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Archimedes` principle is the theory on which this technique for measuring body composition is based. It states that when a body is immersed in water, it is buoyed by a counterforce equal to the weight of the water displaced.
The density of body fat is less than that of water, therefore contributing to the buoyancy of the body, as does air in the lungs. Muscle and bone tissue have a greater density than that of water and can cause a body to sink, therefore an individual with greater fat free mass but the same overall body mass, will weigh more in water due to the higher body density and lower percentage body fat.
Although hydrostatic weighing is considered the "standard" when measuring body composition, there are numerous sources of error associated with the procedure.
The formulas used to convert body density to percent fat assume a density of 0.900 g/ml for the fat mass and 1.100 g/ml for the density of the fat free mass. However the density of the fat free mass varies with age, gender, and ethnicity, use of a single general formula to convert body density to percentage body fat for all populations would therefore result in systematic errors.
Even with flawless measurement technique, hydrostatic weighing results in an error estimate of ±2% body fat due to the individual differences in the density of fat free mass.