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The extensor digitorum longus is a muscle of the anterior compartment of the lower leg. Responsible for extending the four smaller toes as well as dorsiflexing the ankle, it is considered an extrinsic extensor of the foot in that it is located outside of the foot itself. A long and narrow muscle extending from just below the knee joint to the distal phalanges, the last of the toe bones, it is found on the outside of the shin above the fibula bone. Although it runs parallel to the fibula, however, its fibers are pennate, meaning that instead of running lengthwise they angle in toward a midline like the veins in a leaf.
As its fibers converge from either side of the muscle instead of from a narrow point at the top, the extensor digitorum longus has its origins on several different structures in the lower leg. Some of its fibers arise from the tibia bone — specifically, the lateral condyle, which is the outermost of the two rounded bony eminences at the top of the bone. Others originate on the front of the fibula along the topmost 75 percent of the shaft of the bone. This muscle also arises from several membranous structures surrounding and dividing the muscles in this region, including the interosseous membrane, which separates the anterior and posterior compartments of the lower leg, the deep fascia, which envelops the muscle like a sausage casing, and the intermuscular septa, which isolate the extensor digitorum longus from the nearby tibialis anterior and peroneus longus and brevis.
These fibers converge well above the ankle to form a tendon that crosses vertically in front of the ankle joint and behind a Y-shaped ligament known as the cruciate crural ligament. From here the tendon diverges into four smaller tendons that attach at the middle and distal phalanges, or last two bones, of the four smaller toes. The pull on these tendons by contractions of the extensor digitorum longus is what extends the toes, or pulls them upward from a curled position.
In addition to this muscle’s action at the lesser four toes, it plays a role in dorsiflexing the ankle joint. Dorsiflexion is the act of hinging the foot at the ankle so that the dorsal or top surface of the foot is brought closer to the shin. While the tibialis anterior is the primary dorsiflexor of the ankle, the other muscles with tendons crossing the joint — among them the extensor digitorum longus and extensor hallucis longus, which extends the big toe — assist in this movement.