Origin and Insertion
- Origin: Arises from the head and upper two-thirds of the lateral surface of the fibula, as well as from the deep crural fascia.
- Insertion: The tendon inserts onto the plantar aspect of the medial cuneiform and the base of the 1st metatarsal bone.
Tendon Course and Retinacula
The tendon passes posterior to the lateral malleolus. The crural fascia is reinforced here by two retinacula passing to the calcaneus:
- Superior fibular retinaculum (retinaculum mm. fibularium superius): The tendon of the fibularis longus shares a common synovial sheath with the fibularis brevis tendon. The sheath extends 4–5 cm proximal to the retinaculum.
- Inferior fibular retinaculum (retinaculum mm. fibularium inferius): The tendons occupy separate synovial sheaths, separated by the fibular trochlea (trochlea fibularis) of the calcaneus.
From there, the tendon of the fibularis longus runs along the lateral side of the foot, turns into the groove of the cuboid bone, and crosses obliquely to the medial side of the sole. In the tarsal region, the tendon is enclosed within the fibers of the long plantar ligament (lig. plantare longum), which stretches from the calcaneus to the bases of the 2nd–5th metatarsals.
Topography and Adjacent Structures
- In the sole, the tendon belongs to the deep, fourth layer of the foot. It lies posterior to the oblique head of the adductor hallucis muscle (caput obliquum m. adductoris hallucis).
- The synovial sheath of the m. fibularis longus serves as an origin site for the opponens digiti minimi muscle (m. opponens digiti minimi).
- In the subcutaneous tissue posterior to the lateral malleolus, the small saphenous vein (v. saphena parva) and the sural nerve (n. suralis) are found.
Function, Blood Supply, and Innervation
- Function: The muscle performs plantarflexion and eversion (pronation) of the foot, depresses the medial border, and supports the transverse arch of the foot.
- Blood supply: Supplied by the anterior tibial artery (a. tibialis anterior) and the fibular artery (a. fibularis/peronea). The latter is a major branch of the posterior tibial artery and runs laterally along the fibula.
- Innervation: Superficial fibular nerve (n. fibularis/peroneus superficialis, nerve roots L₄–S₁).
Clinical Significance
- The muscle serves as an indicator for evaluating the S₁ spinal nerve root. Lesions of this root also affect the Achilles reflex, with sensory changes typically involving the posterior thigh and leg, the lateral edge of the foot, and the sole.
- Testing the strength of the fibularis longus is recommended to objectify motor deficits when assessing lower extremity weakness.