The structure indicated is the brachioradialis muscle of the forearm.

The brachioradalis muscle is one of the muscles of the posterior compartment of the forearm. The posterior compartment consists of a superficial layer of muscles comprised of 7 muscles. The deep layer is comprised of 5 muscles.

Origin: lateral supracondylar ridge of humerus

Insertion: radial styloid process

Innervation: radial nerve

Action: flexes the forearm at the elbow – strongest when forearm is midway between pronation and supination.

Learn more about the muscles of the forearm in this tutorial.