Biceps femoris

Where is the biceps femoris?

It is one of the three hamstrings, the long muscles of the back of the thigh, with semitendinosus and semimembranosus. (StatPearls, Posterior Thigh Muscles; OpenStax A&P 2e, 11.6)

Above the knee it forms the upper outer border of the popliteal fossa; semimembranosus and semitendinosus form the upper inner border, and the gastrocnemius the lower one. (StatPearls, Hamstring Muscle)

What are the two heads of the biceps femoris?

The long head arises from the ischial tuberosity by a common tendon with semitendinosus. The short head arises from the lateral lip of the linea aspera, the lateral intermuscular septum and the lateral supracondylar ridge of the femur. (StatPearls, Posterior Thigh Muscles)

Both heads insert mostly on the head of the fibula, with fibers to the lateral collateral ligament of the knee and the lateral condyle of the tibia. (StatPearls, Posterior Thigh Muscles)

The short head is the one part of the hamstrings that does not cross the hip, because it arises on the femur below the hip joint. (StatPearls, Hamstring Muscle)

Biceps is Latin for two-headed, and the muscle arises by two heads, as biceps brachii does in the arm. (OpenStax A&P 2e, 11.2; Lewis and Short, biceps; Gray’s Anatomy (1918), Posterior Femoral Muscles and Biceps Brachii)

What does the biceps femoris do?

Both heads flex the knee and rotate the tibia laterally. The long head also extends the hip. (StatPearls, Posterior Thigh Muscles)

In walking, the hamstrings switch on in the last quarter of the swing phase, extending the hip and braking knee extension. With the anterior cruciate ligament they resist forward slide of the tibia at heel strike. (StatPearls, Hamstring Muscle)

Which nerves and muscles work with the biceps femoris?

Its two heads have different nerves, both parts of the sciatic nerve: the tibial nerve supplies the long head, and the common fibular nerve the short head. (StatPearls, Hamstring Muscle and Posterior Thigh Muscles)

The sciatic nerve runs down the back of the thigh deep to the long head and usually divides into the tibial and common fibular nerves near the top of the popliteal fossa. (StatPearls, Posterior Thigh Muscles and Tibial Nerve)

The hamstrings bend the knee and the quadriceps femoris straightens it, so the two groups pull against each other at that joint. (OpenStax A&P 2e, 11.1; Gray’s Anatomy (1918), Posterior Femoral Muscles and Anterior Femoral Muscles)

Does the biceps femoris vary?

Dissections have found a third head of biceps femoris, and in one body the short and long heads never joined, though both still inserted on the head of the fibula. (StatPearls, Hamstring Muscle and Posterior Thigh Muscles)

Where the sources differ

Sources place the division of the sciatic nerve at different levels. The StatPearls posterior thigh article puts it at the upper border of the popliteal fossa and its hamstring article at the level of the knee joint. Its tibial nerve article puts it a few centimeters above the fossa, and in some people inside the pelvis. (StatPearls, Posterior Thigh Muscles, Hamstring Muscle and Tibial Nerve)