Which muscle is the primary extensor, adductor, and medial rotator of the humerus?

• A. Latissimus dorsi
• B. Pectoralis major
• C. Teres minor
• D. Subscapularis

Answer: (A)

Latissimus dorsi is the muscle that can extend, adduct, and medially rotate the humerus all in one smooth action. Its broad back origin from the thoracolumbar spine, iliac crest, and lower ribs, and its insertion into the intertubercular groove of the humerus, give it a powerful pull downward and inward on the upper arm. When it contracts, the arm moves backward (extension), toward the body's midline (adduction), and the front of the arm twists inward (medial rotation). This combination is most pronounced when you perform actions like pulling down, climbing, or rowing, where the shoulder needs to extend and pull the arm in while rotating it inward.

Pectoralis major can help with adduction and medial rotation, especially when the arm is already in some flexion, but its primary role isn’t extensor; teres major assists with extension, adduction, and medial rotation as a close collaborator to latissimus dorsi, while subscapularis mainly medially rotates and stabilizes the joint rather than extending the arm or pulling it back into adduction.