Which of the following muscle acts as unlocker of knee?

Popliteus rotates the tibia medially on the femur or, when the tibia is fixed, rotates the femur laterally on the tibia. At the beginning of flexion of the fully extended knee, lateral femoral rotation by popliteus muscle unlocks the joint.Must know:Locking of knee joint is due to the the action of quadriceps femoris, that brings about medial rotation of femur on tibia in later stages of extension. Ref: Snells, Clinical anatomy, 7th Edition, Page 650.