Which muscle is primarily responsible for extending the humerus?

• A. Latissimus dorsi
• B. Trapezius
• C. Rhomboid minor
• D. Levator scapulae

Answer: (A)

Extending the humerus means moving the upper arm backward at the shoulder joint. The latissimus dorsi is the primary muscle for this action because its broad origin from the lower spine and pelvis and its insertion on the humerus allow a strong posterior pull when it contracts. This makes it the main driver of arm extension, and it also assists with adduction and medial rotation of the arm. Movements like pulling yourself up or rowing rely on this action.

The other muscles mainly affect the scapula or shoulder girdle rather than extending the humerus itself: trapezius moves and stabilizes the scapula and can influence neck and shoulder posture; rhomboid minor retracts the scapula; levator scapulae elevates the scapula. Their roles don’t center on extending the arm at the shoulder joint. Innervation for the latissimus dorsi is the thoracodorsal nerve (C6–C8).