Which muscle on the chest is responsible for moving the arm toward the midline?

• A. Pectoralis major
• B. Deltoid
• C. Triceps brachii
• D. Rectus abdominus

Answer: (A)

The main idea here is adduction of the arm—moving the arm toward the body's midline. The chest muscle that does this is the pectoralis major. It runs across the chest from the sternum and clavicle to the upper arm bone, and when it contracts it pulls the humerus inward toward the midline, helping draw the arm toward the chest. It also can help bend (flex) the arm and rotate it inward.

The deltoid is the shoulder muscle that mainly lifts the arm away from the body (abduction) and has other actions, not the primary move toward the midline. The triceps brachii focuses on extending the elbow and assisting in other shoulder movements rather than adduction. The rectus abdominis is an abdominal muscle that flexes the torso and does not move the arm. So the chest muscle responsible is the pectoralis major.