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