Final answer:
The hamstrings consist of the biceps femoris with two heads: the long head and the short head. No medial or lateral head exists for the biceps femoris. These muscles facilitate flexing the knee and extending the thigh.
Step-by-step explanation:
The muscles of the posterior thigh are known as the hamstrings, which are responsible for flexing the leg at the knee and extending the thigh. Specifically, the biceps femoris muscle of the hamstring group has two heads: a) the long head and b) the short head. There is no medial or lateral head for the biceps femoris; those terms are more commonly associated with muscles like the quadriceps femoris. The long head of the biceps femoris originates from the ischial tuberosity and the short head originates from the femur. Both heads work together to perform movements such as bringing the back of the lower legs up and back toward the buttocks, as in kneeling and also contribute to outward twisting of the thigh.