Fibularis Longus

The fibularis longus, also known as peroneus longus, is the larger and more superficial of the two principal muscles of the lateral compartment of the leg. It arises from the proximal fibula, descends along the lateral side of the leg, passes posterior to the lateral malleolus, and then follows a distinctive course across the plantar aspect of the foot before inserting on the medial cuneiform and base of the first metatarsal.^[1][2]

Fibularis longus primarily produces eversion of the foot and assists plantarflexion of the ankle. Its long plantar course also gives it an important role in supporting the arches of the foot, particularly the transverse arch and lateral-to-medial stability of the forefoot.

The muscle is innervated by the superficial fibular nerve and works closely with fibularis brevis. Its tendon has important relationships with the lateral malleolus, calcaneus, cuboid, first metatarsal, and medial cuneiform.

Location

Fibularis longus occupies the lateral compartment of the leg. It is superficial to fibularis brevis through much of the compartment and forms a substantial part of the muscular contour of the proximal lateral leg.

The muscle belly begins near the head and proximal shaft of the fibula and extends distally before becoming tendinous. Its tendon passes behind the lateral malleolus and continues along the lateral side of the foot.

Unlike fibularis brevis, which terminates at the fifth metatarsal, the fibularis longus tendon continues onto the sole and crosses obliquely toward the medial side of the foot.

General Anatomy

Feature	Anatomy
Muscle	Fibularis longus
Alternative name	Peroneus longus
Compartment	Lateral compartment of leg
Origin	Head and proximal lateral surface of fibula, adjacent fascia and intermuscular septa
Insertion	Plantar aspect of medial cuneiform and base of first metatarsal
Innervation	Superficial fibular nerve
Root values	L5, S1, S2
Principal action	Eversion of foot
Secondary action	Assists plantarflexion of ankle
Arch support	Contributes to transverse and longitudinal arch stability

Origin

Fibularis longus arises from the head of the fibula and the proximal portion of the lateral surface of the fibular shaft.^[1][3]

Additional fibers arise from the deep fascia and the anterior and posterior intermuscular septa that define the lateral compartment.

The broad proximal attachment gives fibularis longus a relatively large muscle belly compared with fibularis brevis.

Insertion

The tendon inserts onto the plantar aspect of the medial cuneiform and base of the first metatarsal.

This insertion is remarkable because the muscle originates on the lateral side of the leg but ultimately attaches to the medial side of the foot.

The oblique plantar course of the tendon allows fibularis longus to exert force across the width of the foot and contributes to stabilization of the first ray and transverse arch.

Course

Fibularis longus descends along the lateral side of the fibula and becomes tendinous in the distal leg. Its tendon then passes posterior to the lateral malleolus.

After curving around the lateral malleolus, the tendon passes along the lateral surface of the calcaneus and approaches the cuboid.

It then turns onto the plantar surface of the foot and enters the groove associated with the cuboid. From there, the tendon crosses the sole obliquely toward the medial cuneiform and first metatarsal.

Lateral Compartment of the Leg

The lateral compartment contains two principal muscles:

• Fibularis longus
• Fibularis brevis

Both muscles are supplied by the superficial fibular nerve and act primarily as evertors of the foot.

The compartment is bounded by the fibula, deep fascia of the leg, and anterior and posterior intermuscular septa.

Relationship to Fibularis Brevis

Fibularis longus lies superficial to fibularis brevis through much of the lateral compartment. The two muscles share a common nerve supply and perform similar actions at the ankle and foot.

Their tendons pass together behind the lateral malleolus, where the fibularis brevis tendon generally lies anterior to the fibularis longus tendon.

Distal to the malleolus, the tendons separate. Fibularis brevis travels toward the fifth metatarsal, while fibularis longus continues toward the plantar surface of the foot.

Relationship to the Common Fibular Nerve

The common fibular nerve winds around the neck of the fibula near the proximal attachment of fibularis longus.

