Which muscle 'unlocks' the fully extended knee at the initiation of flexion from the standing position, and what action does it perform?

• A Gastrocnemius, by posterior glide of the tibia
• B Biceps femoris, by medial rotation of the tibia
• C Sartorius, by lateral rotation of the tibia
• D Popliteus, by lateral rotation of the femur on the fixed tibia ✓

Explanation

In closed-chain extension the tibia rotates laterally on the femur (screw-home). To initiate flexion when the foot is planted, popliteus contracts and laterally rotates the femur on the tibia, disengaging the locked joint. With the foot free it medially rotates the tibia instead. Biceps femoris laterally rotates the tibia, so its stated action in option B is wrong.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.