ADducts, extends and internally rotates the arm at the shoulder. The latissimus dorsi muscle ORIGINATES along the spinous processes of T-7 to L-5, iliac crest, inferior 3rd and/or 4th ribs, inferior angle of the scapula and the thoracolumbar fascia. The INSERTION is into the bicipital groove (intertubercal groove) of the humerus between the teres major located medially and the pectoralis major located laterally. This insertion point between the two major muscles has earned the latissimus dorsi the moniker, "The lady between two majors." The ACTION of the latissimus dorsi is to internally (medially) rotate the arm, ADduct the arm and then extend the arm.