The nerve then divides into superficial and deep fibular branches. The superficial fibular nerve enters the lateral compartment and supplies fibularis longus and fibularis brevis.

The close relationship of the common fibular nerve to the fibular neck makes this region clinically important because the nerve is relatively superficial and vulnerable to injury.

Superficial Fibular Nerve

The superficial fibular nerve is the motor nerve of the lateral compartment. It descends between or in close relationship to fibularis longus and fibularis brevis.

After supplying these muscles, the nerve becomes superficial in the distal leg by piercing the deep fascia.

Its terminal branches provide cutaneous innervation to much of the dorsum of the foot, with important exceptions supplied by neighboring nerves.

Relationship to the Lateral Malleolus

The fibularis longus tendon passes posterior to the lateral malleolus. This bony prominence redirects the tendon from a vertical course in the leg toward the lateral and plantar aspects of the foot.

The tendon shares the retromalleolar groove with fibularis brevis and is held in place by the superior fibular retinaculum.

Because both tendons make a significant change in direction at this location, the retromalleolar region is a common site of fibular tendon pathology.

Fibular Retinacula

The superior and inferior fibular retinacula stabilize the fibularis longus and brevis tendons around the lateral ankle.

The superior fibular retinaculum holds the tendons behind the lateral malleolus. More distally, the inferior fibular retinaculum secures them against the lateral calcaneal region.

These structures prevent bowstringing and excessive displacement of the tendons during ankle movement.

Relationship to the Calcaneus

After passing behind the lateral malleolus, fibularis longus travels along the lateral surface of the calcaneus.

A bony prominence called the fibular trochlea may separate the courses of fibularis brevis and fibularis longus. Fibularis brevis generally passes superior to the trochlea, while fibularis longus passes inferior to it.

The inferior fibular retinaculum helps maintain the tendons against the calcaneus.

Relationship to the Cuboid

The cuboid is a key landmark in the course of the fibularis longus tendon. The tendon passes through a groove on the plantar aspect of the cuboid as it changes direction toward the medial side of the foot.

The cuboid therefore functions as a pulley for the tendon, redirecting its force across the plantar surface.

A sesamoid-like ossification called the os peroneum may occur within the fibularis longus tendon near the cuboid.

Plantar Course

After passing around the cuboid, the fibularis longus tendon crosses the sole of the foot obliquely from lateral to medial.

It travels deep within the plantar region toward the medial cuneiform and first metatarsal.

This transverse course distinguishes fibularis longus from nearly every other extrinsic muscle tendon of the foot and explains its important contribution to transverse arch stability.

Relationship to Tibialis Anterior

Fibularis longus and tibialis anterior insert near opposite surfaces of the same medial foot region.

Tibialis anterior approaches the medial cuneiform and first metatarsal from the anterior and medial side, while fibularis longus reaches this region after crossing the plantar surface from the lateral side.

The two muscles have opposing actions at the ankle and subtalar region. Tibialis anterior dorsiflexes and inverts, while fibularis longus plantarflexes and everts.

Their complementary attachments help stabilize the medial column of the foot.

Relationship to the First Metatarsal

The fibularis longus tendon inserts on the plantar aspect of the base of the first metatarsal.

Contraction can exert a plantarward force on the first ray, helping stabilize the medial forefoot against the ground during weight-bearing.

This function is particularly important during later stages of stance when forces are transmitted toward the forefoot.

Relationship to the Medial Cuneiform

The medial cuneiform provides the other major distal attachment of fibularis longus.

The tendon reaches the bone from its plantar surface after traversing the width of the foot.

This attachment contributes to the muscle's ability to stabilize the first tarsometatarsal region and support the architecture of the foot.

Innervation

Fibularis longus is innervated by the superficial fibular nerve, with root contributions commonly described as L5, S1, and S2.

The superficial fibular nerve arises from the common fibular nerve near the proximal fibula and enters the lateral compartment.

It supplies both fibularis longus and fibularis brevis before becoming cutaneous in the distal leg.

