During the 'screw home' mechanism of the knee joint, what motion occurs at the tibiofemoral joint at the end of full extension, and which muscle is responsible for 'unlocking' the knee to begin flexion?
- A Medial rotation of the femur on the tibia to lock; biceps femoris unlocks
- B Anterior glide of the tibia to lock; quadriceps unlocks
- C Posterior displacement of the femur to lock; gastrocnemius unlocks
- D Lateral rotation of the femur on the tibia (or medial tibial rotation relative to femur) to lock; popliteus 'unlocks' by medially rotating the femur ✓
Explanation
As the knee reaches full extension (closed-chain activity with foot on ground), the femur laterally rotates on the fixed tibia (equivalent to the tibia internally rotating relative to the femur) — this is the screw-home mechanism that tightens all ligaments and creates a 'close-packed' stable knee. Articular geometry (larger medial femoral condyle), ACL, and collateral ligaments drive this. To begin flexion, the popliteus contracts to medially rotate the femur on the tibia (or externally rotate the tibia), unlocking the joint. The popliteus is therefore known as the key to unlock the knee. Biceps femoris laterally rotates the tibia and would not unlock; quadriceps is an extensor.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.