Extensor digitorum longus muscle

Origin and insertion

Extensor digitorum longus originates from the inferior part of the lateral tibial condyle, the proximal half of the medial surface of fibula and the anterior surface of the interosseus membrane (its most superior part). The part of its origin at tibial condyle is fused with the originating fibers of the fibularis longus muscle.

After its origin, the muscle descends inferiorly and just above the ankle gives off a tendon that passes under the superior extensor retinaculum and through the inferior extensor retinaculum. Since all the muscle fibers attach on one side of the tendon, this muscle is classified as an unipennate muscle.

Within the inferior extensor retinaculum, the tendon splits into the four smaller tendons that all together are wrapped with a synovial sheath. After exiting the retinaculum, the four tendons diverge towards the toes. Superficially to the proximal phalanges, each tendon widens and forms the triangular dorsal digital expansions (or extensor hoods). The medial portion of each extensor hood blends with the respective tendons of the lumbrical muscles, while the lateral parts related to the digits 2-4 are joined by the tendons of the extensor digitorum brevis muscle.

The extensor hoods course over the proximal phalanges, and at the level of the base of middle phalanges, each of them further divides into a central band and two lateral bands. The central bands insert to the base of the distal phalanges of lateral four toes, respectively. The lateral bands continue over the middle phalanges and just proximally to the distal phalanges, each pair of them merges into a single tendinous band. These newly formed bands insert to the bases of middle phalanges of lateral four digits.

Relations

This muscle is the most lateral of all the muscles in the extensor compartment and lies laterally to the tibialis anterior and extensor hallucis longus. Anterior tibial artery and vein pass between the extensor digitorum longus and the tibialis anterior.

Being within the inferior extensor retinaculum, the distal part of the muscle crosses the anterior side of the ankle joint. While traversing the retinaculum, the tendon of extensor digitorum longus sits medially to the tendon of fibularis tertius and laterally to the extensor hallucis longus tendon. In the dorsum of the foot, tendons of the muscle course superficially to the extensor digitorum brevis muscle.

Innervation

The extensor digitorum longus is innervated by the deep fibular nerve (L5, S1), a branch of the common fibular nerve.

Blood supply

The leg portion of the muscle is supplied by two arteries of the leg; the proximal part is supplied by the anterior tibial artery, while the distal part receives blood from the fibular artery.

The tendons of the muscle are vascularized by the anterior lateral malleolar, lateral tarsal, metatarsal, plantar and digital arteries.

Function

The primary action of the extensor digitorum longus is to extend the lateral four toes at metatarsophalangeal joint. This means that when acting independently, it is unable to extend the entire length of the toes, extending only at metatarsophalangeal, while at the interphalangeal joints the toes remain flexed. However, contracting together with lumbricals which are the main extensors of the interphalangeal joints, this muscle contributes to extension at every joint between the bones of the lateral four toes.

Acting in synergy with tibialis anterior, extensor hallucis longus and fibularis tertius, this muscle participates in dorsiflexion of the foot when their proximal attachments are fixed. When the distal attachments are fixed and the body is in the anatomical position, all four muscles bring the trunk and lower limb to the front. This action moves the body weight bearing point from the proximal to distal part of foot.

All of these actions are important for the gait cycle; the dorsiflexion angulates and lifts the foot above the walking surface, while the toe extension keeps the toes extended until the heel hits the ground.

Go ahead and quiz yourself on the extensor digitorum longus and other muscles of the leg to consolidate what you learned so far!