Methods for Assessing Body Composition
Numerous techniques have been used to estimate body composition. None of the methods currently used actually measure %BF; the only way to truly measure the volume of fat in the body would be to dissect and chemically analyze tissues in the body. The techniques routinely used to estimate %BF are based on the relationship between %BF and other factors that can be accurately measured, such as skinfold thicknesses or underwater weight. Because of the predictable relationship between the measured value and body composition, %BF can be estimated through these indirect methods.
Each of the techniques described in the following sections has advantages and disadvantages. Knowing these characteristics will help you decide wisely when choosing the method for body composition assessment. A comparison of important considerations can be found in table 8.2. For fitness professionals, ease of measurement, relative accuracy, and cost are the primary considerations when choosing a technique. In other situations (for research or clinical applications), the accuracy of the measurement may outweigh other considerations.
Anthropometric Methods: Body Mass Index
A widely used clinical assessment of the appropriateness of a person’s weight is the body mass index (BMI), or Quetelet index. This value is calculated by dividing the weight in kilograms by height in meters squared.
BMI is a quick and easy method for determining if body weight is appropriate for body height. Table 8.3 lists the adult BMI categories. In the past, height–weight charts were used for this purpose, but BMI is currently the accepted method for interpreting the height–weight relationship. BMI does not differentiate between fat and fat-free weight, which is problematic when testing athletic individuals with a large lean mass. For example, a football linebacker who is 6 ft 2 in. (1.9 m) and weighs 220 lb (100 kg) is considered overweight according to BMI standards (BMI = 28.3 kg · m−2), when in fact he may have a very low %BF. On the other hand, an inactive person with a similar height and weight is probably carrying excess adipose tissue.
Even with its limitations, for most adults there is a clear correlation between elevated BMI and negative health consequences (26). The recommended BMI range is 18.5 to 24.9 kg · m−2. Overweight is classified as a BMI of 25 to 29.9 kg · m−2, and a BMI of 30 kg · m−2 or higher is considered obese (1, 26). In screening situations where estimating body fat is impossible or impractical, BMI can be useful for providing feedback to people about the appropriateness of their body weight. It is important to note that BMI standards for children are age and sex specific (28, 29). Children at the 95th percentile or greater for their age and sex are considered obese. Consult the CDC growth charts for BMI standards for children (29).
Several girth measurements (body and limb circumferences) are used to either estimate body composition or describe body proportions. Girth measurements provide quick and reliable information. They are sometimes used in equations to predict body composition and may also be used to track changes in body shape and size during weight loss. The major disadvantage is that they provide little information about the fat and fat-free components of the body. For example, a bodybuilder’s thigh can have a larger circumference yet less fat than that of an individual who is obese. A list of several commonly measured girths follows; refer to the Anthropometric Standardization Reference Manual (22) for additional circumference sites.
- Waist—narrowest part of the torso between the xiphoid process and the umbilicus
- Abdomen—circumference of the torso at the level of the umbilicus
- Hips—maximal circumference of the buttocks above the gluteal fold
- Thigh—largest circumference of the right thigh below the gluteal fold
WHR is a frequently used clinical application of girth measurements. This value is often used to reflect the degree of abdominal, or android-type, obesity. A WHR greater than 0.95 for young men or 0.86 for young women is associated with elevated health risks, while for men and women 60 to 69 yr old, a WHR of >1.03 and >0.9, respectively, is linked with disease risk (30).
Waist circumference (WC) alone can also provide valuable information about disease risk (13, 17, 26). A WC of greater than 102 cm (40 in.) in men or 88 cm (35 in.) in women significantly increases the risk of obesity-related disease (1). ACSM now uses BMI and WC as the techniques for classifying obesity for risk stratification (1).
When assessing girths, use the following procedures to standardize the measurements:
- Make sure the measuring tape is horizontal when measuring trunk circumference and is perpendicular to the long axis of the limb when measuring limbs. Use either a mirror or an assistant to help ensure that the tape is placed properly.
