What Is Hydrostatic Underwater Weighing

Woman taking body fat test (hydrostatic weighing), underwater view
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Hydrostatic underwater weighing, or hydrostatic testing, is a method of determining body composition (body fat to lean mass). It measures a person's total body density using Archimedes' Principle of displacement. Hydrostatic underwater weighing has been considered the gold standard for body composition assessment. However new, more sophisticated methods may make underwater weighing obsolete in the near future.

Hydrostatic Underwater Weighing Principles

Underwater weighing is based upon Archimedes Principle which states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object. You can use this principle to determine a person's percentage of body fat because the density of fat mass and fat-free mass are constant. Lean tissue, such as bone and muscle, is denser than water, and fat tissue is less dense than water. Basically, muscle sinks and fat floats. Therefore, a person with more body fat will weigh less underwater and be more buoyant. Someone with more muscle will weigh more underwater.

Hydrostatic Underwater Weighing Method

To perform underwater weighing, a person is first weighed on dry land. Next, the person will get into a large tank of water. While sitting on a special scale, he is lowered underwater and asked to expel all the air from his lungs and remain motionless while the underwater weight is measured.

This procedure is repeated three times and averaged.

A special calculation is then used to determine lean weight and fat weight and determine a person's percentage of body fat. By volume, fat weighs less than muscle, and pound for pound, fat, and muscle each have a constant mass and displace a specific amount of water.

Accuracy

This method of body composition analysis is considered to be very accurate and is used to judge whether other methods are accurate. It usually is consistent and so it can be used to measure progress. However, the person administering the test needs to understand the principles and do the calculations correctly.

A source of error in the test is the need to get all air out the subject's lungs when weighing underwater, as any air remaining will make them float and throw off the results. This is one reason that three underwater weights are taken so they can ensure they match.

Where to Get a Hydrostatic Underwater Weighing Test

Many universities offer hydrostatic weight tests and several companies offer testing for the public. Some, such as Body Fat Test provides mobile hydrostatic body composition testing service. The lack of easy access to this testing makes is less than ideal to use, especially since bioelectric impedance measurement is now widely available.

Other Forms of Body Fat Testing

Hydrostatic weighing is considered to be accurate, but there are other technologies being developed that may replace it. One is the Bod Pod, which uses air displacement much as hydrostatic weighing uses air displacement.

Another is dual energy X-ray absorptiometry (DEXA), which measures bone mineral content as well as estimating body fat and muscle.

Bioelectric impedance body fat monitors are widely available for home use and at health clubs. They use a low voltage electric current. However, they are less accurate and dependent on several variables. Using fat calipers and making several measurements is also available and easy to do.

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