Biceps Femoris

The biceps femoris is a large muscle of the posterior compartment of the thigh and forms the lateral component of the hamstring region. As its name indicates, the muscle has two heads: a long head arising from the ischial tuberosity and a short head arising from the femur. The two heads converge distally into a common tendon that inserts principally onto the head of the fibula.^[1][2]

The long head is one of the true hamstring muscles and crosses both the hip and knee joints. It therefore contributes to extension of the thigh and flexion of the knee. The short head crosses only the knee and contributes to knee flexion. Together, the two heads produce lateral rotation of the flexed leg.

Biceps femoris is unusual because its two heads receive innervation from different divisions of the sciatic nerve. The long head is supplied by the tibial division of the sciatic nerve, while the short head is supplied by the common fibular division.

Location

Biceps femoris occupies the posterolateral aspect of the thigh. It lies lateral to semitendinosus and semimembranosus and extends from the pelvis and posterior femur toward the lateral side of the knee.

The long head is superficial through much of its course. The short head lies deep to the long head and becomes visible distally as it joins the common biceps femoris tendon.

Near the knee, the tendon becomes a prominent landmark on the posterolateral side and forms the superolateral boundary of the popliteal fossa.

General Anatomy

Feature	Anatomy
Muscle	Biceps femoris
Compartment	Posterior compartment of thigh
Heads	Long head and short head
Long head origin	Ischial tuberosity
Short head origin	Linea aspera and lateral supracondylar region of femur
Insertion	Primarily head of fibula
Long head innervation	Tibial division of sciatic nerve
Short head innervation	Common fibular division of sciatic nerve
Primary knee action	Flexion
Rotation	Lateral rotation of flexed leg
Hip action	Long head assists extension of thigh

Long Head

The long head of biceps femoris is the larger and more superficial of the two heads.

It arises from the ischial tuberosity and descends along the posterolateral thigh. Its proximal tendon shares a close relationship with the origin of semitendinosus.

Because it crosses both the hip and knee, the long head is capable of acting at both joints and meets the classical anatomical definition of a hamstring muscle.

Short Head

The short head of biceps femoris arises from the posterior femur and lies deep to the long head.

Unlike the long head, it does not originate from the ischial tuberosity and does not cross the hip joint. It therefore acts only at the knee.

The short head is also distinguished by its innervation from the common fibular division of the sciatic nerve rather than the tibial division.

Origin

Long Head Origin

The long head originates from the ischial tuberosity, principally from its posteromedial region, in close association with the origin of semitendinosus.^[1][3]

The ischial tuberosity provides a strong proximal attachment for the hamstring muscles and transmits substantial forces during hip extension and knee movement.

Short Head Origin

The short head arises from the lateral lip of the linea aspera and the lateral supracondylar line of the femur, with additional attachment to the lateral intermuscular septum.

Its extensive femoral origin allows the fibers to converge toward the long head and common distal tendon.

Insertion

The two heads converge into a strong tendon that inserts principally onto the head of the fibula.

The distal tendon surrounds and attaches around portions of the fibular head and may send expansions toward adjacent fascia and the lateral side of the proximal tibia.

The close relationship between the tendon and fibular head makes the insertion readily identifiable on surface examination.

Course

The long head descends from the ischial tuberosity through the posterolateral thigh. The short head joins it from the posterior and lateral femur.

The combined muscle narrows distally into the prominent biceps femoris tendon, which crosses the posterolateral aspect of the knee.

The tendon passes toward the fibular head, with the common fibular nerve closely related to its medial and posterior aspects near the knee.

Posterior Compartment of the Thigh

The principal muscles of the posterior thigh include:

• Biceps femoris
• Semitendinosus
• Semimembranosus

The hamstring portion of adductor magnus is also functionally associated with this region.

The posterior compartment is supplied predominantly by branches of the sciatic nerve and receives arterial supply from perforating branches of the profunda femoris artery.

Hamstring Classification

The long head of biceps femoris is classified as a true hamstring because it arises from the ischial tuberosity, crosses both the hip and knee, and receives innervation from the tibial division of the sciatic nerve.

The short head is not considered a true hamstring under the traditional anatomical definition because it arises from the femur, crosses only the knee, and is supplied by the common fibular division of the sciatic nerve.

Relationship to Semitendinosus

Semitendinosus lies medial to the long head of biceps femoris.

The two muscles share a close proximal relationship at the ischial tuberosity but diverge distally. Semitendinosus travels toward the medial proximal tibia, while biceps femoris travels toward the fibular head on the lateral side.

Their opposing distal positions also produce different rotational actions at the flexed knee.

Relationship to Semimembranosus

Semimembranosus lies medial and deeper than much of biceps femoris.

Both muscles extend the hip and flex the knee, but semimembranosus produces medial rotation of the flexed leg, whereas biceps femoris produces lateral rotation.

The divergence of the medial and lateral hamstrings contributes to control of rotational movements at the knee.

Relationship to Adductor Magnus

The adductor magnus lies anterior to the hamstring muscles through much of the posterior thigh.

Biceps femoris therefore has an important deep relationship with the large adductor mass, particularly proximally and in the middle thigh.

