What is the action of latissimus dorsi?

• A. Extends, adducts, and medially rotates the humerus.
• B. Abducts the humerus.
• C. Flexes the elbow.
• D. Laterally rotates the scapula.

Answer: (A)

Latissimus dorsi primarily acts on the shoulder joint to extend the arm (move it backward), adduct it (pull it toward the body), and medially rotate it (turn the arm inward). Its fibers run from the lower spine and pelvis up to the humerus, so when it contracts it pulls the arm down and back and toward the midline. This combination of movements is why it’s described as extending, adducting, and medially rotating the humerus. It’s not responsible for abducting the arm (that’s done by muscles like the deltoid and supraspinatus), nor for elbow flexion (elbow flexors like the biceps brachii do that), nor for lateral rotation of the scapula (scapular rotation involves trapezius and serratus anterior).