The triceps runs along the back of the upper arm and helps extend the elbow, straightening the arm. Its full anatomical name is the triceps brachii, meaning the “three-headed muscle of the arm.”
The triceps has three parts: the long head, lateral head and medial head. All three cross the elbow joint and attach through a tendon to the olecranon process of the ulna, the pointed bone at the back of the elbow. The long head also crosses the shoulder joint, giving it additional roles in shoulder movement and stability.
When you push open a door, perform a push-up or straighten your arm against resistance, the triceps contracts. It works as an antagonist to the biceps, which primarily flexes the elbow and bends the arm. The two muscles can therefore produce opposite movements at the elbow.
The diaphragm is the main breathing muscle, the rectus abdominis is a major abdominal muscle, and the biceps is chiefly associated with elbow flexion. Those muscles are not the primary answer for arm extension.