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)
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.
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