Skinfold Test: Protocol, Equipment, Sites, and Interpretation

What is the protocol for skinfold test?

The skinfold test protocol involves precise measurement of subcutaneous fat thickness at specific anatomical sites using calibrated calipers, following standardized procedures for subject preparation, site identification, fold grasping, and caliper application to estimate body density and subsequently, body fat percentage.

Introduction to Skinfold Testing

Skinfold caliper testing is a widely utilized and practical method for assessing body composition by estimating the amount of subcutaneous fat. This technique relies on the principle that approximately half of the body's total fat is located directly beneath the skin. When performed correctly by a trained professional, skinfold measurements can provide valuable insights into changes in body fat over time, making it a useful tool for tracking progress in fitness and health interventions. While less precise than laboratory-based methods like DEXA or hydrostatic weighing, its accessibility and cost-effectiveness make it a cornerstone in many fitness and clinical settings.

Essential Equipment

Accurate skinfold testing requires specific equipment to ensure precision and reliability.

• Skinfold Calipers: These are the primary tools. They come in various types, including:
  • Harpenden Calipers: High-end, precise, and durable, often considered the gold standard.
  • Lange Calipers: Another high-quality option, commonly used in research and clinical settings.
  • Plastic Calipers (e.g., Accu-Measure): More affordable and portable, suitable for general use but may have less consistent pressure and jaw tension.
  • Calibration: Regardless of type, calipers must be regularly calibrated to ensure they apply consistent pressure (typically 10 g/mm²) across their measurement range.
• Non-Stretch Measuring Tape: Used for precise landmark identification (optional but recommended for marking).
• Non-Toxic Marking Pen: For clearly marking the measurement sites on the skin.
• Data Recording Sheet: To meticulously record measurements and track progress.
• Privacy Screen/Area: To ensure client comfort and confidentiality.

Pre-Test Preparations and Considerations

Proper preparation of both the subject and the technician is crucial for obtaining accurate and reliable skinfold measurements.

• Client Preparation:
  • Avoid Exercise: The client should refrain from vigorous exercise on the day of the test, as it can cause fluid shifts that temporarily alter skinfold thickness.
  • Hydration: Ensure the client is normally hydrated. Dehydration can affect skin turgor.
  • Showering/Lotions: The client should not apply lotions, creams, or oils to the skin, as these can make it difficult to grasp the skinfold firmly.
  • Clothing: Minimal, loose-fitting clothing (e.g., shorts, sports bra) that allows easy access to all measurement sites is ideal.
  • Comfort: Ensure the testing environment is private and comfortable to minimize client anxiety and muscle tension.
• Technician Considerations:
  • Training and Practice: The technician must be well-trained and highly practiced in identifying anatomical landmarks, grasping skinfolds, and applying calipers consistently.
  • Consistency: The same technician should ideally perform serial measurements on an individual to minimize inter-tester variability.
  • Calm Demeanor: A professional and reassuring approach helps put the client at ease.
• Ethical Considerations:
  • Informed Consent: Obtain verbal and written informed consent, explaining the procedure, its purpose, and the right to withdraw.
  • Privacy and Modesty: Always prioritize client privacy and modesty during the procedure.

General Principles of Skinfold Measurement

Adhering to these fundamental principles ensures the standardization and accuracy of skinfold measurements.

• Right Side Measurement: All measurements should consistently be taken on the right side of the body. This is the universally accepted standard for comparison and consistency.
• Grasping the Fold:
  • Use the thumb and forefinger of your non-dominant hand to firmly pinch a double fold of skin and subcutaneous fat.
  • Lift the fold approximately 1 cm above the site where the caliper will be placed.
  • Ensure that only skin and subcutaneous fat are grasped, separating it from the underlying muscle. This can be verified by asking the client to contract the muscle beneath the fold; if the fold remains loose, it's correctly isolated.
• Caliper Placement:
  • Place the jaws of the caliper perpendicular to the long axis of the skinfold.
  • Position the caliper approximately 1 cm below your thumb and forefinger, midway between the crest and base of the skinfold.
• Reading the Caliper:
  • Release the caliper jaws slowly so that the full tension is applied to the skinfold.
  • Read the measurement to the nearest 0.5 mm or 1 mm (depending on caliper precision) approximately 1 to 2 seconds after the jaws have fully closed. Waiting too long can lead to "creep" (compression of tissue) and inaccurate readings.
• Repeat Measurements:
  • Take a minimum of two measurements at each site. If the two measurements differ by more than 1-2 mm, take a third measurement.
  • Measurements should be taken in a rotational sequence (e.g., measure triceps, then biceps, then subscapular, then return to triceps for the second measurement). This allows the skin to regain its normal thickness and turgor between readings at the same site, preventing tissue compression from affecting subsequent measurements.
• Consistency: The most critical factor for reliable results is consistency in landmark identification, fold grasping, and caliper application.

Standardized Skinfold Sites and Procedures

Various skinfold equations use different combinations of sites. Below are descriptions of commonly used sites, often part of 3-site, 7-site, or 9-site protocols.

