Which muscle acts as the antagonist to horizontal adduction of the shoulder?

• A. Pectoralis Major, Deltoid (Anterior)
• B. Latissimus Dorsi, Teres Major
• C. Deltoid (Posterior)
• D. Serratus Anterior, Pectoralis Minor

Answer: (C)

Moving the arm horizontally across the chest (toward the midline) is horizontal adduction. The opposing action—horizontal abduction—is mainly produced by the posterior deltoid. So, the posterior fibers of the deltoid are the main antagonist to horizontal adduction because they drive the arm back out to the side in the transverse plane, opposing the across-the-chest movement.

Other muscles listed aren’t the primary antagonists here. The pectoralis major and the anterior deltoid are key contributors to horizontal adduction, not its opposite. Latissimus dorsi and teres major help with adduction and internal rotation, so they don’t oppose horizontal adduction. Serratus anterior and pectoralis minor mainly stabilize or move the scapula rather than directly counteracting horizontal shoulder movement.