Skinfolds

Skinfold calipers, which vary in cost from free to $500, are used to measure a double thickness of the fat layer under the skin. This fat layer (called subcutaneous fat) is hypothesized to represent approximately 50 percent of a person's total body fat. Therefore, if you can get a good estimate of the subcutaneous fat layer, you should be able to predict the total body fat level. The prediction equations commonly used to determine body composition from skinfolds are based on the body composition determinations derived from hydrodensitometry. It works something like this: You measure a group of people using hydrodensitometry to determine their body fat percentages. Then you measure these same people with a series of skinfolds, which are used in statistics to predict the body fat percentage obtained from hydrodensitometry. If the skinfolds, when applied to the newly created equation, can successfully predict the hydrodensitometry value, then you have a skinfold equation for predicting body fat percentage.

A number of different equations are available for the general population, and several equations have been developed specifically for athletes. In general, using an equation that is more specific to the person you've measuring yields more accurate results. Also, equations that use more skinfold measurements are generally more accurate. For instance, one equation may require height, weight, age, triceps skinfold, and abdomen skinfold. Another equation may require height, weight, age, and skinfolds at the triceps, subscapular, midaxillary, suprailiac, abdomen, and midthigh sites.

It's important to say a word about the values that are derived from skinfold equations and used to predict body fat percentage. Many of the equations used for athletes are actually meant for the general (i.e., nonathlete) population. Since many athletes are considerably leaner than the average nonathlete, the results derived from skinfold equations are unrealistically low. Many athletes come to the lab saying they have a body fat percentage of 2 or 3 percent, and I know immediately that these are estimates from equations that have not been normalized on athletes. It's simply not possible to have such a low body fat percentage. When these athletes are given the true value from a more realistic assessment (using either better, more population-specific equations or a more accurate technique), they don't usually respond positively when they receive the new number (usually somewhere between 8 and 18 percent). It's important for you to remember that when skinfold equations are used, the single number you get is not going to be perfectly accurate. However, that number can be used as a baseline to determine change over time if the same technique and same equation are used to get the second value. It is completely inappropriate to compare the first value with one that was obtained using a different set of skinfolds and a different equation.