The lower fibers of the pectoralis major primarily perform which action at the shoulder?

• A. Extend the shoulder
• B. Flex the shoulder
• C. Abduct the shoulder
• D. Medially rotate the shoulder

Answer: (A)

The lower (sternocostal) fibers of the pectoralis major are oriented so that when they contract, they pull the humerus posteriorly. This makes them best at extending the shoulder, especially from a flexed position where the arm is lifted in front of the body and these fibers pull it back toward neutral. In other words, their line of pull favors moving the arm backward rather than upward or outward.

Clavicular fibers are the ones that flex the shoulder, so flexion is not the primary action of the lower portion. Abduction isn’t driven by the pectoralis major, which mainly pulls the arm toward the midline (adduction) and internally rotates it; extension is the action best matched to the orientation and attachments of the lower fibers.