The Teres Minor is a narrow, elongated skeletal muscle located in the posterior shoulder region. Although relatively small, it is a component of a larger structure that plays a major part in arm movement and stability. It runs across the back of the shoulder joint, nestled deep beneath the larger Deltoid muscle. The muscle bridges the gap between the shoulder blade and the upper arm bone, helping control the precise positioning of the upper arm.
The Anatomy: Origin and Insertion Points
The physical location of the Teres Minor is defined by its two attachment points: the origin and the insertion. The muscle begins its course at the upper two-thirds of the lateral border of the scapula (shoulder blade). The lateral border is the edge closest to the armpit, placing the muscle on the back side of the body.
From this origin point, the muscle fibers travel upward and laterally, crossing the shoulder joint. The Teres Minor then attaches to the greater tubercle of the humerus, the large bony prominence on the top of the upper arm bone. Specifically, it inserts into the inferior facet of this greater tubercle, often blending with the shoulder joint capsule.
This connection between the shoulder blade and the upper arm bone makes the Teres Minor an intrinsic muscle of the shoulder. It is positioned inferior to the Infraspinatus muscle and superior to the larger Teres Major muscle.
Primary Role in Shoulder Movement
The Teres Minor’s primary action is the external, or lateral, rotation of the humerus at the shoulder joint. When the muscle contracts, it pulls the upper arm bone away from the midline of the body, rotating the arm outward. This motion is necessary for tasks like turning a doorknob or reaching for a high shelf.
The muscle also contributes to adduction of the arm, which is the movement of bringing the arm closer to the side of the body. Working with other muscles, it helps to coordinate the complex movements of the shoulder.
The Rotator Cuff System
The Teres Minor is one of four muscles that collectively form the rotator cuff. These muscles and their tendons create a cuff-like structure that surrounds the glenohumeral joint. The other three muscles in this group are:
- Supraspinatus
- Infraspinatus
- Subscapularis
The collective function of this system is to provide dynamic stabilization to the shoulder joint. The shoulder is inherently unstable, so the rotator cuff muscles contract to hold the head of the humerus securely against the shallow socket of the scapula as the arm moves. This prevents dislocation and allows the larger, stronger muscles to move the arm effectively.
The Teres Minor and the Infraspinatus muscles work together to perform external rotation. The Subscapularis handles internal rotation, and the Supraspinatus initiates arm abduction (lifting the arm away from the side). This coordinated action keeps the upper arm bone centered in the socket throughout a full range of motion.