The structure indicated is the extensor digitorum longus muscle.

The extensor digitorum longus is one of the four muscles of the anterior compartment of the leg. The muscles of the leg consist of three compartments:

1. Anterior
2. Posterior
3. Lateral

In the anterior compartment are the four following muscles:

1. Tibialis anterior
2. Extensor hallucis longus
3. Extensor digitorum longus
4. Fibularis tertius

The muscles of the anterior compartment of the leg serve to dorsiflex the ankle joint, extend the toes and invert the foot, and they are supplied by the deep fibular (peroneal) nerve.

Origin: Proximal ½ of medial surface of fibula. Lateral tibial condyle.

Insertion: Via 4 tendons into the bases of distal and middle phalanges of lateral four toes.

Action: Extension of lateral four toes. Dorsiflexion of foot.

Innervation: Deep fibular nerve

Learn more about the anatomy of the leg muscles in this tutorial.