All Measurements Are Not Created Equal

Awareness of the importance of maintaining a healthy percentage of body fat (percent BF) is growing. The relationships between carrying too much body fat and the onset of many obesity-related disorders are well-documented.^{3, 4, 8} Similarly, an individual’s health may be compromised when percent BF is lower than what is recommended for healthful living.^{5, 11} Body fat is also related to successful athletic performance; decisions about competing at a certain weight classification (i.e., boxing and wrestling) are influenced by the athlete’s ability to drop fat weight without sacrificing muscle. Therefore, fitness professionals rely on percent BF estimations when creating exercise prescriptions, making dietary recommendations and monitoring change for those enrolled in weight-loss or -gain programs.

Methods available for assessment

Many methods are available for assessing percent BF. Generically, these methods are categorized as being either laboratory methods or field methods. Compared to field methods, laboratory methods tend to be more accurate, expensive, time-intensive, limited in availability and reliant on skilled technicians. Examples of laboratory methods include underwater weighing (UWW), air displacement plethysmography, whole-body bioelectrical impedance analysis (BIA), dual-energy x-ray absorptiometry (DXA), isotope dilution and combinations of these methods to derive multi-component models. Field methods, on the other hand, provide quick and relatively easy, albeit less accurate, estimations of percent BF. Examples of field methods include skinfold (SKF) assessment, anthropometry (height, weight, circumferences, BMI) and segmental (upper-body or lower-body) BIA. Many fitness facilities offer their clients a body composition assessment as part of their membership or basic fitness package. The SKF method is frequently used to provide baseline measures and to track individual progress toward percent BF or fat-free mass goals. A review of scientific literature reveals that the SKF method had its start in the early 1900s.² Those pioneering SKF investigators observed that the thickness of the SKF varied from anatomical site to anatomical site; later investigations looked at the relationship between the sum of SKF values from multiple sites to body density derived from UWW. Consequently, prediction equations were created to estimate body density from the sum of SKF values. Since body density holds little meaning to those outside the scientific community, an equation is needed to convert body density into percent BF. The first SKF equation was developed in 1951;2 many SKF equations and nomograms have subsequently been developed. Needless to say, the SKF method has undergone more than 50 years of usage by practitioners and scrutiny by researchers.

Standardized skinfold technique

Although the SKF thickness assessment is a long-standing and inexpensive field method used to estimate body fatness, it requires an extensive level of technician skill. Given the likelihood that a client may have baseline and follow-up SKF measurements taken by different members of your staff, it is crucial that the measurements are taken at the same anatomical locations and in the same way each time. Standardized SKF assessment procedures have been identified, and following them increases the accuracy of the percent BF estimation. According to the Anthropometric Standardization Reference Manual⁶, these procedures require the following:

1. All measurements are taken on the right side.
2. All sites are carefully identified through anatomical landmarks and measurements. Marking the SKF site with something like a ballpoint pen is recommended for novice technicians.
3. The thumb and index finger of the left hand are spanned approximately 8 centimeters (may be more for clients having higher levels of percent BF) and positioned 1 centimeter above the mark, with the mark positioned in the middle between the thumb and index finger.
4. The skinfold is formed by simultaneously “pinching” and lifting the skin and underlying layer of fat away from the muscle beneath.
5. The “pinch” is maintained with the left hand while the SKF caliper jaws are opened and positioned 1 centimeter below the thumb and index finger, level with the mark, perpendicular to the long axis of the skinfold, and midway between the top and base of the fold.
6. The jaws of the caliper are allowed to gradually close, while the “pinch” continues to be maintained.
7. The SKF measurement is determined by reading the caliper two to four seconds after the pressure on the caliper jaws is released.
8. The caliper jaws are re-opened and moved away from the site; the “pinch” is released and steps 3 through 8 are repeated at the next site in rotational order; for each site, a minimum of two measurements should be taken.

It is worth noting that there are slight differences in the standardized procedures for the SKF technique depending on the authority you choose to follow. For example, the Anthropometric Standardization Reference Manual6 recommends that the duplicate measures be within plus or minus 10 percent of each other, while the American College of Sports Medicine (ACSM)1 recommends that duplicate measures be within plus or minus 1 to 2 millimeters. Furthermore, ACSM recommends that the calipers be read within one to two seconds.

Importance of standardized techniques

Researchers have found that up to 9 percent of the variability in SKF measurements can be linked to differences in technique between SKF technicians¹³, with some sites (abdomen and thigh) yielding larger intertester differences than other sites (triceps, subscapular, suprailiac).¹² The Jackson and Pollock⁹ and Jackson, Pollock and Ward¹⁰ equations are commonly used, generalized, three-site SKF equations that include the abdominal and thigh sites; the former is used for assessing percent BF for men, while the latter is used for assessing women. Given that the SKF method produces a percent BF measure that is within plus or minus 3.5 percent of that obtained via UWW,¹ additional error introduced through intertester differences may further erode the predictive accuracy of the SKF assessment. An additional benefit to following standardized techniques is to eliminate any confusion regarding whether a horizontal, vertical or diagonal fold should be taken at a given site.

