Introduction

Electrical conductivity offers the prospect of a rapid, safe, non-invasive, non-contact method for assessing body composition in humans. It was originally conceived in the USA in 1973 for the rapid assessment of the fat and lean proportions of meat packages, carcasses and livestock. The electronic meat measuring equipment (EMME) was adapted for human measurement in the early 1980s and renamed TOBEC. This operates on the principle that the impedance of a radiofrequency coil is changed when a human body is inserted and the change of impedance is related to the volume of body electrolytes. This was further developed in the mid 1980s to a scanning facility, in which the change of coil impedance as a function of the position of the subject was Fourier analysed to attempt to eliminate the influence of body shape on the estimate of fat-free mass. An alternative approach, named the tissue resonant impedance monitor (TRIM), was pioneered at the University of Swansea, Wales. The change of radiofrequency resulting from the insertion of the subject into a helical coil was related to the subject's lean body mass. This principle has been developed further at Leeds General Infirmary into a scanning facility driven with a free-running oscillator known as electromagnetic resonance (EMR). The average frequency shift that occurred when a subject was traversed completely through the coil was related to the subject's water space (r = 0.87). Subsequent analysis showed excellent correlation between frequency shift and surface area in calibration phantoms (r = 0.998), suggesting that only superficial induction of eddy currents was occurring and that the measurement was therefore relative to water at the surface.