The hamstring muscles run along the back of your thigh, from your pelvis to your lower leg. There are three hamstrings: the semimembranosus, semitendinosus and biceps femoris. All three bend the knee and extend the hip joint, meaning they enable you to straighten your upper leg from a bent position and move your thigh back toward the buttocks. The hamstring muscles play important roles in many common activities, including walking and running.
Semimembranosus and Semitendinosus
Two of the hamstring muscles -- the semimembranosus and semitendinosus -- lie on the inner side of the back of your thigh. Both muscles originate from the sitting bone, or ischial tuberosity, on the back of the pelvis. The semimembranosus inserts onto the back of the upper shin bone, or tibia. The semitendinosus -- named for its long tendon -- attaches to the inner shaft of the tibia and helps form the bulge of tissue on the inner knee known as the pes anserinus, or "goose foot."
The biceps femoris, named for its two muscle bellies, or heads, lies on the outer side of the back of the thigh. The long head originates on the ischial tuberosity, close to the attachment point of the semitendinosus. The short head originates from the back of the lower thigh bone. Both heads insert onto the head of the fibula, the long bone that runs down the outer shin.