Physiological unlocking is caused by -
During last stages of extension, femur rotates medially on tibia due to quadriceps femoris to lock the knee joint. Femur rotates laterally on tibia during flexion. 
Quadriceps femoris- Vastus medialis, vastus lateralis, vastus intermedius, rectus femoris. Unlocking occurs with popliteus.
Popliteus
Rectus femoris
Semimembranosus
Sartorius