Ace the AFAA Personal Trainer Test 2026 – Pump Up Your Certification Power!

The anterior and medial deltoids are considered antagonistic to the posterior deltoid because they serve opposite functions during arm movement. The posterior deltoid is primarily responsible for shoulder extension and external rotation, while the anterior deltoid is involved in shoulder flexion and internal rotation. The medial deltoid assists with shoulder abduction.

When the posterior deltoid contracts to pull the arm backward, the anterior deltoid must relax and vice versa. This relationship is part of the overall function of muscles in the shoulder complex, where the balance between opposing muscles is crucial for maintaining proper joint mechanics and function. Understanding these muscle dynamics is essential for personal trainers, as it informs exercise programming and helps prevent muscular imbalances.