Final answer:
The serratus anterior is the muscle antagonistic to the pectoralis major. These muscles work in opposition to provide movement and stability of the shoulder joint.
Step-by-step explanation:
The pectoralis major is responsible for pulling the humerus anteriorly, while the serratus anterior pulls the scapula posteriorly. By contracting and relaxing in opposition, these two muscles allow for the smooth movement and stability of the shoulder joint.