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.