For years, hydrodensitometry or underwater weighing has been regarded as a gold standard for body composition analysis. This laboratory technique is based on Archimedes' principle, which states that the volume of an object submerged in water is equal to the volume of water the object displaces. Additionally, it assumes that the density and specific gravity of lean tissue (muscle and bone) are greater than those of fat tissue. Therefore, lean tissue will sink and fat tissue will float.
In order to perform hydrodensitometry the subject is first placed in a temperature-regulated tank or pool and submerged. After complete exhalation the subject is weighed underwater on a suspended chair or frame for approximately 10-15 s. Archimedes' principle is applied by comparing the mass of the subject in air to the mass of the subject in water. Of note, corrections for the density of water corresponding to the water temperature are made. Because body density is simply a ratio of body mass to body volume it can easily be calculated from the above measurements. Once body density is known, percent body fat can be easily estimated by one of two different equations . Because the density of fat-free mass is known to differ with age, gender, ethnicity, level of body fitness and activity level, population-specific formulas for the conversion of body density to %BF have been developed . Overall test-retest reliability for hydrodensitometry has been reported to be good (r = 0.99) .
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