Which muscle is used to unlock the knee from full extension and is active during knee flexion?

• A. Popliteus
• B. Rectus Femoris
• C. Gastrocnemius
• D. Tibialis Anterior

Answer: (A)

Unlocking the knee from full extension is done by the popliteus. When the leg is straight, the knee is held in a locked position by the screw-home mechanism, with the tibia externally rotated relative to the femur. The popliteus contracts to rotate the tibia medially (or, in a closed-chain context, to rotate the femur laterally), thereby breaking that lock so the knee can begin to flex. It’s located at the posterior knee and specifically facilitates the initial phase of flexion, often continuing to assist as the joint moves through bending. The other muscles don’t perform this unlocking role: the rectus femoris is a knee extender and would oppose unlocking; the gastrocnemius can assist knee flexion but isn’t the key unlocker; the tibialis anterior acts on the ankle and doesn’t contribute to unlocking the knee.