Skinfold Measurement: Method, Science, Tools, and Interpretation

What is the method of measuring skinfold?

Skinfold measurement is a widely used, practical, and non-invasive method to estimate body fat percentage by measuring the thickness of subcutaneous fat at specific anatomical sites using a specialized caliper.

Understanding Skinfold Measurement

Skinfold measurement is a foundational technique in exercise science and health assessment for estimating body composition, specifically body fat percentage. The premise is that a significant proportion of total body fat is stored directly beneath the skin (subcutaneously). By precisely measuring the thickness of these skinfolds at various standardized locations on the body, and applying specific mathematical equations, one can derive an estimate of overall body density, which is then converted into a body fat percentage. This method provides valuable insights for tracking fitness progress, assessing health risks, and guiding nutritional and exercise interventions.

The Science Behind Skinfolds

The efficacy of skinfold measurement relies on several scientific principles and assumptions:

• Subcutaneous Fat as an Indicator: It assumes that the amount of subcutaneous fat is proportional to the total body fat.
• Constant Density: It assumes that the density of fat mass (approximately 0.9 g/cm³) and fat-free mass (approximately 1.1 g/cm³) are relatively constant across individuals.
• Population-Specific Equations: The technique utilizes regression equations derived from large population studies. These equations correlate the sum of various skinfold measurements with body density, which is typically measured more accurately through methods like hydrostatic weighing or DEXA. Once body density is calculated, it is converted into body fat percentage using established formulas, such as the Siri equation (Body Fat % = (495 / Body Density) - 450).

Tools of the Trade

The primary tool for skinfold measurement is the skinfold caliper. These come in various models, ranging from research-grade precision calipers (e.g., Harpenden, Lange) to more affordable plastic versions (e.g., Accu-Measure). While plastic calipers are less precise, they can be adequate for tracking individual changes over time, provided the same caliper and technique are consistently used. Regardless of the type, a good caliper should maintain a constant pressure throughout its range of motion.

Other useful tools include:

• Measuring tape: For locating anatomical landmarks precisely.
• Non-toxic marker: To mark the measurement sites.
• Data sheet: To record measurements accurately.

Preparing for Measurement

To ensure accurate and reliable skinfold measurements, both the subject and the tester should adhere to certain pre-measurement guidelines:

• Subject Preparation:
  • Avoid intense physical activity on the day of the measurement.
  • Do not consume alcohol or caffeine within 12 hours prior.
  • Ensure adequate hydration.
  • Avoid lotions or creams on the skin that could affect caliper grip.
  • Measurements should ideally be taken at the same time of day for consistency in follow-up assessments.
• Tester Preparation:
  • Be proficient in identifying anatomical landmarks.
  • Be skilled in the proper pinching and caliper application technique.
  • Maintain consistency in all aspects of the measurement process.
  • Use the same caliper for all measurements on an individual.

The Measurement Protocol: A Step-by-Step Guide

The accuracy of skinfold measurement hinges on meticulous technique. Here's a generalized protocol for common sites:

1. Site Selection: Choose a standardized set of sites based on the chosen prediction equation (e.g., Jackson & Pollock 3-site, 7-site, Durnin & Womersley 4-site). Common sites include:

   • Triceps: Vertical fold on the posterior midline of the upper arm, halfway between the acromion and olecranon processes.
   • Biceps: Vertical fold on the anterior midline of the upper arm, directly above the cubital fossa.
   • Subscapular: Diagonal fold (45-degree angle) just below the inferior angle of the scapula.
   • Suprailiac: Diagonal fold, anterior superior iliac spine (ASIS) at a 45-degree angle to the inguinal crease.
   • Abdominal: Vertical fold 2 cm to the right of the umbilicus.
   • Midaxillary: Vertical fold at the level of the xiphoid process on the midaxillary line.
   • Thigh: Vertical fold on the anterior midline of the thigh, halfway between the inguinal crease and the proximal border of the patella.
   • Calf: Vertical fold on the medial aspect of the calf, at the maximal circumference.

