A patient with a locked knee cannot fully extend it. Which muscle actively unlocks the extended knee, and by what action?

• A Gastrocnemius, by pulling the femoral condyles posteriorly
• B Biceps femoris, by flexion of the knee
• C Sartorius, by medial rotation of the tibia
• D Popliteus, by lateral rotation of the femur on the fixed tibia ✓

Explanation

In full extension the knee undergoes terminal locking by medial rotation of the femur on the tibia (or lateral rotation of the tibia when the foot is free). Popliteus initiates unlocking by laterally rotating the femur on the fixed tibia during closed-chain movement, freeing the condyles so flexion can proceed. Biceps femoris merely flexes and laterally rotates the tibia and does not perform the unlocking rotation itself.

Reference: Gray's Anatomy, 42nd ed.

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Written and medically reviewed by the StethoPrep medical team.