Selected Methods for Measuring Body Composition

Hydrostatic Weighing

Hydrostatic weighing, one of the most accurate of the measurement techniques, involves weighing subjects while they are completely submerged in water. Subjects may contribute to optimcal accuracy if they can exhale the maximum amount of air possible from the lungs and can sit still for 6 to 10 seconds while completely submerged. Accuracy is further enhanced if the technician has the equip ­ ment to measure residual air (the amount of air remaining in the lungs after a maximum exhalation).

The equipment required for hydrostatic weighing includes an autopsy scale with a capacity of approximately 8 kg. The scale is suspended over a tank of water that is at least 3 feet deep. The subject sits suspended chin-deep, exhales completely, and bends forward from the waist until entirely submerged. This position is maintained for 6 to 10 seconds to allow the scale to stabilize. From 5 to 10 trials are required, and the underwater weight is attained by averaging the three heaviest readings. The subject’s net underwater weight is calculated by subtracting the weight of the seat, its supporting structure, and a weight belt (if needed) from the gross underwater weight.

Bioelectrical Impedance Analysis (BIA)

Bioelectrical impedance analysis (BIA) is a relatively new and simple method of determining body composition. The equipment is portable, computerized, and expensive, but it is safe, noninvasive, quick, and convenient to use. A harmless electrical current is sent through the supine body via electrodes attached to the right hand and foot. Water is an excellent conductor of electricity, whereas fat, which is essentially anhydrous (lacking \illater), is a nonconductor. Lean body mass includes all tissues of the body except fat, so the two components can be separated. The measurement of electrical conductance or impedance is the tissue resistance to the transmission of an electrical current. The determination of total body water is used to calculate lean weight, fat weight, and percentage of body fat. Bioelectrical impedence accurately measures total body water, but the major source of error with this method is in the conversion of this factor to the estimation of body fat percentage and fat-free mass. This technique is about as valid as skinfold measures, however, it is easier to learn and perform proficiently.

A major limitation is that this technique does not accurately estimate fat-free mass in very lean or very fat subjects. Body fatness is generally overestimated for lean subjects and underestimated in obese subjects.

Skinfold Measurenents

Skinfold measurements are one of the least expensive and most economical methods of measuring body composition. The cost of skinfold calipers ranges from $10 to as much as $450 for computerized models. The most accurate calipers maintain constant jaw pressure of 10 g/mm 2 of jaw surface area.

The thumb and index finger are used to pinch and lift the skin and the fat beneath it. The caliper is placed beneath the pinch. When performed by skilled technicians, skinfold measurements correlate quite well (0.80 or greater) with body density calculated from underwater weighing.

Determining Desirable Body Weight from Body fat

Calculating desirable body weight is a simple procedure when the percentage of body fat is known.

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