Extensor digitorum longus muscle
Extensor digitorum longus (EDL muscle) is a feather-like muscle of the anterior (extensor) compartment of leg. Besides EDL muscle, this compartment also contains the tibialis anterior, extensor hallucis longus and fibularis (peroneus) tertius muscles.
Since all these muscles cross the dorsal aspect of the ankle joint, their common function is dorsiflexion of the foot. As the extensor digitorum longus also crosses the subtalar, metatarsophalangeal and interphalangeal joints of the foot, it also everses the foot and extends the toes.
This article will discuss the anatomy and function of the extensor digitorum longus muscle.
|Origin||(Proximal half of) medial surface of fibula, lateral tibial condyle, interosseus membrane|
|Insertion||Distal and middle phalanges of digits 2-5|
Metatarsophalangeal and interphalangeal joints 2-5: Toe extension;
Talocrural joint: Foot dorsiflexion;
|Innervation||Deep fibular nerve (L5, S1)|
|Blood supply||Anterior tibial, fibular, anterior lateral malleolar, lateral tarsal, metatarsal, plantar, and digital arteries|
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.
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.
The tendons of the muscle are vascularized by the anterior lateral malleolar, lateral tarsal, metatarsal, plantar and digital arteries.
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.