AFAA Personal Trainer Certification Practice Exam 2025 – All-in-One Guide to Master Your Certification!

The posterior deltoid primarily functions to extend and laterally rotate the shoulder joint, and it works in opposition to the muscles responsible for shoulder flexion and internal rotation. The pectoralis major, particularly its clavicular head, is a major muscle involved in flexing the shoulder and rotating the arm inward. Therefore, in the context of muscle function, the posterior deltoid opposes the actions of the pectoralis major.

Understanding the functional anatomy of these muscles highlights the importance of balance in shoulder movements. As the posterior deltoid works to extend the arm backward, the pectoralis major contracts to pull the arm forward and across the body. This interaction is vital for maintaining proper shoulder dynamics during various physical activities.

In contrast, while the anterior deltoids and latissimus dorsi are involved in similar movements, they do not directly oppose the posterior deltoid in the way that the pectoralis major does, making the latter the primary antagonist in terms of shoulder joint actions. The mid-trapezius primarily functions in retracting the scapula and is not directly involved in opposing the posterior deltoid’s specific movements as effectively as the pectoralis major.