Biceps femoris muscle

The biceps femoris () is a muscle of the thigh located to the posterior, or back. As its name implies, it consists of two heads; the long head is considered part of the hamstring muscle group, while the short head is sometimes excluded from this characterization, as it only causes knee flexion (but not hip extension) and is activated by a separate nerve (the peroneal, as opposed to the tibial branch of the sciatic nerve).

Structure

It has two heads of origin:

the long head arises from the lower and inner impression on the posterior part of the tuberosity of the ischium. This is a common tendon origin with the semitendinosus muscle, and from the lower part of the sacrotuberous ligament.

the short head, arises from the lateral lip of the linea aspera, between the adductor magnus and vastus lateralis extending up almost as high as the insertion of the gluteus maximus, from the lateral prolongation of the linea aspera to within 5 cm. of the lateral condyle; and from the lateral intermuscular septum.

The two muscle heads joint together distally and unite in an intricate fashion. The fibers of the long head form a fusiform belly, which passes obliquely downward and lateralward across the sciatic nerve to end in an aponeurosis which covers the posterior surface of the muscle and receives the fibers of the short head. Inferiorly, the aponeurosis condenses to form a tendon which predominantly inserts onto the lateral side of the head of the fibula. There is a second small insertional attachment by a small tendon slip into the lateral condyle of the tibia.