The Teres Major muscle is a powerful, thick muscle located in the upper back and shoulder girdle. It is often nicknamed “the little helper of the lat” because its actions mirror those of the much larger Latissimus Dorsi muscle, with which it shares a close anatomical relationship. Understanding the location and function of the Teres Major is important for comprehending the full range of motion of the shoulder joint and upper body. Its placement makes it a major player in many pulling and rotating movements.
Pinpointing the Teres Major
The Teres Major is situated on the posterior side of the torso, connecting the shoulder blade (scapula) to the upper arm bone (humerus). It originates on the posterior surface of the inferior angle of the scapula, which is the lowest, pointed part of the shoulder blade. The muscle fibers then travel laterally and upward, running underneath the armpit region.
The muscle inserts onto the medial lip of the intertubercular sulcus of the humerus, a specific groove on the front of the upper arm bone. This long trajectory means the Teres Major moves from the back of the body to the front of the arm, allowing it to exert force on the shoulder joint. To feel this muscle, locate the bottom tip of the shoulder blade; the muscle can be palpated as a thick, rounded band near this area when the arm is rotated inward against resistance.
Key Movements and Role
The Teres Major is classified as a powerful mover of the arm, contributing three distinct actions at the glenohumeral joint. The first action is adduction, which involves pulling the arm directly toward the midline of the body or down from a raised position. This action is seen in the downward movement when performing a pull-up or pulling a rope toward the torso.
The second function is internal (medial) rotation of the humerus, which turns the arm inward toward the chest. This action is integral to movements like reaching across the body or performing an underhand throw. Finally, the muscle assists in extension, pulling the arm backward from a forward-flexed position. The Teres Major often works in synergy with the Latissimus Dorsi muscle to execute strong pulling and rotating movements.
The Teres Major vs. Teres Minor
Despite their similar names and close proximity, the Teres Major and Teres Minor have fundamentally different roles in shoulder mechanics. The Teres Major is a prime mover, contributing to large-scale, forceful actions. The Teres Minor, however, is one of the four muscles that comprise the rotator cuff.
The primary function of the Teres Minor is to stabilize the head of the humerus within the shoulder socket and perform external (lateral) rotation of the arm. This is the opposite of the Teres Major’s internal rotation function. The distinction is also evident in their nerve supply: the Teres Major is innervated by the lower subscapular nerve, while the Teres Minor receives its signal from the axillary nerve.
This difference in function and innervation shows that the Teres Major is functionally aligned with large muscles responsible for arm movement, while the Teres Minor is dedicated to shoulder stability and controlled rotation. Although both muscles attach to the scapula and humerus, their insertion points differ: the Teres Minor inserts on the posterior side of the humerus, and the Teres Major inserts on the anterior side, explaining their opposing rotational effects.