A broad muscle that narrows to a pulling point
The latissimus dorsi—usually called the “lats”—covers much of the lower back. Its fibers begin across a wide area of the back and pelvis, then angle upward and outward toward a relatively narrow tendon attached to the upper arm bone. That layout lets a large sheet of muscle transmit force through one attachment near the shoulder.
When the muscle shortens, it draws the upper arm down and behind the body and rotates it inward. In a rowing motion, that helps bring the arm and whatever it is holding toward the torso. The lats do not work alone: muscles around the shoulder blade and upper arm help guide the movement, while the trunk stabilizes the body.
The same pull works in reverse
The muscle’s action depends on which end of the body is held still. With the arms fixed overhead, as in climbing or a pull-up, the lats can help pull the torso upward toward the arms. That is the same basic force used in a row, with the moving side reversed.
This broad-to-narrow arrangement is useful because the shoulder needs to move through many directions, while the body still needs strong pulling muscles. The lats help connect those demands: a wide area of the back gathers into a tendon that can act on the upper arm.
