A patient has a 10-degree knee extension lag after knee surgery. What is the most plausible reason for the extension lag?

• A. Tightness of the hamstrings
• B. Inhibition of the quadriceps
• C. Capsular restriction
• D. Inhibition of the quadriceps is the correct rationale

Answer: (D)

A knee extension lag after surgery is most often due to inhibition of the quadriceps. After procedures around the knee, swelling and pain trigger arthrogenic inhibition, reducing the muscle’s ability to generate force to fully straighten the joint. As a result, the patient can passively extend the knee to full extension, but active extension falls short by several degrees, such as a 10-degree lag.

Hamstring tightness would more likely limit knee flexion or resist rapid lengthening rather than cause a pure extension lag. Capsular restriction tends to reduce passive range as well, so you’d expect limited passive extension too, not just an active lag.