Triceps brachii muscle

The triceps brachii is a large, thick muscle that is located in the posterior compartment of the arm. It is the only muscle in this compartment, extending almost the entire length of the humerus. As its name suggests, it has three heads - long, lateral, and medial. The medial head lies deeper and is covered by the other two heads. All three heads merge into a single distal tendon that inserts into the proximal aspect of the olecranon of the ulna.