Clinical Methods for Evaluating Body Composition
Near Infrared Interactance (NIR) The use of Infrared Interactance (NIR) light to measure fat is not a new technique. The United States Department of Agriculture (USDA) first developed the technique to measure the fat contained in 1 cubic centimeter sections of beef and pork carcasses after slaughter in the early eighties. The methodology is based on the ability of fat tissues to "absorb" more IR light than lean tissue which can then be measured as a change in the infrared level. Age level: age 12 through college level. Reliability, Objectivity, and Validity: The original application of this IR technology was developed on "skinned" carcasses. No research is available about IR penetration through the skin. The actual contribution of the IR wand measurement and input into the height and weight calculations used in the device’s program has also been questioned. Equipment required: The only commercially available unit to predict human body composition is manufactured by Futrex. Since the Futrex device was first marketed for clinical use, many research articles have been published stating that the device is not accurate and is not recommended for clinical use in the assessment of body composition. Procedure: A fiber-optic probe is placed over the bicep and an infrared light beam is emitted. The light passes through subcutaneous fat and muscle and is reflected by bone back to the probe. Although NIR devices are becoming more common in gyms and clubs worldwide, recent studies suggest that this technology needs continued work to merit use. Obviously it is something would not be appropriate for most if not all physical education programs. Still, now you know what it is.
Dual energy X-ray absorptiometry (DXA, previously DEXA) A relatively new technology that is very accurate and precise is Dual Energy X-ray Absorptiometry (DXA). Dual Energy X-ray Absorptiometry is based on a three-compartment model that divides the body into total body mineral, fat free soft (lean) mass, and fat tissue mass. This technique is based on the assumption that bone mineral content is directly proportional to the amount of photon energy absorbed by the bone being studied. DXA uses a whole body scanner that has two low dose x-rays at different sources that read bone and soft tissue mass simultaneously. The sources are mounted beneath a table with a detector overhead. The scanner passes across a person's reclining body with data collected at 0.5 cm intervals. A scan takes between 10-20 minutes. It is safe and noninvasive with little burden to the individual, although a person must lie still throughout the procedure. DXA is fast becoming the new "gold standard" because it provides a higher degree of precision in only one measurement and has the ability to show exactly where fat is distributed throughout the body. It is very reliable and its results extremely repeatable; in addition, the method is safe and presents little burden to the subject. Although this method is not as accurate in measuring the extremely obese and the cost of equipment is high, DXA is quickly moving from the laboratory setting into clinical studies.
Nuclear Magnetic Resonance (NIMR) With Nuclear Magnetic Resonance (NIMR) electromagnetic waves are transmitted through tissues and are absorbed by selected nuclei, which then release energy at a particular frequency or resonance. The frequency characteristics are related to the type of tissue. Computer analysis of the signal can provide detailed images and the volumes of specific tissues can be calculated. A fiber optic probe is connected to a digital analyzer that indirectly measures the tissue composition (fat and water) at various sites on the body. This method is based on studies that show optical densities are linearly related to subcutaneous and total body fat. The biceps is the most often used single site for estimating body fat using the NIMR method. The NIMR light penetrates the tissues and is reflected off the bone back to the detector. The NIMR data is entered into a prediction equation with the person's height, weight, frame size, and level of activity to estimate the percent body fat. This method has become popular outside of the laboratory because it is simple, fast, noninvasive, and the equipment is relatively inexpensive. However, the amount of pressure applied to the fiber optic probe during measurement may affect the values of optical densities, and skin color and hydration level may be potential sources of error. To date, studies conducted with this method have produced mixed results; a high degree of error has occurred with very lean and very obese people; and the validity of a single-site measurement at the biceps is questionable. Numerous sources report that more research is needed to substantiate the validity, accuracy and applicability of this method. All of the aforementioned clinical methods for evaluating body composition require trained administrator, and very expensive equipment. Consequently, these methods are of little value to the physical educator as testing procedures, but at least now you know what they are and that they are available.
Summary People are different‌you might have noticed that the last time you were in the shower‌.they come in all shapes and sizes. Some individuals seem to be all legs with a very small trunk and shoulders. Others have massive shoulders, arms, and chest and legs that make them look like a pair of pliers in shorts. Still others appear to be evenly proportioned. To judge everyone on the same standard, for example, to decide whether someone is overweight or underweight on the basis of a single factor such as height or weight is in the words of Mike Tyson ludicrous. Even more ludicrous is the use of age-height-weight tables as indicators of physical fitness. Although some of these charts and tables allow for basic differences in body framework, they do not account for differences in proportion of muscle and fat. In the past decade or so, a great deal of attention has been directed toward the measurement of body density. From body density, percent fat and lean body weight can then be estimated. This approach is undoubtedly more sound than one that uses just height and weight to evaluate an individual’s fitness level. However, underwater weighing and other
laboratory techniques are impractical for field testing. Generally, the equipment for such testing is extremely expensive. Also, most of this type of laboratory analysis requires trained administrators, and considerable time to conduct. Consequently, estimates of body density are generally made with skin folds, circumferences, and diameters. As with all forms of anthropometry, such measurements require knowledge, skill, and a great deal of practice for results to be reliable and valid. Unfortunately, there are no universal standards for the location of the various body sites, and relatively small differences in location can result in large differences in body density and percent fat predictions. In short, the objectivity and validity of these tests are questionable. Obviously, this is one area in physical assessment were more practical and valid tests need to be developed.