- Apply constant pressure to the tape without pinching the skin. Use a tape measure fitted with a handle that indicates the amount of tension exerted.
- When measuring limbs, measure on the right side of the body. Alternatively, measure on both sides and record the values for both right and left.
- Ensure that the person stands erect, relaxed, and with feet together.
- When measuring girths of the trunk, take the measurement after the person exhales and before the next breath begins.
Measuring skinfold thickness is one of the most frequently performed tests to estimate %BF. This quick, noninvasive, inexpensive method can provide a fairly accurate assessment of %BF. The value obtained by skinfold measurement is typically within 3.5% of the value measured with underwater weighing (30). Skinfold measurement is based on the assumption that, as a person gains adipose tissue, the increase in skinfold thickness is proportional to the additional fat weight.
Because of the widespread use of skinfold measurement, fitness professionals should master the skills involved. Accurately assessing skinfold thickness requires the correct performance of several steps: locating the skinfold site, pinching the skinfold away from the underlying tissue, measuring with the caliper, and choosing the proper equation. The following sections address each of these concerns.
Locating the Skinfold Site
It is critical to accurately determine the site of the skinfold measurement. To increase the accuracy of the measurement, especially for the inexperienced technician, the site should be located and then marked with a washable marker. This helps ensure that the calipers are placed in the correct position each time the skinfold is measured. All skinfold measurements should be taken on the right side of the body unless otherwise specified. Refer to table 8.4 and figure 8.1 for some of the most commonly used measurement sites. For a more complete description of determining skinfold sites, refer to the Anthropometric Standardization Reference Manual (22). Measuring skinfolds immediately after exercise should be avoided because exercise can shift fluid volume, leading to inaccurate results.
Pinching the Skinfold
Once the correct location for the skinfold measurement is determined, the tester gently but firmly pinches and lifts the skinfold away from the underlying muscle in order to measure it. The following are guidelines for measuring skinfolds correctly:
1. Place the fingers perpendicular to the skinfold approximately 1 cm from the site to be measured.
2. Gently yet firmly pinch the skinfold between the thumb and the first two fingers and lift away from the underlying tissues. Place the jaws of the caliper perpendicular to the skinfold at the measurement site. The jaws of the caliper should be halfway between the bottom and top of the fold. Maintain the pinch while taking the measurement.
3. Read the measurement on the caliper 1 to 2 sec after the jaws contact the skin.
4. Wait at least 15 sec before taking a subsequent measurement. To allow time for the fold to return to normal, take one measurement at each site and then repeat measurements. If the second measurement varies by more than 1 to 2 mm, repeat the measurement a third time.
Measuring the skinfolds of individuals who are obese can be difficult, if not impossible. If the jaws of the caliper will not open wide enough to measure the skinfold, use an alternative method for assessing body composition. Girth measurements for predicting %BF (33, 34) along with BMI, WC, and WHR may be used instead.
Measuring With the Caliper
Skinfold thickness is measured with a skinfold caliper. The numerous commercially available calipers vary in price and accuracy. Lange and Harpenden calipers are the ones most often used in research settings because of their precision and reliability; however, other calipers may also be used effectively (2). Obviously, if the calipers do not measure skinfolds accurately, they will compromise the estimate of body fat. Measure with calipers that closely match those used in the development of the equation you are using.
Choosing the Proper Equation
Most skinfold equations were developed using underwater weighing as the criterion method and actually are designed to estimate body density. To develop skinfold equations, the body density of many people was measured (typically using hydrostatic weighing), and these values were compared with skinfold thickness through a statistical method called regression analysis. This statistical technique results in the development of an equation that reflects the relationship between skinfolds and body density. Inserting a client’s skinfold measurements (and sometimes other information such as age) into these equations produces an estimate of the client’s body density. Body density then is converted to %BF by using a two-compartment model equation such as the Siri equation, described later in this chapter.
