What is Hydrostatic Weighing & How and Why is it Done?

What is Hydrostatic Weighing?

Hydrostatic weighing is one of the most accurate ways of measuring body fat. It is also known as underwater weighing.

It measures the body’s density using the Archimedes principle of Displacement.(1) During the Hydrostatic weighing test, the bodyweight is measured underwater a well as on the land. Body fat density and body fat percentage are measured by finding the difference between the land and underwater weight.

If performed properly, hydrostatic weighing can be 1.8% to 2.8 % more accurate than other advanced methods to measure body fat.(2) Hydrostatic weighing is done mostly on the athletes.

How Is Hydrostatic Weighing Done?

Archimedes principle states that the amount of liquid displaced when an object is submerged is equivalent to the volume of the object. In hydrostatic weighing, measuring the weight on the land and underwater can help calculate the body’s density.

As fat is lighter than bones and muscles, the density can be used to calculate the body fat percentage.

Hydrostatic weighing is a process with 2 main components: dry land weighing and underwater weighing.

Before the weighing procedure, the person is instructed to not eat or exercise 4 hours beforehand.

To start with the hydrostatic weighing procedure, a person is weighed on dry land that can include your bathroom or a similar place in the testing facility. Then the person is asked to slowly enter a water tank that has an underwater seat hanging from a scale. The administrator instructs to fully submerge in water and to breathe out fully. It is repeated three times.

The average of the three tests is taken to measure the body fat percentage.

Why Is The Hydrostatic Body Fat Test Done?

For a highly accurate measurement of body fat, hydrostatic body fat is measured.

Athletes use this measurement technique to monitor their progress. This method is also used in researches where accurate measurement of body fat is required.

It is not a commonly performed test but is done free of cost by certain gymnasiums and universities.

Pros of Underwater Weighing

The advantages of underwater weighing include:

  • It is more accurate than the method available for body composition testing.
  • It can be easily used in the people with pacemaker
  • There are no side effects of this test.

Limitations of Hydrostatic Weighing:

There are certain limitations of hydrostatic weighing test, such as:

  • It is a time-intensive and labor-intensive procedure
  • It makes participants uncomfortable
  • Cannot be done on the elderly, chronically ill, and disabled population
  • Require specially trained professional set up to perform the procedure
  • Participants should be able to exhale the air completely
  • There is the possibility of miscalculation of residual volume

The test does not identify the exact place of fat deposition in the body

Other Ways To Measure Body Fat:

  1. Skinfold Caliper: It is the simplest way to measure body fat. While performing the test a pair of caliper is used to measure the thickness of the skin folds and the fat underneath. It is done at three or seven different locations on the body. This test is accurate to about 3-4 percent for 70 % of the population.(3) It can be inaccurate if administered by someone inexperienced.
  2. Bioelectric Impedance: Bioelectric impedance analysis is a common feature for body fat measurement. The electric currents estimate body fat on how it moves easily through the body. Fat is a poor conductor of electricity and leads to resistance.
  3. Dual-Energy X-ray Absorptiometry (DEXA): In DEXA, X-rays are used to measure body fat, muscle mass, and bone mineral density.
  4. MRI and CT Scan: These are the most accurate methods of measurement of body fat. But, these methods are limited to medical research and are very expensive.

Hydrostatic weighing is the most accurate way to measure body fat. It is the most commonly used method in researches and with athletes. Although the equipment used to perform this test is expensive, some gyms and university area offer this test free of cost.

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