Biceps Femoris Muscle

Anatomy and Origin

The muscle is situated along the lateral margin of the posterior thigh, forming a prominent lateral contour superior to the popliteal fossa.

It arises from two distinct heads:

• Long head (caput longum): originates from the ischial tuberosity (tuber ischiadicum) via a narrow, flat tendon shared in common with the m. semitendinosus.
• Short head (caput breve): originates from the lateral lip of the linea aspera (linea aspera) in the lower third of the femoral shaft.

Both heads converge and transition into a robust tendon that passes inferiorly and laterally. This tendon inserts primarily onto the head of the fibula (caput fibulae), with additional fibers blending into the crural fascia and the lateral condyle of the tibia.

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

Topography and Neighboring Structures

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

At the knee, the biceps femoris tendon forms the superolateral boundary of the popliteal fossa (fossa poplitea). The superomedial boundary is formed by the tendons of the m. semitendinosus and m. semimembranosus. The popliteal fossa contains:

• Tibial nerve (n. tibialis): located more superficially and medially;
• Common fibular nerve (n. fibularis communis): located more deeply and laterally, coursing toward the neck of the fibula;
• Popliteal vein (v. poplitea): positioned anterior to the tibial nerve;
• Popliteal artery (a. poplitea): the deepest structure, representing the continuation of the femoral artery from the adductor canal.

Functions and Biomechanics

The biceps femoris acts across both the hip and knee joints and plays a critical role in gait and trunk stabilization:

• Hip joint: The long head extends the flexed thigh and assists in pelvic tilt stabilization.
• Knee joint: Flexes the leg and laterally rotates it when the knee is flexed.
• Closed kinetic chain: When the foot is fixed, it assists in extending the trunk against resistance.
• Gait cycle: Near the end of the swing phase, the biceps femoris contracts eccentrically to decelerate forward limb momentum prior to heel strike. Simultaneous activation of flexors and extensors creates a rigid lever known as the "pillar leg" phenomenon.

Blood Supply and Innervation

The muscle receives arterial blood from several sources:

• Medial femoral circumflex artery (a. circumflexa femoris medialis);
• Perforating branches of the deep femoral artery (aa. perforantes);
• Popliteal artery (a. poplitea).

Innervation is dual and head-specific:

• Long head: Innervated by the tibial division of the sciatic nerve (n. ischiadicus, spinal segments S1–S3).
• Short head: Innervated by the common fibular (peroneal) division of the sciatic nerve (n. ischiadicus, spinal segments L4–S1).

Clinical Significance

The biceps femoris is frequently involved in hamstring injuries, particularly hamstring strains during high-speed sprinting or explosive deceleration. Typical symptoms include localized pain near the ischial tuberosity or posterior to the knee joint.

Manual muscle testing for the hamstrings includes:

• Prone knee flexion against resistance;
• Knee flexion at 30–90° angles combined with internal or external tibial rotation.

In mononeuropathies or lumbosacral radiculopathies (affecting L4–S2), needle electromyography (EMG) of both the short and long heads of the biceps femoris helps localize lesions of the sciatic nerve or its branches. Diagnostic studies performed 3–4 weeks post-injury reveal maximal denervation changes, such as positive sharp waves and fibrillation potentials.