Biceps Femoris

Introduction

Introduction to Biceps Femoris

The biceps femoris is the most lateral of the three hamstring muscles, occupying the posterior compartment of the thigh. It has two distinct heads of origin: the long head and the short head. The long head arises from the ischial tuberosity, sharing a common origin with the semitendinosus, while the short head originates from the linea aspera of the femur. The two heads converge to form a common tendon that inserts onto the head of the fibula. The biceps femoris is a powerful flexor of the knee, an extensor of the hip (long head only), and a lateral rotator of the leg when the knee is flexed. Its dual innervation—tibial nerve for the long head and common peroneal nerve for the short head—is unique among the hamstrings and clinically significant. The muscle is commonly injured in sprinting and kicking sports, and its distal tendon forms the superolateral border of the popliteal fossa, where it is closely related to the common peroneal nerve.

Study

1. Origin and Insertion

The long head of biceps femoris originates from the upper medial facet of the ischial tuberosity via a conjoint tendon with the semitendinosus, and also from the sacrotuberous ligament. The short head arises from the lateral lip of the linea aspera, the lateral supracondylar line of the femur, and the lateral intermuscular septum. The two heads unite in the distal third of the thigh to form a common tendon that crosses the posterolateral aspect of the knee joint and inserts primarily onto the head of the fibula. Additional slips pass to the lateral tibial condyle and to the deep fascia of the leg.

2. Relations

The biceps femoris forms the lateral boundary of the popliteal fossa. Superficially, it is covered by the deep fascia of the thigh. The long head is superficial to the short head. Deep to the muscle lie the sciatic nerve, the profunda femoris artery, and the adductor magnus. The common peroneal nerve descends along the medial border of the biceps tendon in the distal thigh and popliteal fossa, making it vulnerable during injuries to the lateral knee or during surgical approaches.

3. Nerve Supply

The biceps femoris is the only hamstring muscle with dual innervation. The long head is supplied by the tibial division of the sciatic nerve (L5, S1, S2). The short head is innervated by the common peroneal division of the sciatic nerve (L5, S1, S2). This distinction is important in nerve conduction studies: denervation of the short head of biceps femoris is an early sign of common peroneal nerve pathology.

4. Blood Supply

The biceps femoris receives its blood supply from multiple sources: the perforating branches of the profunda femoris artery, the inferior gluteal artery (to the long head), and the popliteal artery. The rich anastomotic network ensures adequate perfusion even after injury.

5. Actions

Both heads flex the knee and laterally rotate the leg when the knee is flexed. The long head also extends the hip joint, acting synergistically with the other hamstrings (semitendinosus and semimembranosus). During gait, the long head decelerates knee extension at the end of the swing phase and assists in hip extension at heel strike. The short head, as a purely knee flexor, helps fine-tune knee motion.

6. Clinical Relevance

Hamstring strains: The biceps femoris is the most commonly injured hamstring muscle, typically at the myotendinous junction during eccentric overload (sprinting). The injury usually occurs at the proximal portion of the long head. Avulsion fractures of the ischial tuberosity may involve the proximal hamstring origin, including the long head of biceps femoris. Proximal hamstring tendinopathy causes deep buttock pain worsened by sitting and stretching.

Common peroneal nerve injury: The close relationship of the biceps tendon to the common peroneal nerve means that fibular head fractures, knee dislocations, or tight casts can injure the nerve. In biceps femoris tendon repairs, the nerve must be identified and protected.

Baker's cyst (popliteal cyst): A fluid-filled mass in the popliteal fossa may develop between the semimembranosus tendon and the medial head of gastrocnemius; the biceps femoris tendon is the lateral landmark. The cyst can compress the tibial nerve. Distal biceps femoris tendon ruptures are rare but can occur, presenting as pain and weakness of knee flexion with a palpable defect laterally.

Summary

Summary of Biceps Femoris

The biceps femoris is a lateral hamstring muscle with two heads: a long head from the ischial tuberosity and a short head from the linea aspera, both inserting on the fibular head. It flexes the knee and laterally rotates the leg; the long head also extends the hip. The long head is innervated by the tibial nerve, the short head by the common peroneal nerve. It is the most commonly injured hamstring muscle and its tendon guides the common peroneal nerve at the knee.