There's two groups of muscles in the upper arm, the biceps and the triceps. The biceps is a large muscle on the upper arm, formally known as biceps brachii muscle, and it rests on the top of the humerus bone. It rotates the arm and also flexes the elbow. The triceps is a large muscle in the back of the upper arm, formally known as triceps brachii muscle, and it helps in strengthening the arm.
