The structure indicated is the anconeus muscle of the forearm.

The anconeus 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 epicondyle of humerus

Insertion: olecranon, proximal surface of ulna

Innervation: Radial nerve

Action: assists elbow extension, stabilises the elbow joint, abducts ulna during pronation

Learn about the muscles of the forearm in this tutorial.