The triceps brachii and the anconeus are the two muscles that extend the elbow, or forearm. According to Inner Body, these muscles work in conjunction with one another to straighten the arm by increasing the angle between the radius, ulna and humerus.
The triceps brachii muscle is located behind the upper arm. It extends the arm and holds the elbow still when necessary. The much smaller and triangular anconeus muscle helps the muscle extend and rotates the forearm; however, the elbow relies on the triceps brachii and not the anconeus. Because of this, the anconeus is considered part of the triceps brachii by some.