The extensor digitorum brevis is a small muscle of the foot. Situated on the dorsal or upper aspect of the foot, it runs lengthwise between the heel and the toes. Its fibers also run in this direction, although as they approach the toes they turn slightly inward. The extensor digitorum brevis is responsible for extending, or straightening, the middle three toes.
This muscle originates on the calcaneus, or heel bone, on its dorsal aspect toward the outer or lateral side. From here it extends across the top surface of the tarsal bones toward the toes, where it divides into four tendons. Three of these tendons insert on toes two, three, and four on the tops of the middle phalanges, which are the second of the three bones in each of the four smaller toes. As for the fourth tendon — the one that attaches to the big toe — it inserts on the proximal or first phalanx. In fact, this division of the muscle along its medial side in essence forms an entirely separate muscle known as the extensor hallucis brevis, or extensor of the big toe.
The extensor digitorum brevis attaches to the middle toes by converging with the tendons of the extensor digitorum longus. A much larger muscle that begins in the anterior compartment of the lower leg, the extensor digitorum longus forms a tendon that crosses the ankle joint and similarly divides into four smaller tendons. These tendons attach to the middle and distal, or farthest, phalanges in the toes. The tendons of the extensor digitorum brevis approach the toes from beneath those of the extensor digitorum longus, each one converging along the lateral side of its adjacent longus tendon. As the brevis tendons only extend to the middle phalanges, however, the longus tendons extend slightly farther than those of the brevis, reaching the distal phalanges.
The attachment of these tendons means that when the extensor digitorum brevis muscle contracts, the shortening of the muscle pulls upward and backward on the tops of the toes, straightening them as it lifts them. Likewise, the extensor digitorum longus performs this movement synergistically with the brevis. As it crosses the ankle joint, contraction of this muscle first pulls upward on the foot at the ankle, an action known as dorsiflexion. At the same time, the longus extends the toes in conjunction with the action of the extensor digitorum brevis. If the foot remains in a fixed position at the ankle and the toes are extended, only the brevis performs the action.