Which muscle's long head contributes to shoulder extension and adduction?

• A. Triceps Brachii
• B. Deltoid
• C. Latissimus Dorsi
• D. Pectoralis Major

Answer: (A)

The key idea is that a muscle’s cross-joint action depends on whether a portion of it crosses that joint. The long head of the triceps brachii does cross the shoulder joint, so it can influence movements at the glenohumeral joint in addition to its elbow action. When the triceps contracts via its long head, it can pull the humerus backward (extension) and toward the body (adduction), especially when the arm is in a flexed position or when other shoulder movers are active. Other options either don’t rely on a long head crossing the shoulder to produce these actions (latissimus dorsi and pectoralis major contribute to extension and adduction, but they aren’t described by a long head crossing the shoulder), or their primary function is not shoulder extension/adduction in the same way. Therefore, the long head of the triceps brachii best accounts for shoulder extension and adduction.