Blood Supply

Fibularis longus receives blood from regional muscular branches, particularly branches associated with the fibular artery and neighboring arterial networks.

The fibular artery arises from the posterior tibial artery and descends in close association with the fibula within the deep posterior region of the leg.

Perforating and muscular branches contribute to the vascular supply of the lateral compartment and lateral ankle.

Actions

Eversion

The primary action of fibularis longus is eversion of the foot. Its tendon passes behind the lateral malleolus and exerts force from the lateral side of the foot.

Contraction elevates the lateral border and turns the plantar surface away from the midline.

Fibularis longus acts with fibularis brevis as one of the principal evertors of the foot.

Plantarflexion

Because the tendon passes posterior to the transverse axis of the ankle joint, fibularis longus assists plantarflexion.

Its plantarflexion force is weaker than that of gastrocnemius and soleus, but it contributes when plantarflexion occurs together with eversion.

Stabilization of the First Ray

The distal attachment of fibularis longus allows it to exert a plantarward force on the first metatarsal.

This helps stabilize the first ray during weight-bearing and contributes to an effective base for force transmission through the medial forefoot.

The action is especially relevant during push-off in walking and running.

Support of the Transverse Arch

The oblique course of the fibularis longus tendon across the sole allows it to contribute to support of the transverse arch of the foot.

The tendon acts like a dynamic sling extending from the lateral side of the foot toward the medial cuneiform and first metatarsal.

Its action complements the intrinsic muscles, ligaments, and bony configuration that maintain transverse arch stability.

Support of the Longitudinal Arches

Fibularis longus also contributes to dynamic support of the longitudinal architecture of the foot.

By stabilizing the lateral side of the foot and plantarflexing the first ray, it helps maintain appropriate relationships between the forefoot and hindfoot during weight-bearing.

Its role occurs in combination with tibialis posterior, tibialis anterior, intrinsic foot muscles, the plantar aponeurosis, and plantar ligaments.

Role During Walking

During gait, fibularis longus contributes to lateral stability and helps control inversion and eversion as body weight passes over the foot.

During stance, the muscle helps stabilize the first metatarsal against the ground and supports transfer of load toward the medial forefoot.

Its contribution becomes particularly important as the heel rises and the body advances toward push-off.

Role in Ankle Stability

Fibularis longus contributes to dynamic lateral ankle stability by opposing excessive inversion.

Rapid activation of the fibular muscles can help resist inversion forces that place the lateral ankle ligaments under stress.

This active muscular control complements the passive stabilization provided by the lateral ligament complex.

Comparison with Fibularis Brevis

Feature	Fibularis Longus	Fibularis Brevis
Relative position	Superficial	Deep
Proximal attachment	Head and proximal fibula	Distal fibula
Distal attachment	Medial cuneiform and first metatarsal	Fifth metatarsal tuberosity
Course across sole	Yes	No
Primary action	Eversion	Eversion
Innervation	Superficial fibular nerve	Superficial fibular nerve

Comparison with Tibialis Posterior

Fibularis longus and tibialis posterior both contribute to plantarflexion and arch support, but their effects on inversion and eversion differ.

Fibularis longus everts the foot, whereas tibialis posterior is a strong inverter. Their coordinated contraction can therefore contribute to plantarflexion and stabilization while balancing frontal-plane movements.

Both muscles have extensive relationships with the architecture of the foot and contribute dynamically to maintaining the arches during weight-bearing.

Os Peroneum

The os peroneum is an accessory ossicle that may be present within the fibularis longus tendon near the cuboid.

When present, it lies at a point where the tendon changes direction around the lateral plantar foot.

The ossicle can be visible on radiographs and should be distinguished from an acute fracture fragment. It can also participate in disorders involving the fibularis longus tendon.

Anatomical Variations

Fibularis longus can vary in the extent of its muscular origin, tendon morphology, and distal attachment.

The size and morphology of the fibular trochlea and cuboid groove may also vary and can influence the local course of the tendon.

