Hydrostatic Weighing
Underwater (hydrostatic) weighing, based on Archimedes' Principle, is generally regarded as the "gold standard" for body composition assessment, although this claim is being made less of late because of newer and more sophisticated procedures. Because body fat is less dense than water, it increases one's buoyancy while the fat-free mass, which has a density greater than water, makes one sink. After correcting for residual volume–which increases buoyancy and decreases the underwater weight–percent fat can be calculated based on the underwater weight. The largest source of error in underwater weighing is thought to be the determination of residual volume (RV; the amount of air remaining in the lungs following maximal expiration). When RV is estimated rather than measured, the precision of underwater weighing is little better than anthropometric (skinfold) determination.
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