Latissimus dorsi ("widest muscle of the back") originates on the spinous processes of the lower six thoracic vertebrae and all of the lumbar vertebrae, the sacrum, the iliac crest, and the lower three or four ribs. It inserts with teres major on the medial edge of the intertubercular groove of the humerus. Its principal actions are extension, adduction, internal rotation and depression of the humerus.

          
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Translation: Latissimus dorsi means "widest (muscle) of the back". It attaches to your spinal column from halfway down your chest all the way to your sacrum, to the upper edge of your pelvis, to the bottom three or four ribs, and to the inner, upper surface of your humerus. Its main jobs are to pull your arm back, down, and toward your body, and to help rotate it inwardly. This means you would use it in rowing, chinning, or pulling yourself upward or forward with your arms.

NOTE: Because latissimus dorsi is such a powerful and well-developed muscle in most of us, and because it attaches to the lower thoracic and lumbar vertebrae and the pelvis, it is an extremely important muscle in postural alignment. It can bear a large part of the responsibility for a misaligned pelvis, and can be significantly involved in the curves of scoliosis.

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