The main muscles of the anterior arms are made up of the biceps brachii aka the guns, the brachialis and the brachioradialis. The main muscle of the posterior arms is the triceps brachii. The biceps and triceps are a perfect example of an agonist-antagonist muscle group. The biceps are responsible for flexion whereas the triceps are responsible for extension of the arms. The brachialis and brachioradialis are synergist muscles to the biceps.