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For decades, hydrostatic weighing has been the most common technique used in determining body composition in exercise physiology laboratories. In essence, a person's regular weight is compared with a weight taken underwater. Because fat is more buoyant than lean tissue, comparing the two weights can determine a person's percent of fat.
In addition to requiring a considerable amount of time, skill, space, equipment, and a well-trained technician, this technique has several drawbacks:
First, because each individual assessment can take as long as 30 minutes, hydrostatic weighing is not feasible when testing a lot of people.
Furthermore, the person's residual lung volume (amount of air left in the lungs following complete forceful exhalation) should be measured before testing. If residual volume cannot be measured, as is the case in some laboratories and health/fitness centers, it is estimated using the predicting equations-which may decrease the accuracy of hydrostatic weighing.
Also, the requirement of being completely under water makes hydrostatic weighing difficult to administer to aquaphobic people. For accurate results, the individual must be able to perform the test properly.
Finally, forcing all of the air out of the lungs is not easy for everyone but is important to obtain an accurate reading. Leaving additional air (beyond residual volume) in the lungs makes a person more buoyant. Because fat is less dense than water, overweight individuals weigh less in water. Additional air in the lungs makes a person lighter in water, yielding a false, higher body fat percentage.
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