Hydrostatic Weighing

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During the past 68 years many laboratory procedures have been developed to analyze the body in relation to its three major structural components: fat, muscle, and bone. Some of the procedures are time-consuming and require the use of sophisticated, expensive laboratory equipment, while other procedures are fairly simple and inexpensive. There are several ways of measuring body fat: measuring skin-fold thickness with calipers, hydrostatic weighing, where a person's underwater weight is compared with his dry-land weight to determine body density, which is inversely proportional to body fat, and the impedance system, which measures the body's resistance to electrical current. The term densitometry refers to the general procedure of estimating body composition from body density. Although several methods can be used to estimate body density, densitometry has become synonymous with underwater weighing, also called hydrostatic weighing. About 2000 years ago the Greek mathematician Archimedes discovered a basic principle that is currently applied in the evaluation of body composition. Through experimentation, he developed the concept of specific gravity, that is, ratio of the weight of an object to the weight of an equal volume of water. By applying this principle (specific gravity = weight of object/weight of an equal volume of water), it is possible to determine the specific gravity of any object merely by weighing the object and then weighing an equal volume of water. However, determining the weight of an equal volume of water presents some difficulty. This problem was solved when Archimedes realized that the volume of water that overflowed his bath was equal to the volume of his submerged body. He also concluded that an object submerged in water must be buoyed up by a counterforce that equals the weight of the water it displaces. This buoyancy force helps to support an object in water against the downward pull of gravity. In turn, the object is said to lose weight in water. Because the object's loss of weight in water equals the weight of the volume of water it displaces, we can redefine specific gravity as the ratio of the weight of an object in air divided by its loss of weight in water (specific gravity = weight of an object in air/loss of weight in water). The hydrostatic weighing method is the most common laboratory method used to measure body composition. The objective is to find body volume, which is used with bodyweight to determine body density. Percentage of fat is calculated from body density. The underwater weighing method is based on the principle for measuring the density of an object. When a person is submerged, the difference between the weight in air (on land) and underwater equals the weight of water displaced. Through several mathematical equations body density, percent body fat (%BF) and fat-free mass (FFM) are determined. The density of lean tissue varies by age, gender, race, athletic conditioning and bone density, among other factors. With hydrostatic weighing, bodyweight, underwater weight, residual lung volume and water density are needed to calculate body density. There are several methodological issues to consider when estimating body volume from underwater weight. These include subject position, residual volume, number of trials and selection criteria, alternative lung volumes, and head placement. The underwater weight depends on the amount of air in the lungs when the person is submerged; the major potential sources of measurement error are the volume of air left in the lungs after expiration (residual volume) and air trapped elsewhere in or around the body. Residual lung volume is often estimated from height-weight charts or measured indirectly in a lab. For most accurate values, the subject should completely exhale while underwater. Using predicted residual lung volume rather than measuring it makes underwater weighing less accurate, increasing the error factor by 1% to 3.5%. Residual volume is also commonly measured using either the closed-circuit approach, "where there is a dilution and eventual equilibration of an inert tracer or indicator gas such as nitrogen, oxygen, or helium" (Roche, 10), or the open-circuit approach where "nitrogen is 'washed out' of the lungs during a specified period of oxygen breathing" (Roche, 10). Direct comparisons between the two approaches "have shown them to be very reliable (r ? 0.97, mean differences = 5 ml) and give very similar (mean difference = 26 ml) and highly correlated (r = 0.96) estimat

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