Bioelectrical impedance analysis (BIA) involves passing a weak, high-frequency current through the body in order to measure variables such as tissue volume, or the volume of blood, on the basis of electrical resistance. Biometric measurements using BIA date back to the beginning of the 20th century; and since then there have been many technical refinements. BIA is non-invasive, it excels in terms of cost and safety, and it does not require any special measurement skills. It has therefore become an outstanding measurement tool for a range of body composition assessments in areas such as large-scale population studies or sports science. There have already been numerous reports of studies of BIA, and its contribution to sports science is well recognized, so that many researchers have demonstrated an interest in using BIA. However, while BIA is easier to operate than other measurement tools, it requires basic knowledge and precise methodology in order to properly interpret the data. This review summarizes the theory of measuring tissues related to exercise, the method of measuring cardiac output, and the method of estimating skeletal muscle mass. First, while the measurement, by BIA, of cardiac output (CO) at rest yields stable results, previous measurements during dynamic exercise had several limitations. CO measurement during intense exercise has recently become possible as a result of improvements to fast Fourier transform (FFT) and the algorithms. Moreover, the water content and fat mass of the body can now be calculated in terms of electrical models in which resistors and capacitors are arranged in series or in parallel, and cells and tissues have been simplified as far as possible. Finally, new estimation equations using single-frequency BIA and multi-frequency BIA have been developed for measurement of skeletal muscle mass in the limbs, and each are reported to have high reliability and high reproducibility. Development of simple, highly accurate measuring instruments and methods for vital observation, utilizing BIA, and that can contribute to sports science, is expected.
