LATISSIMUS DORSI (Lats)
The latissimus dorsi (lats) is a large flat muscle located just below the shoulder blades which extends out from the spine wrapping around to the side, behind and up under the arm. The upper medial portion is partly covered by the trapezius muscle. The latissimus dorsi attach to the ribs and, in most people, to the illiac crest (hip). A small slip of the lattismus dorsi attaches to the humerous (upper arm bone).
The latissimus dorsi is the largest muscle in the upper body and plays a major role in all motions of the upper body, including shoulder motion.
In the shoulder, the latissimus dorsi works under and deep in the shoulder to extend, adduct 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 influences movement of the scapula (shoulder blade).
Muscular imbalance of the latissimus dorsi affects posture and function of the shoulder. This imbalance can cause chronic shoulder and back pain.