A patient with a fully extended knee takes the first step of walking and the knee must flex. Which muscle performs 'unlocking' of the locked knee?

• A Popliteus ✓
• B Plantaris
• C Gastrocnemius, medial head
• D Semimembranosus

Explanation

Full extension involves terminal medial rotation of the femur on the fixed tibia (screw home mechanism), which pulls the joint into close packing. The popliteus, arising from the lateral femoral condyle and inserting on the posterior tibia, laterally rotates the femur on the tibia (or medially rotates the tibia on the femur when the foot is off the ground) to unlock the knee before flexion begins. It is supplied by the tibial nerve.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.