Muscles whose action lines pass anterior to axis of knee are extensors of knee (quadriceps femoris) most extensors originate of femoral surface and extend along anterior and lateral surfaces of thigh. Muscles whose action lines pass posterior to axis of knee are flexors of knee (hamstrings) originate of pelvic girdle and extend along posterior and medial surfaces of thigh. Adductor magnus works with hamstrings to extend hip. Pes anserinus: bursitis in area where the three hamstring muscles attach. Biceps femoris: part of hamstrings (lateral)(protects sciatic nerve) (flex knee at 90, feel tendon of biceps femoris laterally) O: ischial tuberosity(long head) and linea aspera of femur(short head) I: head of fibula, lateral condyle of tibia. Semimembranosus: part of hamstrings (medial)(wider muscle, deep to semitendinosus) (flex knee, medial tendon) I: posterior surface of medial condyle of tibia. Semitendinosus: part of hamstrings (long tendon) (medial) (flex knee, prominent tendon medially) I: anterior, medial surface of tibia near insertion of gracilis.