The coronoid fossa is a depressed surface found on the lower front portion of the humerus, the long bone of the upper arm. It is situated on the front of the bone where the ulna and radius bones of the forearm meet the humerus at the elbow joint. This depression is where the coronoid process of the ulna curves into the humerus upon bending the arm at the elbow. While no muscles attach to the coronoid fossa, the brachialis muscle of the upper arm inserts at the coronoid process just beyond the fossa and makes arm flexion possible by pulling the process into the receiving fossa as it contracts.
Extending from the shoulder to the elbow, the humerus features several irregularly shaped eminences and depressions at its base. Some serve as sites of attachment for muscles. The medial and lateral epicondyles, for instance, are rounded protrusions on either side of the base of the bone that are origin points for multiple muscles of the forearm. Others — the cartilage-covered trochlea and capitulum — are articular surfaces, the portion of the bone that makes contact with the ulna and radius to form the elbow joint. These are found along the bottom center aspect of the humerus between the epicondyles.
Also between the two epicondyles on the anterior or front aspect of the bottom of the humerus are two fossae, the coronoid fossa and the radial fossa. The radial fossa is the depression into which fits the head of the radius when the elbow flexes, while the coronoid fossa receives the top of the ulna bone. Unlike the rounded head of the radius, however, the upper end of the ulna is shaped like a crescent. With its concave side facing forward, the ulna curves around the posterior aspect of the bottom of the humerus. When the elbow bends, the olecranon process or top of the crescent hooks around the back of the humerus while the coronoid process or bottom of the crescent pushes into the coronoid fossa on the front of the humerus.
While the biceps brachii muscle in the upper arm initiates arm flexion, the brachialis muscle beneath it is an important facilitator of this motion. The biceps does not actually attach to the humerus. The brachialis, however, attaches to the humerus just above the elbow joint, crosses the joint, and inserts at its lower end on the coronoid process of the ulna. When it contracts, it pulls the coronoid process upward and backward into the coronoid fossa just as the biceps pulls upward on the anterior aspect of the radius bone, producing the motion of elbow flexion.