Importance of the prediction equation

Not only is it imperative that standardized techniques be followed to increase the accuracy and repeatability of a client’s percent BF assessment, it is also important that the best SKF prediction equation and body density to percent BF conversion formula be selected. Initial work in body composition assessment relied on UWW as the reference measure for the development of field methods and conversion equations. That earned UWW the title of “gold standard” for body composition assessment. UWW is based on a two-component model that assumes that the total body mass can be divided into either fat mass (FM) or fat-free mass (FFM). Therefore, early research efforts relied on several assumptions regarding the two components, especially the assumptions regarding the fat-free body (FFB). Refinements in research capabilities over the years have led to the realization that, contrary to the initial assumptions, the percentages of water, mineral and protein within the FFB differ based on age, ethnicity, activity level and health status. Variations in these FFB constituents may cause a deviation in the assumed density of the FFB (1.10 g/cc) and, consequently, whole-body density. Such discoveries have led to the development of population-specific equations for estimating percent BF from the sum of SKF. Selecting a population-specific SKF equation based on the age, gender, ethnicity and activity level of your client will increase the predictive accuracy of the SKF technique for estimating body density and percent BF. Generalized equations, developed using a large sample of diverse participants, could also be advantageous, as long as your client’s physical demographics were represented within the original or cross-validation samples. For an extensive list and review of population-specific two-component model formulas for converting body density to percent BF, see the text of Heyward and Wagner.⁷

In summary

Offering baseline and follow-up body composition assessments for your clients provides an invaluable mechanism that concretely reveals alterations in percent BF or FFM due to their individualized training programs. The SKF technique is a well-known, well-researched method for quickly estimating a client’s percent BF. Following standardized SKF procedures will increase the accuracy of the resulting percent BF value. Adhering to standardized procedures helps reduce, if not eliminate, technique error; this increases the likelihood that observed changes in body composition are the result of training and not erroneous measures attributable to the technician(s) involved.

References/Footnotes

1. American College of Sports Medicine. ACSM Guidelines for Exercise Testing and Prescription. Lippincott Williams & Wilkins: Baltimore, Md., 2006 (7th ed.).
2. Brozek, J., and A. Keys. Evaluation of leanness-fatness in man: Norms and interrelationships. British Journal of Nutrition 5: 194-206, 1951.
3. Christou, D.D., C.L. Gentile, C.A. DeSouza, D.R. Seals and P.E. Gates. Fatness is a better predictor of cardiovascular disease risk factor profile than aerobic fitness in healthy men. Circulation 111: 1904-1914, 2004.
4. Gutin, B., Z. Yin, M.C. Humphries, R. Bassall, N. Le, S. Daniels and P. Barbeau. Relations of body fatness and cardiovascular fitness to lipid profile in black and white adolescents. Pediatric Research 58: 78-82, 2005.
5. Haglin, L. The consequences of negative energy balance in anorexia syndrome. Journal of Pediatric and Adolescent Gynecology 18: 319-325, 2005.
6. Harrison, G.G., E.R. Buskirk, J.E.L. Carter, F.E. Johnston, T.G. Lohman, M.L. Pollock, A.F. Roche and J.H. Wilmore. Skinfold thicknesses and measurement technique. In Anthropometric Standardization Reference Manual, eds. T.G. Lohman, A.F. Roche and R. Martorell. pp 55-70. Human Kinetics: Champaign, Ill., 1988.
7. Heyward, V.H., and D.R. Wagner. Applied Body Composition Assessment. Human Kinetics: Champaign, Ill., 2004 (2nd ed.).
8. Hirschler, V., H.L.P. Acebo, G.B. Fernandez, M.L. Calcagno, C. Gonzalez and M. Jadzinsky. Influence of obesity and insulin resistance on left atrial size in children. Pediatric Diabetes 7: 39-44, 2006.
9. Jackson, A.J., and M.L. Pollock. Generalized equations for predicting body density of men. British Journal of Nutrition 40: 497-504, 1978.
10. Jackson, A.J., M.L. Pollock and A. Ward. Generalized equations for predicting body density of women. Medicine and Science in Sports and Exercise 12: 175-182, 1980.
11. Klentrou, P., and M. Plyley. Onset of puberty, menstrual frequency, and body fat in elite rhythmic gymnasts compared with normal controls. British Journal of Sports Medicine 37: 490-494, 2003.
12. Lohman, T.G., M.L. Pollock, M.H. Slaughter, L.J. Brandon and R.A. Boileau. Methodological factors and the prediction of body fat in female athletes. Medicine and Science in Sports and Exercise 16: 92-96, 1984.
13. Morrow, J.R., T. Fridye and S.D. Monaghen. Generalizability of the AAHPERD health-related skinfold test. Research Quarterly for Exercise and Sport 57: 187-195, 1986.

There are slight differences in the standardized procedures for the skinfold technique.
Following standardized skinfold procedures will increase the accuracy of the resulting percent body fat value.

Population-Specific Skinfold Equations
Men (sum of chest, abdomen, thigh skinfolds)
Body density = 1.10938 - 0.0008267(sum of three skinfolds) + 0.0000016(sum of three skinfolds)2 - 0.0002574(age)

Women (sum of triceps, suprailiac, thigh)
Body density = 1.099421 - 0.0009929(sum of three skinfolds) + 0.0000023(sum of three skinfolds)2 - 0.0001392(age)