Validity of hand-to-foot measurement of bioimpedance

Nutrition Research Newsletter, April, 2006

Body impedance is directly related to water distribution and hydration of the body. In normally hydrated people, this relationship can be extended to fat-free mass and fat mass. The common and recommended method of measurement of body impedance and that for which many population-specific prediction equations have been validated is for the subject to be in a recumbent position with arms and legs slightly abducted and electrodes on the hand and foot. In the field, it is more difficult to provide the ideal conditions, because subjects can object, feel insecure, or be embarrassed if asked to lie on the floor, ground, or a raised platform, and it may be difficult to provide a wide enough bed or an appropriate place to put the head. It also is not easy physically for some people to lie down and sustain a static position or to rise because of joint, respiratory, or body size problems. The person doing the measurement also needs to be able to help people up and down and kneel on the ground if that place is used. The ambient temperature of the room and the skin can also be a problem, especially because prolonged lying or standing may cause cooling. So that previously validated equations for hand-to-foot bioimpedance measurement of body water may be modified for use in the standing position. The aims of this study were to investigate the effects of posture, time, sex, and age on the relationship between standing and lying resistance, and provide evidence for a more practical and reliable method of measuring hand-to-foot bioimpedance.

In 205 volunteers, six years to 89 years old, 111 males and 94 females from six ethnic groups, effects of posture, time, and age on hand-to-foot resistance were studied over a range of body size. The effect of time in a position on resistance was also recorded in a small subset (n = 10), and repeat measurements over three days at the same time of the day were recorded in another subset (n = 12).

Lying impedance was consistently higher than standing, with the relationship (resistance lying/resistance standing) for the children (five years to 14 years) being 1.031, progressing to a ratio of 1.016 in those more than 60 years old. The time spent static in either position did change resistance measurements. Making sure that the bladder is empty, that food or fluid have not recently been drunk, that the limbs are adequately abducted, that the subject is relaxed, and that the hands and feet are warm will reduce variation and improve accuracy. The researchers also recommend standardizing the activity before the measurement to ensure consistency. When subjects went from lying to standing, there was a small (1 ohm) difference between that measurement and the first standing measurement. Prolonged stasis is known to cause fluid shifts. This would be corrected by gentle physical activity immediately before any bioimpedance measurement, because movement-assisted venous and lymphatic drainage will help restore fluid distribution to a more balanced state.

In conclusion, in the field, measurements of hand-to-foot bioimpedance can be made in the standing position, and, with appropriate adjustment, previously validated recumbent equations can be used. Given that errors in the measurement of height and weight also affect the reliability of the derivation of body fat from bioelectrical conductance, the errors that may arise from a more practical standing measurement rather than lying are minimal.

E Rush, J Crowley, I Freitas, et al. Validity of hand-to-foot measurement of bioimpedance: standing compared with lying position. Obes Res; 14(2):252-257 (February 2006) [Correspondence: Elaine Rush, Division of Sport and Recreation, Faculty of Health and Environmental Sciences, Auckland University of Technology, Akoranga Campus, 90 Akoranga Drive, Northcote, Auckland, New Zealand. E-mail: elaine.msh@aut.ac.nz]