It is recommended that the sum of the skinfold measurements be determined and used when interpreting results with athletes. A sum of seven or eight measurements is a common calculation. This sum can be compared to normative data that have been established for specific sports. The standard estimate of error for the sum of the skinfold measurements is estimated at about 3 percent when completed using standardized protocols; yet this error is increased to 5 percent when the skinfold measurements are entered into equations to yield percentages (of body fat) due to the inherent error equations introduced into the process. While many athletes really want to know their percent body fat, this can lead to a fixation on the percent number and may result in unhealthy behaviors. Rather, using the sum of the values of the skinfolds can gauge changes and provide useful feedback and does not seem to have some of the qualitative labels (such as “good” or “bad”) often seen when using percent body fat.
Overall, skinfold assessments are one of the most frequently used body composition assessment tools in the field. They have the potential to be reliable when performed correctly, though there is still a margin of error that should be included in interpretation of the data.
Bioelectrical Impedance Analysis (BIA). BIA is another practical tool used in the field for assessing body composition. This is a machine that measures electrical impedance, or opposition to the flow of an electric current through body tissues by placement of electrodes on the hands and feet. BIA measures body composition via assessment of total body water, since muscle has a much higher water content compared to fat and water is a conductor of electricity. While fat is an insulator of electricity, the speed at which the electric current passes from one electrode, through the body, and to the other electrode can be used to estimate water content, thus fat-free mass, of the body. BIA has a wider margin of error than other methods, about 3 to 5 percent when conducted under recommended conditions. While the results are not extremely accurate, they can be reliable for the same individual and can assess changes in body composition over time. Accuracy and reliability of BIA requires individuals to be euhydrated because overhydration and dehydration can change total body water estimations. Dehydration may result in a significant overestimation of body fat and overhydration may underestimate body fat. Additionally, athletes should not have recently consumed a large meal or have exercised before BIA assessment because this too can change total body water; therefore, BIA should probably be completed first thing in the morning, after voiding and prior to training or breakfast or both. Strengths of BIA are that it is noninvasive and portable, it can be completed relatively quickly, and does not require a trained administrator. The machine is relatively inexpensive compared to densitometry equipment, though BIA increases in accuracy with added electrodes (4 or even 8 electrodes on hands and feet), which raises the cost of the machine.
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Densitometry. Underwater weighing and air displacement plethysmography (ADP) are both assessments of body density. Underwater weighing (UWW), also known as hydrostatic testing, is based upon Archimedes principle that the force on an object submerged equals the weight of the fluid that is displaced. Because lean tissue (bone and muscle) is denser than water, and fat mass is less dense than water, percent fat is calculated based upon the underwater weight of an individual. Residual volume, which is the volume of air remaining in the lungs after a maximal expiration, must be corrected for because air increases buoyancy of the body and failing to account for this leads to overestimation of fat mass. Underwater weighing can be uncomfortable for some individuals and the equipment (especially the tank) is fairly expensive, so this method is not always preferred or accessible. Since the UWW method uses equations to calculate fat and fat-free mass that are based on assumptions regarding the density of bone and muscle tissue, body fat results may be underestimated for many strength-trained athletes and overestimated for individuals with osteopenia or osteoporosis (Ackland et al. 2012).
ADP has recently replaced UWW as a more accessible form of densitometry. ADP, commonly referred to by name given by the manufacturer “Bod Pod,” since this is the only commercially available machine measuring ADP, is based upon the same principles as UWW but measures air displacement rather than water displacement. Individuals wear a swimsuit and cap (because clothing and hair displace air thereby reducing one’s estimated percentage of body fat) and sit in a small chamber for a few minutes. Body volume and weight are measured, which allows for calculation of body density; this is then entered into an equation to estimate body fat percentage. The Bod Pod has the advantage of being relatively quick, less invasive than UWW (though individuals prone to claustrophobia may find this procedure very difficult), and is relatively accurate with an error range of 2 to 3 percent, about the same as UWW. Because the machine is quite expensive (~$30,000 to $40,000), not many facilities have a Bod Pod, which can limit the availability of the Bod Pod to some athletes. The equations used to calculate body composition are the same as those used for UWW and therefore have the same limitations as described earlier and are not appropriate for all populations, including certain athletic populations.
Dual Energy X-Ray Absorptiometry. Dual Energy X-Ray Absorptiometry, or DXA, measures bone mineral density and for some time has been used in the diagnoses of osteoporosis. DXA is increasingly being used to assess body composition. Two filtered X-ray beams of different energy levels are passed through the body, which are decreased differentially according to the material it passes through. This information allows assessment of fat mass, fat-free soft tissue, and bone density. DXA has advantages in that it is relatively quick, requires little effort on behalf of the participant, does not require a trained practitioner, and is very precise. Limitations include subjection to small amounts of radiation, cost, and availability (it is usually only found in hospital settings). DXA is not affected by hydration status and can be used any time of day, it is also accurate for all populations regardless of training status or bone density but there is some research to indicate that it may be less accurate when estimating body composition in those who are extremely lean, or excessively small or large (Ackland et al. 2012).
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