Bioimpedance is a simple method permitting to measure FFM as these tissues are electrically conductive due to ions contained in body fluids, while FM is not conductive. Measurements of tissue resistances R were initially made in supine position with medical impedance meters using four non reusable adhesive electrodes and were not invasive. The normal procedure was to place one current electrode on the right hand, a voltage electrode on the right wrist, a distal current electrode on the right foot and a voltage electrode on the right ankle at about 6 cm from current electrodes. The right side was selected to avoid the current to pass through the heart. A large variety of medical impedance meters is now available and several have multi-frequency electronics, in order to measure both ECW from the resistance at low frequency (5 kHz) and TBW from the resistance at high frequency (50 kHz or more). A good example is the Xitron Hydra 4200 (Xitron Technology, San Diego, USA) which operates at 50 frequencies from 5 to 1000 kHz and permit bioimpedance spectroscopy (BIS) and uses standard rectangular electrodes 1 cm wide and 8 cm long. It is calibrated by a phantom. Since actual DXA are considered as reference measurements for FFM, the comparison with DXA data permits to obtain equations giving FFM as function of resistance, body height, weight, sex and, in some cases, age. The body weight is measured by a body scale and subject height by a wall mounted scale. But this method is empirical and may not be accurate in subjects with abnormal morphology.
The body resistance measured is the sum of right arm and right leg resistances, plus the trunk longitudinal resistance. Bioimpedance analysis (BIA) devices use a 50 kHz frequency, while impedance meters measuring resistance of body fluids use several frequencies from 5 kHz to 1 MHz to measure independently ECW(at low frequency) and TBW (at high frequency) . The advantages of these impedance meters are that their electrodes have a standardized shape and area and are placed at well defined positions. BIA devices can use equations from the literature obtained at 50 kHz . Their drawbacks are that they are expensive and their measurements take about 10–15 min for placing electrodes and waiting for fluid equilibrium in supine position.
An important innovation in 1996 was the development of foot-to-foot impedance meters (FFI) consisting in a body scale equipped with two current and two voltage reusable metal electrodes and a software to determine FFM or FM from the same parameters as medical devices. Measurements, taken in standing position, are faster than with medical devices and benefit from body weight supplied by the scale. Main companies are Tanita (Tokyo, Japan) and, in France, Tefal (SEB, Rumilly). Due to their large diffusion in the general public, their prices were low, between 70 and 110 €.They are powered by1.4 V batteries delivering generally a 0.8 mA current. Body resistance measured by FFI is smaller than that measured in supine position by a medical impedance meter as leg resistance is lower than that of arms. In standing position, fluid accumulates in the calves and forearms due to gravity, decreasing limb resistance.FFI only measure the resistance of the lower body, consisting in both legs and waist width, representing one-third of the body, while the trunk represents half the body and only 20% of the total body resistance. But, with appropriate software, they can be at least as accurate as medical impedance meters, since they avoid the variability of arm resistances.
Another interesting event was the introduction in 2003 of eight electrodes impedance meters, such as the Tanita BC 418, consisting in a FFI to which two current and two voltage electrodes, connected by electrical cables, have been added for the hands. It can measure automatically the resistance of five current lines connecting hands and feet and extract the resistance of each limb and the trunk by solving a system of ive equations. Of course, it measures also whole body resistance, FFM and FM. Two Korean companies commercialize multi-frequency eight-electrodes FFI, the InBody 720 (Biospace, Seoul) and the X- Scan (Jawon, Kyungson). Hand electrodes are mounted on a horizontal rod fixed to a vertical column together with a screen to display body composition data. These professional devices are, of course, much more expensive than normal FFI, but Tanita Co commercializes several models of eight-electrodes devices, which cost between 200 and 300€.