Specific resistivity used to estimate fat-free mass from segmental body measures of bioelectric impedance.

This report provides estimates of specific resistivity for a sample of 123 children and adults aged 9-62 y. The mean muscle-specific resistivities for the upper and lower extremities (arm and leg) and trunk of these participants were used to compute estimates of fat-free mass (FFM) from the sum of the corrected total muscle conductive volumes of the arms, legs, and trunk. Mean values for predictions of FFM from S2/Z or from the product of total muscle conductive volume and the density of FFM were not significantly different from means of FFM estimated from body density by underwater weighing. This analysis demonstrates that an estimate of FFM can be obtained by summing the conductive muscle volumes of each body segment derived from the corresponding segment lengths squared, measures of bioelectric impedance, and mean muscle-specific resistivities. These mean estimates of FFM are not as accurate as mean predictions of FFM from S2/Z in regression equations.