Teres major muscle

The Teres Major is a small yet important muscle of the shoulder region that contributes significantly to the movement and stability of the upper limb. Positioned at the posterior aspect of the scapula, it works closely with surrounding muscles to facilitate coordinated shoulder actions. Understanding its anatomy and function is essential for clinicians, anatomists, and physiotherapists.

Anatomy of Teres Major

Origin

The Teres Major muscle originates from the dorsal surface of the inferior angle of the scapula and the lower part of the lateral border of the scapula. This positioning allows it to contribute effectively to movements of the humerus.

Insertion

The muscle inserts into the medial lip of the intertubercular sulcus of the humerus. This insertion point allows it to act on the humerus during various shoulder movements.

Muscle Fiber Orientation

The fibers of the Teres Major run obliquely upward and laterally from the scapula to the humerus. This orientation facilitates its role in adduction, medial rotation, and extension of the humerus.

Relations with Surrounding Muscles and Structures

• Superiorly: Teres Minor
• Lateral: Humerus and lateral border of the scapula
• Medially: Latissimus Dorsi, which shares similar insertion on the humerus
• Posteriorly: Scapula and overlying skin

Blood Supply

Primary Arterial Supply

The Teres Major receives its blood supply mainly from the subscapular artery and the circumflex scapular branch. These vessels provide oxygen and nutrients necessary for muscle function and repair.

Venous Drainage

Venous drainage occurs through accompanying veins that parallel the arterial supply, ultimately draining into the subscapular and axillary veins. Efficient venous return is important for maintaining healthy muscle tissue.

Nerve Supply

Innervating Nerves

The Teres Major muscle is innervated by the lower subscapular nerve, a branch of the posterior cord of the brachial plexus. This nerve carries motor fibers that enable the muscle to perform its primary actions.

Segmental Innervation

The segmental levels of innervation for the Teres Major are primarily C5 and C6. Proper nerve function is essential for coordinated shoulder movements and strength.

Biomechanics and Function

Movements Facilitated

• Adduction of the arm: Pulls the humerus towards the body midline.
• Medial rotation of the humerus: Rotates the upper arm inward towards the torso.
• Extension of the humerus: Moves the arm backward from a flexed position.

Role in Shoulder Stability

The Teres Major contributes to the stabilization of the shoulder joint by supporting the humeral head within the glenoid cavity during dynamic movements. It acts synergistically with the latissimus dorsi and rotator cuff muscles to maintain shoulder integrity, particularly during adduction and extension activities.

Clinical Significance

Injury and Strain

The Teres Major muscle can be affected by strains, tears, or overuse injuries, particularly in athletes performing repetitive overhead or throwing activities. Symptoms may include pain in the posterior shoulder, weakness during adduction or internal rotation, and limited range of motion.

Implications in Shoulder Surgery

Understanding the anatomy of the Teres Major is crucial in surgical procedures involving the shoulder, such as tendon transfers or rotator cuff repairs. Preservation of the muscle and its neurovascular supply is important to maintain post-surgical function and strength.

Physical Examination and Testing

• Manual muscle testing for adduction and medial rotation of the humerus
• Palpation along the lower lateral border of the scapula to assess tenderness or spasm
• Observation for compensatory movements during shoulder exercises

Imaging and Diagnostic Evaluation

Ultrasound

Ultrasound imaging can visualize the Teres Major muscle, detect tears, and assess muscle thickness and fiber integrity. It is a non-invasive and dynamic method for evaluation during movement.

MRI

MRI provides high-resolution images of the muscle, tendinous insertion, and surrounding soft tissues. It is particularly useful in diagnosing partial or complete tears and evaluating post-traumatic or post-surgical changes.

CT Scan

CT imaging is less commonly used for isolated Teres Major assessment but can provide detailed views of bony attachments and relations when combined with contrast studies or in complex shoulder trauma cases.

Rehabilitation and Strengthening

Physical Therapy Exercises

• Resistance band internal rotations: Strengthens the Teres Major and improves medial rotation control.
• Latissimus-assisted pull-downs: Engages the Teres Major during controlled adduction movements.
• Prone horizontal extensions: Promotes extension strength and scapular stabilization.
• Scapular retraction exercises: Enhances coordination with surrounding shoulder muscles.

Post-Injury Rehabilitation Protocols

Rehabilitation following Teres Major injury focuses on gradual restoration of range of motion, progressive strengthening, and functional reintegration into daily or athletic activities. Careful attention is given to avoiding overuse and ensuring proper neuromuscular control during recovery.

Comparative Anatomy and Evolutionary Perspective

Variations in Humans

While generally consistent, the Teres Major can exhibit minor anatomical variations in origin, insertion, or fiber orientation among individuals. These variations may influence strength, range of motion, or susceptibility to injury.

Comparative Musculature in Primates

In non-human primates, the Teres Major plays a more prominent role in climbing and brachiation, reflecting evolutionary adaptations for arboreal locomotion. Its relative size and orientation provide insights into functional demands across species.

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2020.
3. Ramsay AD, et al. The Teres Major Muscle: Anatomy, Function, and Clinical Significance. J Shoulder Elbow Surg. 2019;28(10):2014-2022.
4. Neumann DA. Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation. 4th ed. St. Louis: Elsevier; 2020.
5. Standring S, Ellis H. Anatomy and Variations of the Teres Major Muscle. Clin Anat. 2018;31(5):723-731.
6. Warner JJ, et al. Surgical Implications of Teres Major Muscle Anatomy. Orthop Clin North Am. 2017;48(4):457-467.
7. Willmore JH, Costill DL. Physiology of Sport and Exercise. 7th ed. Champaign: Human Kinetics; 2020.