Hydrostatic testing, or underwater weighing, has always been considered the "gold standard" in body composition measurement to which all other methods are compared. Effective as it may be, hydrostatic testing it is not without its drawbacks; the primary one is the inconvenience of getting dunked in water.
To get your fat measured hydrostatically, you are submerged underwater while sitting on a chair that hangs from a scale (picture yourself sitting in a giant grocery scale as you're dunked underwater in a tank or swimming pool). The basis for hydrostatic weighing is the fact that fat floats and muscle sinks. The fatter you are, the more buoyant you will be, and the more buoyant you are, the less you will weigh underwater. The leaner you are, the more easily you will sink, and the more you will weigh underwater.
Hydrostatic weighing does have other disadvantages. Several factors can affect the accuracy of the test. For example, African-Americans have denser bones than other races and as a result, may appear to have lower body fat percentages when measured hydrostatically. Also, men usually have denser bones than women, and younger people have denser bones than older people.
Unless race, age, and sex are all carefully taken into consideration, the estimate of body fat could be significantly in error. Another factor that can influence the results of the test is your "residual volume." Residual volume is the amount of air left in your lungs after a complete exhalation. Before being lowered into the tank, you have to blow all the air out of your lungs. If you can't blow every bit of air out of your lungs before being submerged, you will appear to have a higher body fat percentage than you really do.
Hydrostatic weighing is most often performed in hospital and university research centers. It can also be expensive, although at some universities you can often volunteer to be tested by exercise science students for research projects. All things considered, underwater weighing is not very practical, although it's always interesting to go get it done once in a while just for "fun."
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