Perforating branches of the profunda femoris artery pass through adductor magnus and contribute to the vascular supply of the posterior compartment.

Relationship to the Sciatic Nerve

The sciatic nerve descends through the posterior thigh deep to the long head of biceps femoris.

As it approaches the popliteal region, its tibial and common fibular components separate. The common fibular component follows the lateral side of the popliteal fossa toward the fibular neck.

This relationship is especially important because the two heads of biceps femoris receive motor branches from different components of the sciatic nerve.

Innervation

Long Head Innervation

The long head is supplied by the tibial division of the sciatic nerve, principally from spinal nerve roots L5, S1, and S2.

Short Head Innervation

The short head is supplied by the common fibular division of the sciatic nerve, principally from L5, S1, and S2.

This dual innervation reflects the different developmental origins of the two heads and is a distinctive feature of biceps femoris anatomy.

Relationship to the Common Fibular Nerve

The common fibular nerve follows the medial border of the biceps femoris tendon near the lateral side of the popliteal fossa.

It then passes toward the posterior aspect of the fibular head and winds around the neck of the fibula before dividing into its superficial and deep fibular branches.

The close relationship between the nerve, biceps femoris tendon, and fibular head is important during examination, surgery, and traumatic injury around the lateral knee.

Blood Supply

Biceps femoris receives its principal blood supply from perforating branches of the profunda femoris artery.

Additional contributions may arise from the inferior gluteal artery proximally and branches associated with the popliteal arterial network distally.

The segmental vascular pattern supplies the long muscle belly throughout the posterior thigh.

Actions

Knee Flexion

Both heads of biceps femoris produce flexion of the leg at the knee.

Because the common tendon passes posterior to the transverse axis of the knee, contraction draws the leg posteriorly relative to the thigh.

Biceps femoris works with semitendinosus, semimembranosus, and several assisting muscles during knee flexion.

Lateral Rotation of the Flexed Leg

When the knee is flexed, biceps femoris produces lateral rotation of the leg.

Its insertion on the lateral side of the proximal leg gives the muscle an effective line of pull for rotating the tibia laterally relative to the femur.

This action contrasts with semitendinosus and semimembranosus, which medially rotate the flexed leg.

Hip Extension

The long head assists extension of the thigh at the hip because it crosses posterior to the hip joint.

This action is particularly important when extending the hip from a flexed position, such as during rising, climbing, running, and powerful acceleration.

The short head does not cross the hip and therefore cannot contribute directly to hip extension.

Biarticular Function of the Long Head

The long head of biceps femoris is a biarticular muscle. Its length and force-generating capacity are influenced by the positions of both the hip and knee.

Hip flexion lengthens the long head proximally, while knee extension lengthens it distally. Combined hip flexion and knee extension therefore place considerable stretch on the muscle.

This anatomical arrangement contributes to the substantial mechanical demands placed on the long head during high-speed running.

Role During Walking

Biceps femoris contributes to coordinated control of the hip and knee during gait.

During late swing, the hamstrings contract eccentrically to decelerate knee extension and hip flexion as the limb prepares for ground contact.

During early stance, the long head can contribute to hip extension and stabilization of the limb as body weight is transferred onto the foot.

Role During Running

Running places considerably greater mechanical demands on biceps femoris than ordinary walking.

During terminal swing, the hamstrings undergo rapid lengthening while generating force to decelerate the advancing leg. The long head of biceps femoris is particularly exposed to high strain during this phase.

The muscle subsequently contributes to hip extension and knee control as the foot approaches and contacts the ground.

Role During Sprinting

The biceps femoris long head is heavily involved during sprinting. It must tolerate substantial eccentric loading during late swing and rapidly transition to force production around ground contact.

This combination of high force, rapid lengthening, and high movement velocity is one reason the biceps femoris long head is a frequent site of hamstring injury.

Role in Knee Stability

Biceps femoris contributes to dynamic stabilization of the lateral knee.

Its tendon crosses the posterolateral knee and attaches strongly to the fibular head. Through this position, contraction can influence rotational and posterolateral stability during movement.

Its function is integrated with the lateral collateral ligament, popliteus, lateral gastrocnemius, and other structures of the posterolateral knee.

Relationship to the Lateral Collateral Ligament

The distal biceps femoris tendon has an important relationship with the lateral collateral ligament, also called the fibular collateral ligament.

Both structures attach in the region of the fibular head, with the biceps femoris tendon splitting around or blending with tissues surrounding the ligament according to the specific attachment plane.

This complex anatomy is important when evaluating or reconstructing the posterolateral corner of the knee.

Popliteal Fossa

The biceps femoris forms the superolateral boundary of the popliteal fossa.

The superomedial boundary is formed principally by semimembranosus and semitendinosus, while the two heads of gastrocnemius contribute to the inferior boundaries.

The popliteal fossa contains the popliteal artery and vein, tibial nerve, common fibular nerve, lymphatic structures, and associated connective tissue.

Relationship to the Fibular Head

The fibular head is the principal distal attachment of biceps femoris and is readily palpable on the lateral side of the proximal leg.

