After prolonged squatting, a patient stands up and the knee is momentarily 'locked' in full extension. Which muscle must contract to initiate flexion by unlocking the knee?
- A Gastrocnemius, lateral head
- B Plantaris
- C Popliteus ✓
- D Biceps femoris
Correct answer: C. Popliteus
Explanation
At terminal extension the knee undergoes the 'screw home' mechanism, with lateral rotation of the femur on the tibia. The popliteus, arising from the lateral femoral condyle and inserting on the posteromedial tibia, laterally rotates the tibia relative to the femur (or medially rotates the femur on a fixed tibia) to unlock the knee. Biceps femoris flexes and externally rotates the tibia but is not the unlocking muscle.
Reference: Gray's Anatomy for Students, 4th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.