The teres major muscle is a small muscle found on the posterior side of the shoulder joint. It works together with the latissimus dorsi muscle to adduct the arm at the shoulder joint. The teres major is sometimes confused with its neighbor, the teres minor muscle, which is a muscle of the rotator cuff.
The teres major is a long, round muscle of the posterior shoulder. It arises from an L-shaped origin on the posterior surface of the inferior angle and lateral border of the scapula. From its origin, it extends laterally and slightly superior and anterior to cross over to the arm inferior to the shoulder joint. The muscle fibers of the teres major run parallel to the fibers of the latissimus dorsi and infraspinatus muscles that border it. In the arm, the teres major inserts into the intertubercular sulcus on the anterior surface of the humerus, medial to the insertion of the latissimus dorsi muscle.
Contraction of the teres major muscle draws its insertion on the humerus medially and posteriorly toward its origin on the scapula. From the anatomical position, the teres major adducts the arm at the shoulder, pulling it closer to the body, and medially rotates the arm, pulling the anterior of the humerus posteriorly toward the scapula. When the arm is in a flexed position, the teres major can also extend the arm at the shoulder by returning the arm to the anatomical position.
The teres major muscle works synergistically with the latissimus dorsi muscle that neighbors it on its inferior border. Both muscles work together to adduct, medially rotate, and extend the arm. The primary difference between these muscles is that the teres major arises from and draws the arm toward the scapula, while the latissimus dorsi arises from and draws the arm toward the lumbodorsal fascia in the lower back.