Chief Extensor at knee joint is
Biceps femoris
Quadriceps femoris
Semitendinosus
Semimembranosus
Quadriceps femoris is knee Extensor. Hamstrings (Biceps femoris long head, Semitendinosus, Semimembranosus and Adductor magnus) are Knee Flexors.
Muscles of Lower Limb