Body Composition Essentials
The assessment of body composition involves using the most appropriate, and accessible, method possible to ‘estimate‘ a person’s body composition (an actual assessment of body composition would be by cadaver analysis). One prevalent system of body composition assessment is the two-compartment model introduced by Brozek, Grande, Anderson, & Keys (1963) and Siri (1961) , which assumes the body is made up of fat and fat-free compartments. The terms fat-free mass and lean body mass are often incorrectly used interchangeably. Fat-free mass contains no lipids whereas lean body mass includes approximately 2% to 3% and 5% to 8%, for men and women, respectively (Heyward & Stolarczyk, 1996).
The percent body fat (%BF), referred to as relative body fat, is obtained by dividing the fat mass by the total body weight. The average %BF is 15 for men and 23 for women (Table 1). Obesity, which may be defined as an excessive amount of total body fat for a given body weight, is identified as body fat in excess of 25% for men and 32% for women.

Practical Uses of Body Composition Assessments for Personal Trainers
Heyward and Stolarczyk (1996) suggest the following ways the personal trainer can use body composition measures:
1. To identify the client’s health risk associated with excessively low or high levels of total fat
2. To identify the client’s health risk associated with excessive accumulation of intra-abdominal fat
3. To promote the client’s understanding of health risks associated with too little or too much body fat
4. To assess the effectiveness of nutrition and exercise interventions in altering body composition
5. To estimate ideal body weight of clients and athletes
6. To formulate dietary recommendations and exercise prescriptions
7. To monitor growth, development, maturation, and age-related changes in body composition

Skinfold Method
The accuracy of the SKF method is affected by the skill of the SKF technician, the type of SKF calipers used, the prediction equation used for the measurement, and subject factors (Lohman, Pollock, Slaughter, Brandon, & Boileau, 1984) . An improperly trained technician will introduce considerable measurement error. It is recommended that you practice your SKF technique on 50 to 100 individuals to develop a high level of skill and proficiency (Jackson & Pollock, 1985) . Additionally, it is important to follow standardized guidelines and testing procedures when using the SKF technique. The personal trainer is referred to the Anthropometric Standardization Reference Manual (Harrison et al., 1988) for detailed descriptions in identifying and measuring SKF sites. However, with the popularly used generalized equations by Jackson and Pollock (1978) and Jackson, Pollock and Ward (1980), follow the guidelines presented in Table 2.

In regards to the SKF calipers, plastic calipers have less scale precision, a smaller measurement scale, nonconstant tension throughout the range of measurement, and show less consistency when used by inexperienced SKF technicians, as compared to high-quality calipers (Heyward & Stolarczyk, 1996) . For consistency with your clients‘ measurements, make sure you monitor your their changes using the same calipers.
Most SKF measurements use two to three SKFs to predict body density (Db), from which body fat can be derived using the appropriate equation. Generalized equations, which take into account the effect of age and distribution of body fat, have been developed and validated for persons varying considerably in body fatness (up to 45 %BF) and age (18 to 60 yr) (Jackson & Pollock, 1978; Jackson, Pollock, & Ward, 1980) (Tables 3 & 4). However, for higher precision, SKF prediction equations that take into account a client’s age, gender, ethnicity, and physical activity level are recommended. Heyward and Stolarczyk (1996) provide specific SKF equations based on the factors cited above. Also, to save time, several computer software programs are available to calculate Db and %BF (See Personal Trainer, March/April 1997).

Your clients will have variation in skin thickness, compressibility of adipose tissue, and hydration level, which will affect the SKF measurement. Keep a record of these observations for follow-up assessements. Exercise also increases the extracellular water accumulation in the subcutaneous tissue and thus will increase skinfold thickness. Therefore, SKFs should not be measured immediately after exercise, especially in warmer environments.
Skinfold Measurement Procedures

The following procedures will increase your reliability and precision when doing SKF measurements (Harrison et al., 1988) :
1. Take all SKF measurements on the right side of the body.
2. Carefully identify, measure and mark the SKF site.
3. Grasp the SKF firmly between the thumb and index finger of your hand. The fold is lifted 1 cm above the site to be measured.
4. Lift the fold by placing the thumb and index finger 8 cm (~3 inches) apart on a line that is perpendicular to the long axis of the skinfold. The long axis is parallel to the natural cleavage lines of the skin. For individuals with extremely large skinfolds, the thumb and finger will need to be separated more than 8 cm in order to lift the fold.
5. Keep the fold elevated while the measurement is taken.
6. Place the jaws of the caliper perpendicular to the fold, approximately 1 cm below the thumb and index finger, and release the jaw pressure slowly.
7. Take the SKF measurement 4 seconds after the pressure is released.
8. Open the jaws of the caliper to remove it from the site. Close the jaws slowly to prevent damage or loss of calibration.
9. Take a minimum of two measurements at each site. If values vary from each other by more than 10%, take additional measurements.
10. Take SKF measurements in a rotational order rather than consecutive readings at each site.
11. Take the SKF measurements when the client’s skin is dry and lotion- free.
12. Do not measure SKFs immediately after physical activity because of fluid shifts to the skin.

