Hamstring muscle of the biceps femoris
The biceps femoris is one of the largest and longest muscles in the hamstring. (The other is semi-tendinosis; the biceps femoris lies lateral to or outside of the semi-tendinosis.)
The biceps femoris has two heads: one long and one short. On the thigh, both the long and short heads twist the thigh outward and pull it back. At the knees, they flex and rotate the joint laterally. Of all the hamstring muscles, the biceps femoris contributes the most to hip extension.
The long head of the biceps femoris originates from the medial side of the sessile bones, which are small bony projections technically called the ischial tubercles. The ischial tubercles are located in the lower part of the pelvis. (You probably feel them when you sit down.)
The short head of the biceps femoris originates from three places on the thigh (i.e., the femur), which are closer to the knee than to the thigh. Note that the short head of the hamstring is the only part of the hamstring group that does not cross two joints. For this reason, some experts do not consider it a hamstring muscle at all. Some people really don't have that.
Both the long and short heads of the biceps attach to the lower leg, also in three places: the head of the fibula, the external (called the lateral) condyle of the tibia, and the fascia of the leg. Multiple attachment points on the lower leg can cause more tears here than other hamstring muscles.