Hydrostatic Underwater Weighing PrinciplesUnderwater 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. We 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, are more dense 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 MethodTo perform underwater weighing, a person is first weighted on dry land. Next the individual 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.
Watch a video of underwater weighing from the University of Vermont.