The latissimus dorsi muscles, commonly called the "lats," are large, flat, fan-shaped muscles located on the back. They span from the upper arm (humerus) across the back, attaching to the spine and pelvis, covering the lower to mid- back area. They extend behind the arm and are partly covered by the trapezius muscle near the midline of the back. The lats are the broadest muscles on the back and play a key role in pulling motions, such as pulling the arms down or back, and also help with posture and movement of the spine.
In simple terms, the lats run from the upper arm down to the spine and lower back, forming the large wing-like muscles on the sides of the back that help in pulling motions and stabilize the torso.