Body density is defined as an object’s total mass divided by its total volume, typically expressed in grams per cubic centimeter (g/cm³). Since fat and fat-free tissues have different densities, calculating whole-body density estimates the body’s composition. Skinfold measurements are a common, non-invasive field method to indirectly calculate this density. This technique relies on the principle that a significant proportion of total body fat is stored as subcutaneous fat beneath the skin. Measuring the thickness of these skinfolds at specific sites allows mathematical formulas to predict overall body density and body composition.
Standardized Skinfold Measurement Sites and Technique
Accurate skinfold measurements provide the necessary input data for density calculations. The technique involves using specialized calipers to pinch and measure the double layer of skin and the underlying subcutaneous adipose tissue. Measurements must be taken on the right side of the body. The skinfold is firmly grasped by the thumb and index finger and pulled away from the underlying muscle. The caliper jaws are applied approximately one centimeter below the fingers, perpendicular to the fold, and the reading is taken within a few seconds to prevent tissue compression.
Standardization requires precise anatomical landmarks to ensure the same site is measured consistently. The specific combination of sites used depends on the prediction equation selected for the calculation. Common measurement sites include:
- The triceps, measured vertically midway between the shoulder and elbow.
- The subscapular, a diagonal fold just below the shoulder blade.
- The suprailiac, a diagonal fold above the hip bone.
- The abdominal, a vertical fold near the navel.
- The thigh, a vertical fold on the front of the leg midway between the hip and knee.
Mathematical Models for Determining Body Density
The raw sum of skinfold thicknesses is used in a regression equation to predict body density. Different mathematical models exist because the relationship between subcutaneous fat and total body fat varies based on age, sex, and population group. The most widely used models are the Jackson and Pollock equations and the Durnin and Womersley equations, developed through comparison with criterion methods like hydrostatic weighing.
The Jackson and Pollock equations offer flexible three-site or seven-site measurement options. For men, a three-site protocol often includes the chest, abdomen, and thigh, while for women it uses the triceps, suprailiac, and thigh. The seven-site version, which includes chest, axilla, triceps, subscapular, abdomen, suprailiac, and thigh, is more comprehensive. These equations often incorporate age as an additional variable to improve density prediction.
The Durnin and Womersley equations utilize a four-site measurement protocol: triceps, biceps, subscapular, and suprailiac skinfolds. This model uses the logarithm of the sum of the four skinfolds in its regression equation, which helps account for the non-linear relationship between skinfold thickness and body density. A specific formula is chosen based on the subject’s sex and age bracket, reflecting differences in fat distribution patterns. The output is a predicted body density value (g/cm³), estimating whole-body density from measured subcutaneous fat.
Converting Body Density to Body Fat Percentage
The calculated body density value (g/cm³) is not the final metric sought by the public or clinicians. A second mathematical step is required to translate this density into a body fat percentage. This conversion relies on the two-compartment model of body composition, which assumes the body is composed only of fat mass and fat-free mass, each having a known, fixed density.
The two most common formulas for this conversion are the Siri equation and the Brozek equation. The widely accepted Siri equation uses the formula: Percent Body Fat = (495 / Body Density) – 450. The Brozek equation is a comparable alternative: Percent Body Fat = (457 / Body Density) – 414.2. Both equations assume the density of fat is 0.900 g/cm³ and the density of fat-free mass is 1.100 g/cm³.
This step is necessary because the density figure alone does not directly represent a percentage of mass. The formulas translate the deviation of the measured whole-body density from the assumed fat-free mass density into a percentage of fat. The accuracy of the final percentage depends entirely on the validity of the two-compartment model’s assumptions for the individual being measured.
Factors Affecting Measurement Reliability
The accuracy of the skinfold method is subject to several limitations that introduce error into the final body density calculation. The most significant factor is inter-rater reliability, referring to the skill and experience of the technician performing the measurements. Novice measurers can produce results with a higher error rate, particularly for complex sites like the iliac crest and abdomen. This error rate is significantly higher compared to highly trained anthropometrists.
Proper calibration of the caliper is necessary; an instrument that does not exert the standard pressure can lead to systematic under- or overestimation of fold thickness. The subject’s hydration status can also influence the measurement. Dehydration decreases the compressibility of the skin and subcutaneous tissue, which may result in a lower skinfold reading than the subject’s true value.
Population specificity is a challenge because prediction equations were developed using specific groups, often young, Caucasian adults. Applying a generalized equation to a population with different body composition characteristics, such as older adults or different ethnicities, means the underlying assumptions about fat distribution may not hold true. This mismatch can result in a systematic bias in the predicted body density.