representation and interpretation of the impedance vector \(Z\) by means of the plot R/H–Xc/H. R: resistance; Xc: reactance; H: height; BCM: body cell mass (adapted from [PERSON] et al.) [24]. Bioelectrical Impedance Analysis (BIA) in Sport  
The accuracy of estimating body compartments (FM, FFM, and TBW) by applying conventional BIA in athletes depends on the predictive models used (specific to the sports population or a particular discipline) and on the previously mentioned assumptions, such as constant tissue hydration, a condition that is not always met in these subjects [15; 25]. Therefore, the most appropriate predictive equations for FFM estimation in sports are those obtained from a multicompartment model containing the TBW determination [25; 26]. To date, most predictive models for estimating FFM from BIA have been developed in the general population and are therefore not suitable for athletes, as they tend to overestimate and underestimate FM and FFM, respectively, depending on the athlete [26]. However, if athlete-specific equations are used, and the BIA analysis is performed under ideal conditions following a standardized measurement protocol, this technique could provide BC estimates similar to other methods based on a bicompartmental model, such as hydrostatic weighing or air displacement plethysmography. Recently, a predictive model to estimate FFM in the adult athlete population (men and women from different sports disciplines: basketball, handball, judo and wrestling, karate and takewondo, pentathlon, rugby, sailing, soccer, swimming, tennis, track and field athletics, triathlon, and volleyball) was developed and validated against a four-compartment model (R\({}^{2}\) = 0.94; concordance correlation coefficient = 0.94) [25]. This formula allows estimating accurately the FFM for all types of athletes, regardless of their somatotype or sport categorization. The same research group previously developed specific equations for estimating TBW and ECW in male and female athletes and in 15 different sports, validating the models against isotope dilution [27]. Over the last few years, the application of BIVA to the study of BCM (body cell mass), hydration, and changes in these variables during training or exercise has been intensively studied because of the limitations of conventional BIA in quantifying TBW or BC and the lack of specific predictive models for athletes [28; 29; 30]. It is important to re-emphasize that BIVA directly analyzes electrical variables and is not based on predictive models; therefore, it is not subject to errors or noncompliance with conventional BIA assumptions. Since impedance vector migration shows changes in TBW and the relation ICW/ECW (determined by the phase angle), some studies have shown that BIVA allowed the detection of the dehydration produced by intense exercise, as well as a more efficient posterior rehydration method (changes in body mass or respiratory and urinary/fecal water losses) than the usual method [30; 31; 32].