###### Abstract  
The study of body composition (BC) allows evaluating nutritional status, optimizing competitive performance, and monitoring the success of training regimens in athletes. Since BC reference techniques are expensive and/or invasive, in practice simpler, less expensive, and safer methods such as anthropometry are often used. The problem is that anthropometry requires a qualified anthropometrist, a rigorous measurement protocol using validated equipment, and a lengthy procedure because of its limited accuracy. Bioimpedance analysis (BIA) is an indirect method for estimating BC in a simple, fast, inexpensive, noninvasive, accurate, and reproducible manner. Despite the limitations of conventional BIA, it is still the most commonly used method for assessing BC in athletes. However, bioelectrical impedance vector analysis (BIVA) allows qualitative assessment of body cell mass (BCM) and hydration status without the need for biological assumptions and without being subject to conventional BIA errors. The purpose of this article was to demonstrate the uses and applications of BIA in the study of the BC and physical characteristics of athletes. For example, several practical cases of athletes from different sports were compared. The results obtained from somatotype studies, conventional BIA, and BIVA were included. BIVA interpretation and somatotype were consistent for all cases analyzed, but BIVA also provided information about hydration and BCM. The application of BIVA in the assessment of athletes can improve BC studies in sports because it is fast, simple, and reliable and neither imposes two-compartment-model limitations on BC estimation nor requires any biological assumptions. s +
Footnote †: journal: Applied Sciences
[PERSON] et al. defined BC research as \"a branch of human biology that has three interconnecting areas: body-composition levels and their organizational rules, measurement techniques, and biological factors that influence body composition\". They proposed a five-level model, with each level having different components (compartments), for organizing BC research [5]. However, depending on the BC model used, the main compartments of each level can be combined in different ways. Let us take as an example how the different components are grouped at the molecular level (Figure 1):  
* Two-compartment model: body mass is the sum of fat mass (FM) and fat-free mass (FFM). * Three-compartment model: body mass combines FM, total body water (TBW), and residual1 (i.e., the sum of glycogen (HCO), minerals (M), and protein); it may also include FM, bone mineral (BM), and lean soft tissue (LST). * Four-compartment model: body mass is measured by adding up FM, TBW, M, and residual2 (i.e., the sum of glycogen and protein). The two-compartment model is most commonly used in practice to estimate BC, despite the fact that it assumes some biological considerations that are not always valid, such as stable FFM, hydration, and density. As a result, the estimation of compartments fails under certain specific physiological and pathological conditions (e.g., childhood, growth, elderly, or disproportionate fluid accumulation) [7,8]. The proper use of the multicomponent model is more useful for accurate estimation of changes in body compartments, especially in non-steady-state conditions [9]. An essential aspect of multicompartment models for BC studies is that they do not make biological assumptions; however, they require the use of different and specific instruments [10]. All these models were developed from equations that may include unknown components, known components, and/or measurable properties. Reference techniques such as hydrostatic weighing, dilution techniques, and dual energy X-ray absorptiometry were used to create these regression models. Considering the high cost, inexperience of practitioners, and potential side effects of the reference techniques, this model is not very commonly used in daily practice.