Bioelectrical impedance analysis
type of analysis
Bioelectrical impedance analysis (BIA) is a method for estimating body composition where a weak electric current flows through the body, and the voltage is measured in order to calculate impedance (resistance and reactance) of the body. These measurements can be used to estimate body fat, body water, and muscle mass. The measurements also produce the electrical phase angle of the body.
Principles of measurement
BIA actually determines the electrical impedance, or opposition to the flow of an electric current through body tissues. The impedance of cellular tissue can be modeled as a resistor (representing the extracellular path) in parallel with a resistor and capacitor in series (representing the intracellular path – the resistance that of intracellular fluid and the capacitor the cell membrane). This results in a change in impedance versus the frequency used in the measurement. Whole-body impedance is generally measured from the wrist to the ipsilateral ankle and uses either two (rarely) or four (overwhelmingly) electrodes. In the 2-electrode (bipolar) configuration a small current on the order of 1–10 μA is passed between two electrodes, and the voltage is measured between the same, whereas in the tetrapolar arrangement resistance is measured between as separate pair of proximally located electrodes. The tetrapolar arrangement is preferred, since measurement is not confounded by the impedance of the skin–electrode interface.
As a general principle, if a person is more muscular, the person will tend have more body water, which leads to lower impedance.
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