Hydrostatic Weighing (Hydrodensitometry)

Hydrostatic weighing is the classic means of determining body composition. It uses what is known as Archimedes'principle,⁹ which states that for an equal weight, lower-density objects have a larger surface area and displace more water than higher-density objects. From a body composition standpoint, this principle is applied in the following way:

• The subject is weighed on a standard scale to get a"land weight."
• Using specialized equipment, the subject's lung volume is estimated (the subject blows into a tube).
• The subject sits on a chair that is attached to a weight scale.
• The chair and weight scale are positioned over water, and the chair is slowly lowered into the water.
• When the subject is lowered into the water just below the chin, he is asked to fully exhale and completely lower his head into the water to be completely immersed.
• While immersed,"underwater weight"is read off the scale that is attached to the chair the subject is sitting on.

Subjects weigh less in water than out of water because body fat (regardless of the amount present) makes the subject more buoyant. The greater the difference between in-water weight and out-of-water weight is a function of how much body fat the subject has. A very obese subject with a high level of body fat appears lighter in water relative to land weight. Lung volume is measured before taking the "water weight,"and there is an adjustment for the buoyancy that can be attributed to the air in the lungs. To minimize the lung'ir effect, the subject is asked to exhale before full submersion, but there is always some remaining air in the lungs, referred to as residual volume.

Although there is some potential for error with hydrodensitometry related to a person's hydration status and residual volume, this technique is useful for determining the change in body composition over time if the technicians performing the measurements are good at precisely replicating the measurement procedure. It is also a useful means of determining the body composition of a population because the errors associated with the technique are likely to average themselves out over many measurements. However, individuals within that population would never be sure if their personal body composition results were accurate. Good laboratories that do research in body composition have invested a great deal for the equipment needed to accurately do hydrostatic weighing. Further, they also invest in making sure they have highly qualified people to take the measurements.