Physiological unlocking is caused by

Locking is produced by continued action of the same muscles that produce extension, i.e. the quadriceps femoris (the vastus lateralis, vastus medialis , vastus intermedius and the rectus femoris ). The locked knee joint can be flexed only after it is unlocked i.e., by lateral rotation of the femur. Unlocking is brought about by the action of the popliteus muscle. Ref : B D Chaurasia's Human Anatomy , seventh edition , volume 2, pg. no. 157