During full extension of the knee, the femur medially rotates on the tibia to produce the locked position. Which muscle is responsible for unlocking the knee at the start of flexion?
- A Popliteus ✓
- B Gastrocnemius
- C Plantaris
- D Biceps femoris
Correct answer: A. Popliteus
Explanation
The popliteus arises from the lateral condyle of the femur and inserts into the posterior surface of the tibia above the soleal line. In the closed chain of standing, it laterally rotates the femur on the fixed tibia, unwinding the screw home mechanism so that flexion can begin. Biceps femoris produces active lateral rotation of the leg when the foot is free but does not perform the unlocking action. Gastrocnemius and plantaris are plantar flexors with no rotatory role at the knee.
Reference: Gray's Anatomy for Students, 4th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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