Biceps femoris muscle

Location and anatomy

The muscle is located along the lateral border of the posterior aspect of the thigh. It forms a prominent lateral cord below the popliteal fossa.

The muscle originates by two heads:

• Long head (caput longum) — originates from the ischial tuberosity (tuber ischiadicum) by a thin, flat tendon. This origin is shared with the semitendinosus muscle (m. semitendinosus).
• Short head (caput breve) — originates from the lateral lip of the linea aspera (linea aspera) in the lower third of the femur.

Both heads merge and continue into a robust tendon that courses laterally and inferiorly. It inserts primarily into the head of the fibula (caput fibulae), and also partially blends into the fascia of the leg and attaches to the lateral condyle of the tibia.

The inferior subtendinous bursa of the biceps femoris (bursa subtendinea m. bicipitis femoris inferior) is located between the muscle tendon and the lateral collateral ligament of the knee joint.

Topography and adjacent structures

In a cross-section of the upper third of the thigh, the m. biceps femoris (its long head) belongs to the posterior compartment muscles along with the semitendinosus, semimembranosus, and gluteus maximus muscles.

In the region of the knee, the tendon of the biceps femoris forms the superolateral border of the popliteal fossa (fossa poplitea). The superomedial border is formed by the tendons of the semitendinosus and semimembranosus muscles. The popliteal fossa contains:

• n. tibialis — more medially and superficially;
• n. fibularis communis — more laterally and deeply, coursing toward the neck of the fibula;
• v. poplitea — anterior to n. tibialis;
• a. poplitea — the deepest structure, a continuation of the femoral artery from the adductor canal.

Functions and biomechanics

The biceps femoris provides movement in the hip and knee joints and participates in trunk movements and walking:

• At the hip joint: the long head extends the thigh.
• At the knee joint: flexes the leg and rotates it externally.
• With a fixed lower limb: participates in trunk extension.
• During walking: at the end of the swing phase, the biceps femoris contracts to halt the forward movement of the limb before ground contact. Simultaneously, extensor muscles are activated; the combined contraction of flexors and extensors makes the leg rigid — the "post-like leg" phenomenon.

Blood supply and innervation

The blood supply to the muscle comes from several sources:

• medial femoral circumflex artery (a. circumflexa femoris medialis);
• perforating arteries (aa. perforantes);
• popliteal artery (a. poplitea).

The innervation of the heads differs:

• The long head is innervated by branches of the tibial nerve (n. tibialis) and sciatic nerve (n. ischiadicus) from segments S1–S3.
• The short head receives branches from the common fibular nerve (n. peroneus communis / fibularis communis) and sciatic nerve (n. ischiadicus) from segments L4–L5, S1.

In the table of lower limb muscle functions and innervation, the sciatic nerve and segments L4–S2 are also listed for the biceps femoris.

Clinical significance

The biceps femoris is frequently involved in hamstring injuries. A typical problem is a hamstring strain (hamstring strain) during sprinting. In this case, pain is described along the course of the ischial tuberosity and posterior to the medial or lateral condyles.

To assess the strength of the posterior compartment muscles, the following tests are used:

• hip extension against resistance in the prone position;
• knee flexion at an angle of 30–90° with evaluation of medial or lateral leg rotation.

In mononeuropathies, to assess the motor function of muscles innervated by the tibial nerve, the short and long heads of the biceps femoris are listed among the examined muscles. To detect denervation-reinnervation changes, needle electromyography (EMG) examining the short and long heads of the biceps femoris is recommended. The study is performed 3–4 weeks after the onset of symptoms; during this period, maximal changes — denervation spontaneous activity — develop. Typical findings include positive sharp waves and fibrillation potentials.

Class and exam questions

What else to study on this topic

• Popliteal fossa (fossa poplitea)
• Sciatic nerve (n. ischiadicus)
• Semitendinosus muscle (m. semitendinosus)
• Semimembranosus muscle (m. semimembranosus)

Other topics in the "Anatomical Structures" section

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