With the foot planted on the ground, the fully extended knee is in the locked, screw-home position. Which muscle initiates flexion by laterally rotating the femur on the tibia to unlock the joint?

• A Sartorius
• B Biceps femoris
• C Gracilis
• D Popliteus ✓

Explanation

When the foot is fixed, the popliteus contracts before knee flexion begins and rotates the femur laterally on the tibia, reversing the medial rotation that produced the screw-home lock. It also retracts the lateral meniscus during flexion, protecting it from femoral compression. Biceps femoris also externally rotates the tibia when the leg is free, but only the popliteus performs the fixed-foot unlocking action and is therefore called the key of the knee joint.

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

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