The tendon can often be followed directly to this bony landmark, particularly when the knee is flexed against resistance.

The common fibular nerve passes close to the fibular head and neck, making this region clinically important in both traumatic and surgical conditions.

Comparison with the Medial Hamstrings

Feature	Biceps Femoris	Semitendinosus	Semimembranosus
Position	Lateral	Medial, superficial	Medial, deeper
Knee action	Flexion	Flexion	Flexion
Rotation of flexed leg	Lateral	Medial	Medial
Hip extension	Long head	Yes	Yes
Distal attachment	Fibular head	Medial proximal tibia	Posterior medial tibial condyle
Primary nerve division	Tibial and common fibular divisions	Tibial division	Tibial division

Comparison of the Long and Short Heads

Feature	Long Head	Short Head
Origin	Ischial tuberosity	Posterior femur
Crosses hip	Yes	No
Crosses knee	Yes	Yes
Hip extension	Yes	No
Knee flexion	Yes	Yes
Innervation	Tibial division of sciatic nerve	Common fibular division of sciatic nerve
True hamstring	Yes	No

Anatomical Variations

Biceps femoris can demonstrate variation in the size and architecture of its two heads, proximal attachments, distal tendon, and accessory muscular slips.

Accessory heads may arise from the femur, ischial region, or neighboring fascial structures and join the main muscle or tendon.

The distal tendon also demonstrates variation in its fibular, tibial, and fascial expansions. These variations can be relevant during imaging and posterolateral knee surgery.

Clinical Significance

Hamstring Strain

The biceps femoris, particularly its long head, is one of the most frequently injured muscles in hamstring strains.

Injury commonly occurs during high-speed running when the hamstrings are exposed to substantial eccentric loading during terminal swing.

Strains may occur near the proximal myotendinous junction, within the muscle belly, or more distally depending on the mechanism and severity.

Proximal Hamstring Injury

The long head shares its proximal region at the ischial tuberosity with other hamstring structures.

Forceful hip flexion combined with knee extension can produce injury ranging from muscular strain to partial or complete proximal tendon avulsion.

Severe injuries may involve multiple hamstring tendons and can produce substantial weakness, bruising, and functional impairment.

Distal Biceps Femoris Injury

Injury can occur near the distal myotendinous region or the fibular attachment of the biceps femoris tendon.

Distal injuries may produce lateral knee pain and weakness during resisted knee flexion or lateral rotation.

Because the tendon is closely related to the common fibular nerve and lateral stabilizing structures, accurate anatomical assessment is important.

Fibular Head Avulsion

Strong traction on the biceps femoris tendon can contribute to an avulsion injury of the fibular head, particularly when associated with trauma to posterolateral knee structures.

A small avulsed fragment from the fibular head may indicate significant injury to the lateral supporting structures of the knee.

The nearby common fibular nerve should also be evaluated when substantial trauma affects this region.

Common Fibular Nerve Injury

The common fibular nerve is vulnerable near the fibular neck because of its superficial position.

Injury in this region can affect dorsiflexion and eversion of the foot and produce sensory abnormalities over portions of the lateral leg and dorsum of the foot.

The close relationship of the nerve to the distal biceps femoris tendon is important during surgical exposure and repair.

Sciatic Nerve Injury

Injury to the sciatic nerve can affect biceps femoris depending on the level and division involved.

A lesion involving the tibial component may impair the long head, while involvement of the common fibular component may affect the short head.

More proximal sciatic injuries commonly affect multiple posterior thigh and leg muscles rather than producing isolated biceps femoris weakness.

Posterolateral Corner Injuries

The distal biceps femoris tendon is an important landmark in the posterolateral corner of the knee.

Injuries in this region can involve the fibular collateral ligament, popliteus complex, capsule, biceps femoris tendon, and associated structures.

Recognition of the layered relationships around the fibular head is essential when interpreting imaging and planning reconstruction.

Hamstring Tendinopathy

The proximal hamstring origin can develop tendinopathy associated with repetitive loading.

Symptoms are typically localized near the ischial tuberosity and may be aggravated by running, prolonged sitting, or activities that place the hamstrings under tension.

Biceps femoris long head may be involved together with semitendinosus and semimembranosus depending on the affected portion of the proximal tendon complex.

Imaging

MRI is particularly useful for evaluating biceps femoris injuries because it demonstrates the muscle bellies, myotendinous junctions, tendons, edema, hematoma, and associated posterior thigh structures.

Ultrasound can provide dynamic assessment of superficial portions of the muscle and distal tendon and can demonstrate tears or tendon abnormalities.

Radiographs may be useful when an avulsion fracture involving the ischial tuberosity or fibular head is suspected.

Surface Anatomy

Biceps femoris is readily identifiable along the posterolateral thigh, particularly during resisted knee flexion.

Its distal tendon becomes prominent immediately proximal to the lateral side of the knee and can be followed to the palpable head of the fibula. The tendon forms the superolateral border of the popliteal fossa.

The common fibular nerve lies closely related to the tendon and fibular head before winding around the fibular neck. These palpable relationships make biceps femoris an important surface landmark for examination of the posterior thigh, lateral knee, and common fibular nerve.