The main antagonists of the triceps brachii are the biceps brachii. However, other muscles act as antagonists during different actions of the triceps brachii, including the anterior deltoid, coracobrachialis, pectoralis major, supraspinatus, middle deltoid, brachialis and brachioradialis.
Antagonistic muscles play an important role in the musculoskeletal system by acting in contrast to one another during movement. They are opposites. That is, when one muscle contracts, the other relaxes. Located on opposite sides of the humerus bone, the biceps brachii are the obvious "opposites" of the triceps brachii. However, depending on the type of movement or action that the triceps brachii take, different muscles may also act as antagonists.
The primary action of the triceps brachii is the extension of the arm at the shoulder. In this case, the biceps brachii are the main antagonistic muscles, but the anterior deltoid, coracobrachialis, and pectoralis major at the clavicular head also work in contrast to the triceps brachii.
The secondary actions of the triceps brachii utilize still different antagonistic muscles. Adduction of the arm at the shoulder, for example, employs the supraspinatus and middle deltoid as antagonists. Extension of the forearm at the elbow again uses the biceps brachii as a primary antagonistic muscle along with the brachialis and brachioradialis.