2. Subject Positioning: Ensure the subject is relaxed and in the correct anatomical position (e.g., standing upright, arm relaxed for triceps).

3. Palpation and Marking: Precisely locate and mark the anatomical landmark for each site using a non-toxic marker.

4. Pinch Technique:

   • Grasp a double fold of skin and subcutaneous fat firmly between the thumb and index finger of your non-dominant hand.
   • The fold should be lifted about 1 cm proximal to the marked site, ensuring muscle is not included in the pinch. Gently roll the fold between your fingers to confirm only skin and fat are grasped.

5. Caliper Placement:

   • Place the jaws of the caliper perpendicular to the fold, approximately 1 cm distal to your thumb and index finger, and at the marked site.
   • Release the caliper jaws slowly, allowing them to exert pressure on the skinfold.

6. Reading the Caliper:

   • Take the reading approximately 1-2 seconds after the caliper jaws have been fully applied and the needle has stabilized. Read to the nearest 0.5 mm or 1 mm, depending on the caliper's precision.

7. Number of Measurements:

   • Take at least two measurements at each site. If the two measurements differ by more than 1-2 mm, take a third measurement.
   • Average the two closest readings for that site.
   • Allow the skin to return to its normal state before taking subsequent measurements at the same site (e.g., by rotating through all sites first).

Interpreting Skinfold Results

Once all measurements are taken and recorded, the sum of the skinfolds is typically used in a specific regression equation. These equations are often sex- and age-specific, and sometimes population-specific (e.g., for athletes vs. general population). The equation will yield an estimated body density, which is then converted into a body fat percentage using a formula like the Siri equation.

It's crucial to understand that skinfold measurements provide an estimate of body fat percentage, not an exact value. The standard error of estimate for skinfold equations is typically 3-5%, meaning the true body fat percentage could be within this range of the calculated value.

Advantages and Limitations of Skinfold Measurement

Advantages:

• Cost-effective: Calipers are relatively inexpensive compared to other body composition methods.
• Portable: Calipers are small and easy to transport, allowing for field-based assessments.
• Non-invasive: The procedure is generally well-tolerated and does not involve radiation or immersion.
• Quick: Once proficient, a complete measurement can be performed in a few minutes.
• Tracking Changes: Excellent for tracking changes in body composition over time in an individual, provided consistency is maintained.

Limitations:

• Requires Skill: Accuracy is highly dependent on the skill, experience, and consistency of the tester.
• Inter-Rater Reliability: Variability between different testers can be significant.
• Discomfort: Some individuals may find the pinching sensation uncomfortable or feel self-conscious.
• Extremes of Body Fat: Can be challenging and less accurate for very obese individuals (difficulty grasping folds) or extremely lean individuals (difficulty isolating skin from muscle).
• Equation Specificity: The prediction equations are population-specific, and using an inappropriate equation can lead to inaccurate results.
• Does Not Measure Visceral Fat: Skinfolds only measure subcutaneous fat, not the metabolically active visceral fat surrounding organs, which is a significant health risk factor.

Who Should Perform Skinfold Measurements?

Given the skill-dependent nature of skinfold measurement, it is strongly recommended that assessments be performed by trained and certified professionals. This includes personal trainers with body composition assessment certifications, clinical exercise physiologists, registered dietitians, or kinesiologists. Proper training ensures accurate landmark identification, consistent pinching technique, and correct interpretation of results, maximizing the utility and reliability of this valuable assessment tool.

Conclusion

The method of measuring skinfold, while seemingly simple, is a nuanced technique requiring precision, knowledge, and practice. When performed by a skilled professional, it serves as a practical, accessible, and informative tool for estimating body composition and tracking changes in subcutaneous fat over time, providing valuable data for health and fitness guidance.