Latissimus dorsi muscle
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The latissimus dorsi (Lats) is the large, flat, dorso-lateral muscle on the trunk, posterior to the arm, and partly covered by the trapezius on its median dorsal region.
The latissimus dorsi is a triangular, flat muscle, which covers the lumbar region and the lower half of the thoracic region, and is gradually contracted into a narrow fasciculus at its insertion into the humerus. This makes up some of the armpit and stretches down near the gluteus.
The latissimus dorsi is responsible for extension, adduction, transverse extension, and internal rotation of the shoulder joint. It also has a synergistic role in extension and lateral flexion of the lumbar spine.