Which large back muscle is the prime mover in many pull-up motions?

The large back muscle that serves as a prime mover in many pull-up motions is the latissimus dorsi.

Often called the lats, the latissimus dorsi is a broad, flat muscle stretching from the lower back and pelvis toward the upper arm. It attaches to the humerus and acts on the shoulder, helping extend, adduct, and internally rotate the arm. When the arms are fixed overhead, these actions help pull the torso upward.

That makes the latissimus dorsi central to vertical pulling exercises such as pull-ups, chin-ups, and lat pulldowns. Pull-ups are compound movements, however, so the lats do not work alone. The biceps brachii, brachialis, teres major, trapezius, forearm muscles, and several stabilizers contribute, with activity varying according to grip, body position, and range of motion.

A common mix-up is treating the pull-up as an isolated lat exercise. Grip changes can alter how much the elbow flexors or upper-back muscles contribute, and a chin-up usually allows greater biceps involvement. The name latissimus dorsi comes from Latin words meaning “broadest of the back,” an apt description of its unusually wide attachment across the torso.