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A Closer Look At Skinfold Assessment
 
 
 

Skinfold analysis is a common field assessment used by fitness professionals to predict body fatness. The technique is based on the fact that 50-70% of stored fat lies between the skin and muscle, referred to as subcutaneous fat. The measurement technique requires the tester to identify gender specific sites which reflect predetermined assessment locations for regression equations calculated to predict body density. Two of the most popular groups of equations identified by their respective name sake are the Jackson & Pollack, and Durnin & Wormersley multi-site skinfold equations. The equations are based on the measure of skinfold at select sites expressed in millimeters of thickness. The skinfold measurements are entered into the population specific regression equation to predict body density which is expressed as a percentage of fat. According to the literature, use of field method prediction equations developed from 2-component model (Siri equation) reference measures of body composition systematically underestimate relative body fatness based on comparisons to hydrostatic (underwater) weighing. Ethnic differences further invalidate the Siri equation as seen in American Indian women, African-American men and women, and Hispanic women when standard equations are used. Researchers suggest that this is due to the fact the average fat free body (FFB) density of these ethnic groups exceeds the assumed value (1.1 g/ml). Unfortunately, the overwhelming majority of field method prediction equations have been developed and cross-validated for Caucasian populations and are based on 2-component model reference measures. Because ethnicity may affect the FFB and regional fat distribution, race-specific prediction equations should be used to enhance the accuracy of the assessment.

 

Literature reviews suggest that these same equations have a tendency to under predict body fatness when applied to multi-compartment comparisons. When compared to hydrodensitometry, the equations demonstrated a greater measurement disparity. According to Aristizábal et al (2007), skinfold thickness equations showed poor validity for body fat assessment as the equations showed significant differences and lower correlation coefficients with hydrodensitometry. The equations “indicated agreement with hydrodensitometry over very wide limits” suggesting a large standard estimation of error for skinfold assessment. When circumference and bone breadth were added to skinfold measures the accuracy improved based on the validity of dual x-ray absorptiometry. Garcia et al. (2005) applied a modified equation method to enhance predictability, thereby reducing the under estimations found from skinfold assessment.

 

BFM (kg) for men = -40.750 + {(0.397 x waist circumference) + [6.568 x (triceps SF + subscapular SF + abdominal SF)]}

 

BFM (kg) for women = -75.231 + {(0.512 x hip circumference) + [8.889 x (chin SF + triceps SF + subscapular SF)] + (1.905 x knee breadth)}.

 

These equations provide additional data to help enhance the accuracy sometimes compromised by heterogenous storage patterns and bone mass differences. The combination of girth and skinfold complement the limitations of each when used independently. The most difficult aspect of the equations above is probably the use of the chin site. To most professionals it is an uncommon site and can be perceived as rather invasive, but this location was actually used for decades by the National Institute of Health (NIH) in cohort data collection.

 

Although equations may lead to some prediction error, two of the larger contributing factors to decreased accuracy using skinfold assessment are technician error and client mass. Literature suggests skinfold expertise requires a large number of practice trials before a tester is considered accurate. Assuming body density estimation from skinfold measurements has a prediction validity to within 3% - 4% for 70% of the population, any errors perpetrated by the tester can widen this gap. Additionally, the amount of mass being measured further hinders the accuracy of the assessment. Large amounts of subcutaneous fat cause additional difficulty in pinching the proper fold. For this reason it is suggested that skinfold be used for those of healthy weight ranges rather than the overweight and obese populations. Larger amounts of fat not only reduce the ability of the tester to be accurate but also heighten the psychological aspect of the assessment. Skinfold is fairly invasive compared to girth measurement and bioelectrical impedance, and therefore should be avoided when dealing with heavier individuals subject to body image factors.

 

To improve testing accuracy the tester should comply with all standard protocols and use population specific sites and equations. Arguments have been made that assessments include segmental or regional data. This means skinfold for both men and women should include body sites reflective of gynoid and android storage patterns such as the thigh for females and abdominal or subscapular fold for males.

 

Commonalities of all testing protocols include identifying and marking the sites, pinching the fold using a pronated grip and fully separating the fold from the underlying muscle before securing the skinfold calipers in the middle of the fold just below the thumb and forefinger; and determining the measure in a matter of seconds before unclipping the calipers and releasing the fold. Once each fold is assessed the tester should record the measurement before moving on to the subsequent fold. A duration of greater than 15 seconds should be used between same-site skinfolds to ensure the compression of the fold is not a factor. A double pinch technique (demonstrated below) can be used to aid in performing the assessment properly. Grasping the fold using a traditional grip just above the marked site along with a coordinated pinch just below the site aids in properly separating the fat from the underlying muscle and ensures a proper depth of skinfold. Once the fold is secure, the bottom hand is removed and replaced by the caliper, which is placed over the marked site parallel to the ground. It is important to maintain the skinfold thickness by applying consistent pressure via the pinch until the calipers have been placed properly, the value read, and the calipers subsequently removed. Not holding the fold during the measurement invalidates the assessment. Double pinch practice on the triceps fold is usually beneficial in learning the technique. Sites such as the thigh and abdomen or suprailium are often more challenging and require additional practice.

Example of the double pinch technique being used for assessing the abdominal fold site: