Skin-fold measurements are used to predict the body composition using a regression equation. The equations are generalized as they produce the most accurate results for individuals with average amounts of body fat. While these equations can be used for individuals who are very lean or obese, the results may be less accurate and specific; more accurate equations have been established for these special populations.
Proper Measurement
Making accurate skin-fold measurements is key in obtaining accurate body fat percentages. Individuals are trained to obtain measurements accurately and appropriate use of this technique can determine body fat percentage as precisely as underwater weighing techniques. In order to take a skin-fold measurement the proper site must be identified. The skin is then pinched between the thumb and forefinger half an inch from the measurement site. The skin is lifted from the muscle and calipers are applied. Four seconds should pass between reading the calipers in order to account for the compressibility of fat.
Measurement Sites
Triceps, chest, abdominal, suprailiac and the thigh are measurement sites used for skin-fold testing. The triceps skin-fold is at the back of the arm and should be pinched vertically. The chest skin-fold is taken near the armpit and usually runs vertically. The abdominal skin-fold is taken approximately one inch to the right and a half inch below the bellybutton. This measurement is typically taken horizontally, however some equations call for a vertical measurement. The suprailiac is taken on the side of the body above the hip bone in line with the bellybutton. The angle of this measurement is usually 30 degrees. The thigh measurement is taken vertically halfway between the kneecap and the top of the hip.
Body Density Estimation
In order to determine body fat percentages, body density must be calculated using the skin-fold measurements. There are different equations for males and females, each with a standard error between 3.6% and 3.8 percent. For females, the Jackson, Pollock and Ward equation is most common and uses triceps, suprailiac and thigh measurements. Body density is equal to 1.0994921 minus 0.0009929 times the sum of skin-fold measurements, plus 0.0000023 times the sum of measurements squared minus 0.0001392 times the person's age. For males, the Jackson and Pollock equation is used using the chest, abdominal and thigh measurements. Body density is equal to 1.1092800 minus 0.0008267 times the sum of measurements plus 0.0000016 times the sum of measurements squared minus 0.0002574 times age.
Body Fat Estimation
Taking the body density calculated from using skin-fold measurements it is possible to estimate body fat percentage. There are two common formulas, Siri and Brozek. The Siri formula calculates body fat percentage by dividing 495 by body density minus 450 and multiplying that by 100. The Brozek formula is calculated by dividing 4.570 by body density minus 4.142 and multiplying by 100. There are also population-specific calculations available.
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