Corpus: Biceps femoris muscle

The biceps femoris muscle is a two-headed muscle belonging to the group of ischiocrural muscles. It is located on the dorsal side of the femur.

The biceps femoris muscle consists of two parts:

• The long head originates from the ischial tuberosity of the ischium bone. It shares a common origin with the semitendinosus muscle on the sacrotuberous ligament, known as the common head.
• The short head riginates from the lateral labium of the linea aspera in the distal third of the femur and the lateral intermuscular septum.

Distally, the muscle bodies of the long and short heads unite and attach to the head of the fibula and, with some fibers, to the lateral condyle of the tibia. The inferior bursa of biceps femoris separates the biceps femoris muscle from the lateral collateral ligament of the knee joint.

The biceps femoris muscle has a double innervation: the long head is innervated by the tibial nerve from the sacral plexus (segments: L5-S2). The short head is innervated by the common fibular nerve, also known as the common peroneal nerve (segments: L5-S2), which also originates from the sacral plexus.

The long head causes extension and slight external rotation in the hip joint and stabilizes the pelvis in the sagittal plane. Both the long and short head flex the knee joint. The biceps femoris muscle is the only muscle capable of externally rotating the knee joint when it is in a flexed position.

Due to the innervation of the short head by the common fibular nerve, the ability to flex the knee joint is retained even in cases of proximal damage to the tibial nerve.