How do the arm muscles work to flex the elbow?

To flex the elbow, there are two main muscles involved - the biceps brachii and the triceps brachii. These two muscles work together in an antagonistic pair in order to complete the movement. Antagonistic pairs are when two muscles work against each other to complete two opposite movements. In this case, when the elbow is flexed, the biceps brachii contract (shorten) and the triceps brachii relax (lengthen). When extending the elbow, the opposite happens. Other muscles called synergists help stabilise the movements that an antagonistic pair perform.