actions of the knee joint and primary muscles involved

The knee is responsible for flexion and extension
The Hamstrings: Biceps Femoris, Semimembranosus,Semitendinosus and The Quads: Biceps Femoris, Vastus Intermedius, Vastus Lateralis, and Vastus Medialis are the muscles involved.Last but not least, the Popliteus is responsible for knee flexion also.