The latissimus dorsi (lats) is a large flat muscle located just below the shoulder blades which extends out from the spine, wraps around to the side, behind and up under the arm. The upper portion is partly covered by the trapezius muscle. The latissimus dorsi attaches to the ribs and the illiac crest (hip). A small slip of the muscle also attaches to the humerus (upper arm bone).
The latissimus dorsi is the largest muscle in the upper body and plays a major role in posture and all motions of the upper body, including shoulder movement.
In the shoulder, the latissimus dorsi extends, pulls together and internally rotate the joint. It plays a key role in extension and lateral flexion of the lumbar spine. Along with the trapezius muscle, the latissimus dorsi moves the scapula (shoulder blade).
Weakness in the latissimus dorsi causes posture issues such as slouching. It also affects the function of the shoulder. Weakness of this muscle can cause chronic shoulder and back pain. To get these muscles properly re-aligned, you would need to use the best back posture corrector.