The Teres Major Muscle is a thick, flat muscle belonging to the scapulohumeral muscle group of the upper extremity. It lies between the scapula and humerus, contributing primarily to extension and medial rotation of the humerus. While it can be confused with the rotator cuff muscles, it differs from this group by not attaching to the shoulder joint capsule.

Anatomy

Morphology and Connection Points

The teres major muscle originates from the dorsal surface of the inferior angle of the scapula and the inferior portion of its lateral border. The muscle inserts onto the medial lip of the intertubercular sulcus of the humerus, inserting into the medial lip. This configuration allows the muscle to effectively participate in arm movements. The teres major's thick, flat structure allows it to play a supportive role in powerful arm movements. The teres major is closely associated with the latissimus dorsi muscle. The tendon of the teres major lies posterior to the tendon of the latissimus dorsi, and a bursa reduces friction between the two structures. The muscles converge for a short distance at their inferior borders, working synchronously, particularly during movements such as humeral extension. Furthermore, together with the teres minor, they form the boundaries of the axillary space, and important vascular and nerve structures pass through this anatomical region.

Embryological Origin and Classification

Embryologically, the teres major develops from somitic mesenchyme derived from paraxial mesoderm. It is a myotome-derived structure involved in the formation of upper extremity muscles. The muscle develops by differentiating from the dorsal muscle mass that forms within the upper extremity bud. As such, it is classified within the scapulohumeral muscle group and is one of the muscles that contribute to shoulder stability during movement.

Nerve Innervation and Vascular Supply

The teres major muscle is innervated by the inferior subscapular nerve (C5–C8). The thoracodorsal nerve (also known as the middle subscapular nerve) also contributes to the muscle's innervation in some cases. Both nerves originate from the posterior cord of the brachial plexus and provide motor innervation to the muscle, supporting functional movements. The muscle's blood supply is primarily provided by branches of the subscapular artery and the posterior circumflex humeral artery. The subscapular artery arises from the third portion of the axillary artery and sends terminal branches to the surface of the muscle. The thoracodorsal artery also contributes to the teres major by sending small vessels, as well as to the latissimus dorsi muscle.

Function and Clinical Relationships

The primary functions of the teres major muscle include medial rotation of the humerus, or internal rotation of the arm. It also plays a role in adduction, or bringing the arm closer to the body. The muscle assists the latissimus dorsi during humeral extension, and together, these two muscles allow the shoulder to move backward. The teres major muscle also stabilizes the humeral head in the glenoid cavity, thus indirectly providing stability to the shoulder joint. Clinically, isolated injuries to the teres major muscle are quite rare. However, such injuries can occur, particularly in individuals participating in professional sports, such as baseball pitchers or those who engage in repetitive shoulder movements. Treatment often requires long-term rehabilitation. While surgical intervention may be necessary in some cases, conservative methods (rest, physical therapy, exercise programs) often yield successful results in most cases.