A patient with a full flexed knee cannot smoothly begin extension from a squatting position, and the surgeon explains that a muscle must first 'unlock' the knee. Which muscle performs this function and how?

• A Plantaris, by flexing the knee and pulling the capsule back
• B Biceps femoris, by medially rotating the tibia on the femur
• C Popliteus, by laterally rotating the femur on the fixed tibia ✓
• D Gastrocnemius, by flexing the knee from its femoral origin

Explanation

In terminal extension the knee locks by medial rotation of the femur on the tibia (or lateral rotation of the tibia on the femur). The popliteus, arising from the lateral femoral condyle and inserting on the posterior tibia, initiates flexion by laterally rotating the femur on the fixed tibia, thereby unlocking the joint. Biceps femoris produces lateral rotation of the tibia, not the unlocking movement described, while gastrocnemius and plantaris are weak flexors with no rotatory role at the locked knee.

Reference: Gray's Anatomy for Students, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.