Unlocking of the knee to allow flexion is done by

Unlocking is the lateral rotation of the femur on the tibia during the initial phase of flexion which is principally done by the popliteus. The unlocked knee can be fuher flexed by the hamstrings. Unlocking and locking of the knee must occur alternately to allow locomotion. Gastrocnemius helps in plantar flexion. Vastus medialis is a component of quadriceps femoris that help in locking of the knee. Reference: Textbook of anatomy abdomen and lower limb, Vishram Singh, 2nd edition, page no447, 414