What is the primary action of the latissimus dorsi on the humerus?

• A. Flexion and abduction
• B. Extension, adduction, and medial rotation
• C. Abduction and lateral rotation
• D. Extension only

Answer: (C)

The latissimus dorsi mainly extends, adducts, and medially rotates the humerus. Its broad attachment along the lower spine and pelvis and its insertion into the intertubercular groove place the pull of the muscle so that the arm moves backward (extension), toward the body's midline (adduction), and rotates inward (medial rotation). This combination is exactly what you feel in pulling movements like rows or a pull-up, where the arm is swept down and back toward the body with internal rotation.

Notice that flexion, abduction, and lateral rotation are produced by other muscles: flexion by muscles such as the anterior deltoid and pectoralis major; abduction mainly by the supraspinatus and deltoid; and lateral rotation by the infraspinatus and teres minor.