Final Thoughts
Becoming a highly skilled SKF technicians takes careful practice and precise adherence to measurement procedures. With some obese or heavily muscled clients the SKF method may not work satisfactorily and alternative measures such as bioelectical impedance may be indicated. One of the greatest values for clients, in doing and explaining body composition measures, is the supplementary educational information the personal trainer can provide them about attaining optimal health.

Table 1. Percent Body Fat Standards for Men and Women
Men Women
At riska&Mac178;5%&Mac178;8%
Below average 6-14% 9-22%
Average 15% 23%
Above average 16-24% 24-31%
At riskb&Mac179;25%&Mac179;32%
Chart adapted from Heyward and Stolarczyk (1996) .
aAt risk for diseases and disorders associated with malnutrition
bAt risk for diseases associated with obesity

Table 2. Skinfold Sites for the Jackson et al Generalized Skinfold Equations
Site Anatomical reference Direction of fold Measurement
Chest Axilla and nipple Diagonal Fold is taken 1/2 the distance between the anterior axillary line and nipple for men and 1/3 of this distance for women
Subscapular Vertical border and inferior angle of scapula Oblique Fold is taken on diagonal line coming from the vertebral border, 1-2 cm below the inferior angle
Midaxillary Xiphoid process of sternum Vertical Fold is taken at level of xiphoid process along the midaxillary line
Suprailiac Iliac crest Diagonal Fold is taken diagonally above the iliac crest along the anterior axillary line
Abdominal Umbilicus Vertical Fold is taken vertically 2 cm lateral to the umbilicus
Thigh Inguinal crease and patella Vertical Fold is lifted on anterior aspect of thigh midway between inguinal crease and proximal border of patella. Body weight is shifted to left foot.
Note: Adapted from Jackson and Pollock (1978), Jackson et al. (1980) and Harrison et al. (1988).

Table 3. Generalized Prediction Skinfold Equation for Women (18-55 yr)
Triceps + suprailiac + thigh Body Density (Db) =1.0994921 - 0.0009929 (&Mac183;3SKF) +
0.0000023 (&Mac183;3SKF)2 - 0.0001392 (age)
To convert to %BF using Siri (1961) equation %BF = [(4.95/Db) - 4.50] x 100
Source: (Jackson et al., 1980)
Note:&Mac183;3SKF = Sum of three skinfolds (triceps, suprailiac, thigh)


Table 4. Generalized Prediction Skinfold Equation for Men (18-61 yr)
Chest + abdomen + thigh Body Density (Db) =1.109380 - 0.0008267 (&Mac183;3SKF) +
0.0000016 (&Mac183;3SKF)2 - 0.0002574 (age)
To convert to %BF using Siri (1961) equation %BF = [(4.95/Db) - 4.50] x 100
Source: (Jackson & Pollock, 1978)
Note:&Mac183;3SKF = Sum of three skinfolds (chest +abdomen + thigh)

References:
Brozek, J., Grande, F., Anderson, J. T., & Keys, A. (1963). Densitometric analysis of body composition: Revision of some quantitative assumptions. Annals of the New York Academy of Sciences, 110, 113-140.
Harrison, G. G., Buskirk, E. R., Lindsay Carter, J. E., Johnston, F. E., Lohman, T. G., Pollock, M. L., Roche, A. F., & Wilmore, J. H. (1988). Skinfold thickness and measurement technique. In T. G. Lohman, A. F. Roche, & R. Martorell (Eds.), Anthropometric standardization reference manual (pp. 55-70). Champaign: Human Kinetics.
Heyward, V. H., & Stolarczyk, L. M. (1996). Applied body composition assessment. Champaign: Human Kinetics.
Jackson, A. S., & Pollock, M. L. (1978). Generalized equations for predicting body density of men. British Journal of Nutrition, 40, 497-504.
Jackson, A. S., & Pollock, M. L. (1985). Practical assessment of body composition. The Physician and Sportsmedicine, 13, 76-90.
Jackson, A. S., Pollock, M. L., & Ward, A. (1980). Generalized equations for predicting body density of women. Medicine and Science in Sports and Exercise, 12, 175-182.
Lohman, T. G., Pollock, M. L., Slaughter, M. H., Brandon, L. J., & Boileau, R. A. (1984). Methodological factors and the prediction of body fat in female athletes. Medicine and Science in Sports and Exercise, 16, 92-96.
Siri, W. E. (1961). Body composition from fluid space and density. In J. Brozek & A. Hanschel (Eds.), Techniques for measuring body composition (pp. 223-244). Washington, DC: National Academy of Science.