Hydrostatic weighting

Hydrostatic (underwater) weighing is used to measure body composition and percentage body fat. This is occasionally done in the assessment of obesity. Is based on Archimedes' principle. The residual volume in the lungs can add error to ones calculations if it is not measure directly.

Underwater weighting gives body density. Percent body fat can then be estimated using the Siri or Brozek formula.

Procedure

*Weigh the subject to the nearest 0.1 kg.
*Measure or estimate the subject's Residual Volume (RV). If possible, directly measure RV using the diluted helium or oxygen techniques. Otherwise, RV can be estimated by measuring vital capacity (VC) and multiplying it by 0.24 or 0.28 for males and females, respectively.
*Tare (zero) the scale in the underwater weighing tank. Doing so automatically subtracts the weight of the chair from the subject's underwater weight. While seated in the tank, instruct the subject to blow out all the air that he/she can exhale, bend slowly forward until the top of his/her head is underwater, and to remain motionless until the scale has settled and the computer indicates that it has a reading. After the subject has settled underwater, it takes the computer approximately 5-10 sec to determine the weight. Have the subject perform 6-10 trials and average the heaviest 2-3 readings.

Equations

Determination of body density

Body Density = dry weight / [((dry weight - wet weight) / water density)- RV - 0.1]

= dry weight / (Body Volume - RV - 0.1)

Note: Units for all weights are in kg. Body Volume and RV are in L. Body Volume is the amount of water that is displaced by submerging body in the water completely and blowing out all air from the lungs. The 0.1 represents an estimated volume (L) of gas in the GI tract.

Estimation of Percent Fat

The two most commonly used equations for estimating percent fat from body density are the Siri (1961) and Brozek (1963) formulae. A limitation to these formulae is that they assume the density of fat-free mass to remain a constant across the population when in fact is varies. Thus, the actual percent fat tend to be slightly higher than the measured percent fat in the lean, muscular individual and the opposite effect in obese individuals.

Siri formula

Percent Fat = [(4.95 / Body Density) - 4.50] x 100

Brozek formula

Percent Fat = [(4.570 / Body Density) - 4.142] x 100

ee also

*Body composition
*Composition of the human body