Body Volume From Underwater Weight example essay topic
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 pul of gravity.
In turn, the object is said to lose weight in water. Because the objects 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) estimates of residual volume in healthy men and women. In addition, the excellent reliability (r = 0.99) of the oxygen dilution technique was confirmed in additional samples of young men (n = 195) and women (n = 102). Thus, for normal healthy adults, the technique gives valid, reliable, and accurate estimates of residual volume (Roche, 12). This also makes the whole test more valid, reliable, and accurate. Personally, as an athlete, I have been subjected to numerous body composition test including skinfold test with calipers, hydrostatic weighing, and the impedance system.
I feel that hydrostatic weighing is still an excellent method of determining body composition, but will either be updated or outdated by technology in the future. It is a valid, reliable, and accurate test to use in place of the impedance test when the necessary equipment is not available. The skinfold test, in my opinion, is the worst test (along with circumference testing) to use because it is determined by set standards and obviously everyone is not the same. Which test would I use instead I would use the impedance test if the equipment were available. 330.