The Teres Major is a muscle of the shoulder girdle that plays a significant part in the movement and stability of the upper arm. Often referred to as the “lat’s little helper,” it works closely with the much larger Latissimus Dorsi muscle. Understanding the Teres Major requires knowing which nerve delivers the electrical signals that command the muscle to contract, a process known as innervation.
Location and Basic Structure
The Teres Major is a thick, somewhat flat muscle located on the back of the torso, underneath the shoulder joint. It originates on the posterior surface of the inferior angle of the scapula, the V-shaped corner at the bottom of the shoulder blade. It is easily visualized as it forms the lower border of the posterior axillary fold, the fleshy back wall of the armpit.
The muscle fibers travel upward and laterally toward the upper arm bone, or humerus. It inserts on the medial lip of the intertubercular groove, a vertical channel on the front of the humerus. Although its name is similar to the Teres Minor, the Teres Major is not considered part of the rotator cuff muscles because it does not attach to the capsule of the shoulder joint.
The Nerve Supply
The specific nerve responsible for innervating the Teres Major muscle is the Lower Subscapular Nerve. This nerve is a branch originating from the posterior cord of the brachial plexus, a complex network of nerves that supplies the entire shoulder and upper limb. The nerve fibers are derived from the spinal nerve roots C5, C6, and C7.
This nerve carries motor command signals from the spinal cord to the muscle fibers, effectively telling the Teres Major when and how strongly to contract. The Lower Subscapular Nerve also innervates the lower portion of the Subscapularis muscle, which is another muscle involved in shoulder movement. This shared innervation highlights the close functional relationship between various muscles that control the shoulder joint.
Primary Roles and Movements
The Teres Major functions as a powerful mover of the upper arm at the shoulder joint, performing three primary actions: adduction, internal rotation, and extension of the humerus. Adduction involves pulling the arm down toward the side of the body, such as when you lower your arm from a raised position. Internal rotation is the movement of twisting the arm inward toward the midline of the body.
Extension is the movement of bringing the arm backward from a position where it was flexed in front of the body. The Teres Major often works in conjunction with the Latissimus Dorsi to perform these movements, contributing significantly to activities like rowing, climbing, or pulling actions.
Understanding Related Injuries
The Teres Major muscle can be subject to strains, which are tears in the muscle fibers, or isolated tears, though the latter is uncommon. These injuries often occur during sports that involve forceful or repetitive overhead movements, such as pitching. An acute strain typically results in pain, tenderness, and difficulty performing actions like reaching backward or sideways.
Damage to the Lower Subscapular Nerve can severely compromise the Teres Major’s function. Pathology affecting this nerve, known as lower subscapular nerve neuropraxia, causes the muscle to lose its ability to contract, leading to weakness and atrophy. This nerve injury can be caused by trauma, blunt force, or excessive stretching of the nerve.