Accessory fibular muscles, particularly fibularis quartus, may alter the anatomy of the retromalleolar region and increase crowding around the fibular tendons.

Clinical Significance

Fibularis Longus Tendinopathy

Fibularis longus tendinopathy can produce pain along the lateral ankle or plantar-lateral aspect of the foot.

Repetitive loading, altered foot mechanics, chronic ankle instability, and athletic activity can increase stress on the tendon.

Symptoms may occur at several points along its long course, including behind the lateral malleolus and near the cuboid.

Fibularis Longus Tendon Tear

The fibularis longus tendon can develop partial or complete tears. Regions where the tendon changes direction, particularly around the lateral malleolus and cuboid, may experience substantial mechanical stress.

Tears can produce pain, weakness of eversion, and impaired stabilization of the first ray.

MRI and ultrasound can help determine the location and extent of tendon injury.

Fibular Tendon Subluxation

Injury to the superior fibular retinaculum can permit fibularis longus and brevis to subluxate or dislocate from their normal position behind the lateral malleolus.

Patients may experience snapping or instability around the posterolateral ankle.

Dynamic ultrasound can be particularly useful because tendon displacement may occur primarily during movement.

Os Peroneum Syndrome

Abnormalities involving the os peroneum and surrounding fibularis longus tendon may produce lateral plantar foot pain.

Potential abnormalities include fracture or displacement of the ossicle and associated injury to the fibularis longus tendon.

Recognition of the normal location and variable appearance of the os peroneum is therefore important on imaging.

Lateral Ankle Sprain

The fibular muscles provide dynamic resistance against excessive inversion, the mechanism responsible for many lateral ankle sprains.

Acute inversion can strain the fibular muscles and tendons in addition to injuring the lateral ligament complex.

Recurrent ankle sprains can also alter neuromuscular control and increase stress on the fibular tendons.

Superficial Fibular Nerve Injury

Injury to the superficial fibular nerve can weaken fibularis longus and fibularis brevis, reducing the strength of foot eversion.

Because the nerve also supplies cutaneous branches to much of the dorsum of the foot, sensory changes may accompany the motor deficit.

More proximal common fibular nerve injury can additionally affect the anterior compartment and produce more extensive motor impairment.

Common Fibular Nerve Injury

The common fibular nerve is particularly vulnerable around the fibular neck, near the proximal origin of fibularis longus.

Injury can affect both the superficial and deep fibular divisions, causing weakness of eversion as well as dorsiflexion and toe extension.

The combination of dorsiflexor and evertor weakness can substantially alter gait and foot positioning.

Chronic Ankle Instability

Fibularis longus and brevis are important dynamic stabilizers in individuals with chronic lateral ankle instability.

Recurrent ligament injury, altered proprioception, and impaired activation of the fibular muscles can reduce resistance to repeated inversion.

The lateral compartment muscles therefore form an important component of functional ankle stability in addition to their role as primary evertors.

Imaging

The fibularis longus muscle and tendon can be evaluated with ultrasound and MRI.

Ultrasound provides dynamic assessment of the tendon behind the lateral malleolus and can demonstrate subluxation, tears, or tenosynovitis. MRI provides detailed visualization of the tendon throughout the lateral ankle and plantar foot.

Radiographs and CT can additionally demonstrate the os peroneum and associated bony abnormalities.

Surface Anatomy

The fibularis longus muscle belly contributes to the contour of the proximal lateral leg. Contraction during resisted eversion can make the lateral compartment more prominent.

The tendon can be palpated posterior to the lateral malleolus, where it lies with the fibularis brevis tendon. Distally, its course becomes deeper as it passes toward the cuboid and plantar surface of the foot.

Its complete anatomical pathway, from the fibular head down the lateral leg, behind the lateral malleolus, around the cuboid, and across the sole to the first metatarsal and medial cuneiform, explains how a muscle originating on the lateral side of the leg can simultaneously evert the foot, assist plantarflexion, stabilize the first ray, and contribute to the structural support of the foot.