Both generalized and population-specific skinfold equations have been developed (15, 30). Generalized equations estimate body composition in groups of people who vary greatly in age, body composition, and fitness. An advantage of these equations is that they can be used to estimate body composition in most people; however, the equations lose accuracy when testing individuals who are dissimilar to those used to develop the equations. These equations are also less accurate for people at either end of the fatness continuum.
Population-specific equations predict body composition in a particular subgroup of the population, such as female runners. The advantage of using population-specific equations is that they tend to have higher accuracy when testing people who fit the physical profile of those in the subgroup of interest.
Because sex influences the areas where fat is stored, separate skinfold equations for men and women have been developed. The Jackson and Pollock (14) equations for men and the Jackson, Pollock, and Ward (16) equations for women are generalized equations that are used widely. Note that the client’s age is used in these equations. This is because the relationship between total body fat and subcutaneous fat changes with age; as a person ages, proportionally less fat is stored subcutaneously. Equations from these authors that require three or seven skinfold sites are listed in the sidebar Equations for Estimating Body Density From Skinfold Thicknesses. In addition, tables 8.5 and 8.6 provide quick references for estimating body fatness from skinfold thicknesses for men and women. To use these tables, total the sum of your client’s skinfolds (chest, abdominal, and thigh sites for men; triceps, suprailiac, and thigh sites for women) and locate the corresponding value in the far left column. Then, locate the client’s age in the top row. The intersection of the row and column is the client’s estimated %BF.
Skinfold measurements are a quick and relatively accurate method for estimating %BF; however, care must be taken in making these measurements if the values are to be reliable. Girth measurements, particularly WHR and WC, can be useful in assessing risk of obesity-related disease, and BMI is useful for classifying individuals into overweight and obese categories. The recommended BMI range is 18.5 to 24.9 kg · m−2.
The density of an object is defined as the ratio of its weight to its volume (Density = weight ÷ volume). Two common procedures that estimate body composition based on densitometry are hydrostatic weighing and air displacement plethysmography. The foundation of these techniques is that various types of tissues in the body have different and consistent densities. Fat tissue has a density far less than either muscle or bone. Each of these techniques results in the assessment of total body volume and subsequently the calculation of body density.
Once the body density is calculated, it must be converted into %BF. To make this conversion, a two-compartment model is used. In a two-compartment model, all body tissues are classified as either fat or fat free. One of the most commonly used equations for this procedure is the Siri (32) equation: %BF = (495 ÷ Db) − 450. In this model, the fat-free portion of the body is composed of all tissues except lipids and is assumed to have a density of 1.1 kg · L−1. Fat is assumed to have a density of 0.9 kg · L−1. It has been suggested that the inherent error (caused by variations in hydration or bone density) of this method is 2% to 2.8% in young Caucasian adults (20).
Fat density is fairly consistent among individuals; however, there are situations in which the density of the fat-free body is different from the assumed 1.1 kg · L−1. For example, if a person’s bone density is different from the standard used by Siri, then the assumption that the fat-free body density equals 1.1 kg · L−1 becomes invalid. This is the case for African Americans, who typically have a higher bone density than their Caucasian counterparts, and Schutte and colleagues (31) proposed that a different equation be used to calculate %BF for African American men. Many equations have been proposed to convert body density into %BF. Some of these are listed in table 8.7. For more equations and information on converting body density into %BF, see ACSM’s Guidelines for Exercise Testing and Prescription (1) and Heyward and Wagner (12). Researchers sometimes use techniques more advanced than two-compartment models. Although these techniques are currently impractical for nonresearch settings, they are briefly described in the sidebar Body Composition Techniques in Research Settings: Multicompartment Models.
Hydrostatic (underwater) weighing is one of the most common means for estimating body composition in research settings and is often used as the criterion method for assessing %BF. A criterion method provides the standard against which other methodologies are compared. In this procedure, the participant is submerged in a tank of warm water and then exhales fully while technicians record the body mass (figure 8.2). The body mass while submerged and the body mass on land are used to calculate %BF.
Hydrostatic weighing is based on Archimedes’ principle, which states that a submerged object is buoyed up by a force equal to the volume of water it displaces.