Understanding Coefficient of Variation (CV) in Functional Capacity Evaluations

In Functional Capacity Evaluation (FCE) testing, one of the recurring questions is: “How consistent was the person’s performance?”

This is where the coefficient of variation (CV) becomes clinically useful. The CV is a statistical measure that describes the amount of variability between repeated trials. In FCE practice, it is commonly applied to repeated force-production tests such as grip strength, pinch grip, static lifting, and isometric push/pull testing.

Mathematically, the coefficient of variation is calculated as:

CV = (SD / mean) × 100

Where:

1. SD = standard deviation of the trials, Mean = average of the trials. The result is expressed as a percentage. In practical terms, the CV answers the question:
2. “How close together were the repeated efforts?”
3. A lower CV indicates the repeated trials were relatively consistent. A higher CV indicates greater variability between attempts.

 

Clinical Importance

The coefficient of variation is not a lie detector. It is a measure of consistency. Within an FCE, CV helps clinicians quantify how stable repeated force-production efforts are across trials. Used appropriately, it strengthens the objectivity of the evaluation process and contributes to defensible clinical reasoning. Like all FCE findings, however, CV values must be interpreted within the total context of the examination rather than as a standalone conclusion.

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Using the image example above, three separate isometric tests were performed:

· Arm force testing, Push force testing, Pull force testing

The “Arm” row demonstrates relatively stable repeated efforts:

· Trial 1 = 27.1 lbs · Trial 2 = 28.4 lbs · Trial 3 = 26.6 lbs CV = 2%

These values are tightly grouped together. The low variability suggests the individual was able to reproduce force consistently across trials.

The “Pull” row demonstrates even greater inconsistency:

· Trial 1 = 56.2 lbs · Trial 2 = 36.5 lbs · Trial 3 = 70.2 lb CV = 25%

The wide spread between repetitions results in a substantially elevated CV.

CV use in an FCE

In FCE methodology, consistency across repeated trials is considered one component of performance validity analysis. Historically, research involving grip strength testing observed that individuals exerting maximal effort tended to produce lower coefficients of variation compared to individuals instructed to feign or intentionally reduce effort.

This led to the commonly cited clinical threshold that:

· CV values below approximately 15% are generally considered more consistent with maximal effort

· CV values above 15% may indicate inconsistent performance

However, this interpretation requires caution. The current APTA/Academy of Orthopaedic Physical Therapy Best Practice Guideline specifically states:

“A CV of 15% or greater may be considered unacceptable variability of grip strength results and not representative of full effort.”

The guideline immediately follows this with an important clarification:

“Low and high CVs do not absolutely correlate to maximal or submaximal effort respectively.”

This distinction is critical for clinicians.

A high CV does not automatically mean symptom magnification, malingering, or intentional deception.

Clinical Interpretation of High CV Values

Variability in force production may occur for many legitimate reasons, including:

• Pain inhibition
• Fear avoidance
• Poor motor control
• Neurological impairment
• Acute flare-ups
• Fatigue
• Guarding behavior
• Limited understanding of instructions
• Coordination deficits
• Anxiety during testing

 

For example, a patient with acute shoulder pain may hesitate during one repetition and push harder during another depending on symptom response. Similarly, patients unfamiliar with isometric testing may improve over repeated trials due to motor learning and increased confidence. This is why CV should never be interpreted in isolation.

In modern FCE practice, CV is best viewed as one data point within a broader performance validity framework that also includes:

• Biomechanical observations
• Heart rate response
• Movement consistency
• Reliability cross-checks
• Behavioral observations
• Functional carryover between tests
• Symptom response patterns
• Test termination criteria

 

Applying CV to Push/Pull Testing

Although CV research originated primarily from grip strength literature, the same underlying principle can be applied to isometric push and pull testing. If an individual demonstrates:

• highly variable force outputs
• inconsistent movement patterns,
• and inconsistent symptom reporting,

 

the evaluator may identify concerns regarding reliability of performance. Conversely, reproducible force output across repeated trials strengthens confidence that the measured performance likely reflects the individual’s demonstrated capacity on that day. The key concept is not whether the evaluator “believes” the patient. The key concept is whether the performance demonstrates acceptable reproducibility.

References

Shechtman O. Using the coefficient of variation to detect sincerity of effort of grip strength: a literature review. Journal of Hand Therapy. 2000;13(1):25-32.

Dvir Z. Coefficient of variation in maximal and feigned static and dynamic grip efforts. American Journal of Physical Medicine & Rehabilitation. 1999;78:216-221.

Robinson ME, Dannecker EA. Critical issues in the use of muscle testing for the determination of sincerity of effort. Clinical Journal of Pain. 2004;20(6):392-398.

Academy of Orthopaedic Physical Therapy, APTA. Current Concepts in Functional Capacity Evaluation: A Best Practice Guideline. 2022.