• Triceps:
  • Location: Posterior midline of the upper arm, halfway between the acromion process (bony point of shoulder) and the olecranon process (bony point of elbow). Mark the midpoint with the arm hanging freely.
  • Fold Orientation: Vertical fold.
  • Client Position: Arm relaxed and hanging freely by the side.
• Biceps:
  • Location: Anterior midline of the upper arm, directly above the triceps site, at the same midpoint between the acromion and olecranon processes.
  • Fold Orientation: Vertical fold.
  • Client Position: Arm relaxed and hanging freely by the side.
• Subscapular:
  • Location: Just below the inferior angle of the scapula (shoulder blade). The fold runs diagonally downwards and laterally.
  • Fold Orientation: Diagonal fold, running at approximately a 45-degree angle following the natural skin lines.
  • Client Position: Standing erect, arms relaxed by the side. Client may gently place their arm across their back to make the scapula more prominent, then relax it for measurement.
• Chest/Pectoral (Men and Women):
  • Location (Men): Diagonal fold taken halfway between the anterior axillary line (armpit fold) and the nipple.
  • Location (Women): Diagonal fold taken one-third of the distance from the anterior axillary line to the nipple.
  • Fold Orientation: Diagonal fold.
  • Client Position: Standing erect, arm abducted slightly if necessary to access the site.
• Midaxillary:
  • Location: Vertical fold on the midaxillary line (line directly down from the center of the armpit) at the level of the xiphoid process (bottom tip of the sternum).
  • Fold Orientation: Vertical fold.
  • Client Position: Standing erect, arm abducted slightly (e.g., resting on a chair or examiner's shoulder) to expose the area.
• Abdominal:
  • Location: Vertical fold taken 2 cm (approximately 1 inch) to the right of the umbilicus (navel).
  • Fold Orientation: Vertical fold.
  • Client Position: Standing erect, relaxed abdomen.
• Suprailiac:
  • Location: Diagonal fold taken immediately superior to the iliac crest (top of the hip bone) in the anterior axillary line. The fold follows the natural line of the skin.
  • Fold Orientation: Diagonal fold.
  • Client Position: Standing erect, arm abducted slightly if necessary.
• Thigh:
  • Location: Vertical fold on the anterior midline of the thigh, midway between the inguinal crease (where the thigh meets the torso) and the proximal border of the patella (top of the kneecap).
  • Fold Orientation: Vertical fold.
  • Client Position: Standing with weight shifted to the left foot, right leg relaxed with the knee slightly flexed.
• Medial Calf:
  • Location: Vertical fold on the medial aspect of the calf, at the level of the maximum calf circumference.
  • Fold Orientation: Vertical fold.
  • Client Position: Standing with the right foot flat on a bench or chair, knee flexed at 90 degrees.

Data Interpretation and Body Composition Calculation

Once all measurements are accurately taken, the data is used to estimate body density and then body fat percentage.

• Sum of Skinfolds: The individual measurements (usually the average of two or three readings per site) are summed.
• Regression Equations: The sum of skinfolds is then input into a population-specific regression equation. Common equations include:
  • Jackson & Pollock Equations: Widely used, available for various combinations of sites (e.g., 3-site for men and women, 7-site). These equations are derived from specific populations and convert skinfold sums into body density.
  • Durnin & Womersley Equations: Another set of commonly used equations, often based on four sites (biceps, triceps, subscapular, suprailiac), with separate equations for different age groups and genders.
• Body Density to Body Fat Percentage: The calculated body density is then converted into a body fat percentage using a generalized equation, most commonly the Siri equation (BF% = (495 / Body Density) - 450) or the Brozek equation (BF% = (457 / Body Density) - 414.2).
• Considerations: It's crucial to use the correct equation for the specific set of skinfold sites measured and for the client's demographic (age, sex, ethnicity) to ensure the most accurate estimation.

Enhancing Accuracy and Reliability

Maximizing the accuracy and reliability of skinfold measurements requires diligence and attention to detail.

• Experienced Tester: The single most important factor is a skilled and experienced technician who consistently applies the protocol.
• Standardized Protocol Adherence: Deviating from the established landmarks, fold orientations, or caliper application techniques will compromise results.
• Calibrated Equipment: Regularly check and calibrate calipers to ensure they apply consistent pressure.
• Client Comfort and Relaxation: A relaxed client with untensed muscles facilitates accurate fold isolation.
• Avoiding Consecutive Measurements at the Same Site: Allow adequate time (30 seconds to 1 minute) between measurements at the same site for the skin to return to its normal state.
• Consistent Time of Day: For serial measurements, test the client at the same time of day under similar conditions to minimize diurnal variations.

Limitations and Alternatives

While valuable, skinfold testing has inherent limitations.

• Tester Skill Dependency: Highly dependent on the skill and experience of the technician.
• Subject Variability: Factors like hydration status, recent exercise, and skin elasticity can influence measurements.
• Population-Specific Equations: Equations are derived from specific populations, and their accuracy may decrease when applied to individuals outside those demographics (e.g., very lean athletes, morbidly obese individuals, specific ethnic groups).
• Not Ideal for Extremes: Can be challenging to measure accurately in very obese individuals (due to large folds) or extremely lean individuals (due to minimal fat).
• Privacy Concerns: Some individuals may feel uncomfortable with the required exposure of certain body areas.

Alternatives for Body Composition Assessment:

• Dual-energy X-ray Absorptiometry (DEXA): Considered a gold standard, providing detailed information on bone mineral density, fat mass, and lean mass.
• Hydrostatic Weighing (Underwater Weighing): Measures body density based on Archimedes' principle.
• Air Displacement Plethysmography (Bod Pod): Measures body volume by air displacement to determine body density.
• Bioelectrical Impedance Analysis (BIA): A more accessible method that estimates body fat by measuring the resistance of electrical flow through the body.

Conclusion

The skinfold test, when executed with meticulous adherence to its standardized protocol, remains a valuable and practical tool for estimating body composition. Its strength lies in its ability to track changes in subcutaneous fat over time, providing actionable insights for individuals pursuing health and fitness goals. For accurate and reliable results, emphasis must always be placed on proper technician training, consistent technique, precise anatomical landmark identification, and the use of appropriate regression equations for data interpretation. While not without limitations, its accessibility ensures its continued relevance in the field of exercise science and kinesiology.