The posterior thigh muscles have their origins on the inferior ischial ramus.

http://en.wikipedia.org/wiki/Pelvis

The posterior thigh muscles have their origins on the inferior ischial ramus, with the exception of the short head of the biceps femoris. The semitendinosus and semimembranosus are inserted on the tibia on the medial side of the knee, while biceps femoris is inserted on the fibula, on the knee's lateral side.