## 2 Bioelectrical Impedance Analysis (BIA)
In this method, two injector electrodes should be located dorsally on the right hand proximal to the metacarpophalangeal joint and on the right foot proximal to the metatarsophalangeal joint, and two voltage detector electrodes should be placed: one on the midline between the prominent ends of the right radius and ulna of the wrist, and one on the midline between the prominent ends of the medial and lateral malleoli of the right ankle. The current injection and detection of electrodes should be 5 cm apart [18]. Some BIA equipment uses a hand-to-hand or foot-to-foot configuration. After the BIA application, the obtained data can be interpreted using two different patterns:  
_Conventional BIA_: TBW and FFM are estimated by predictive models and.When FFM is subtracted from the total weight of the body, FM is obtained. This modality assumes some considerations that are not always true:  
1. The human body (or body segments: trunk, arms, and legs) is considered as a cylindrical conductor with variable length, uniform cross-section, and homogeneous composition. However, it is composed of five cylinders with different lengths and cross-sections (the trunk and four extremities), with each one contributing in a different proportion to total body Z. This assumption is not respected in most cases of anomalies in body geometry (amputated limbs, etc.) [16]. 2. FFM is estimated by assuming that, in a healthy body, hydration is a constant, so TBW is 73.2% of FFM (FFM = TBW/0.732 and FM = Weight-FFM). However, this is not true for certain diseases that present hydration alterations; in addition, FFM hydration changes with age [16, 19]. These limitations necessitate the application of predictive models developed for predicting BC in similar populations (in terms of sex, age, race, physiological situation, pathology, etc.) [17, 20]. This is one of the main reasons why the use and interpretation of the phase angle has been extended during the last years. Phase angle is defined as an angle consisting of the R and Xc vectors, quantified as the arctangent of Xc/R, and expressed in degrees. The phase angle is an indicator of the nutritional and functional status of the elderly, and it provides information about prognosis and mortality [15, 20, 21]. Its interpretation is based on its gross value and does not depend on predictive models. Conventional BIA presents less intraobserver variability compared to anthropometry (intraobserver variability <2 Ohm for R and Xc and <1\({}^{\circ}\) for AF; test-retest variability = 1%) [22, 23], provided that the assumptions previously commented on are respected and the most appropriate estimation models are used in each case. Furthermore, this technique can be used in bedridden patients and does not require specialized personnel or the active cooperation of the subjects. These aspects have contributed to the greater incorporation of conventional BIA into clinical practice and population studies in the last decades. Bioelectrical impedance vector analysis (BIVA or vector-BIA) is a semiquantitative analysis method, because it does not employ mathematical models to estimate (absolutely or relatively) one or more body compartments. Hydration status and body cell mass (BCM) are evaluated by expressing the impedance vector, measured at 50 kHz, normalized to standing height (R/H and Xc/H), in a graph of R/H-Xc/H of the reference population (in terms of age, race, and sex) (Figure 2). This reference graph is made up of three concentric ellipses corresponding to the 50 th, 75 th, and 95 th percentiles of the normal distribution of the values of the reference population (tolerance ellipses). Vectors that fall outside the 75 th tolerance ellipse indicate an abnormal impedance [24]. The impedance vector displacements in the reference diagram provide information about the hydration and BCM of the subject.