The subscapularis muscle is the largest and strongest muscle within the group known as the rotator cuff. This muscle is a major contributor to shoulder health and movement, generating a significant portion of the total force required for arm function. Because of its size and location, understanding its anatomy is important for anyone experiencing shoulder discomfort or interested in the biomechanics of the upper body.
Exact Anatomical Placement
The subscapularis is positioned on the anterior surface of the scapula, or shoulder blade. This large, triangular muscle entirely fills the subscapular fossa, a shallow concavity on the inner side of the shoulder blade that rests against the rib cage. The muscle’s origin spans the majority of the subscapular fossa.
From its wide origin on the shoulder blade, the muscle’s fibers converge and pass laterally toward the shoulder joint. The tendon of the subscapularis then inserts onto the lesser tubercle of the humerus. The lesser tubercle is a small, round prominence located on the front portion of the humerus, near the ball of the joint. This specific location means the muscle is largely hidden between the shoulder blade and the chest wall, making it challenging for a clinician to feel directly.
Movement and Stability Role
The subscapularis performs two primary actions at the shoulder joint necessary for smooth, controlled arm movement. Its most powerful function is internal rotation, which is the motion of turning the arm inward toward the body. This action is used in many daily tasks, such as reaching across the chest, fastening a seatbelt, or reaching behind the back.
The muscle also acts as a dynamic stabilizer of the glenohumeral joint, which is the main ball-and-socket joint of the shoulder. It works with the other rotator cuff muscles to keep the head of the humerus centered within the shoulder socket during arm movement. By providing dynamic support to the front of the joint, the subscapularis helps prevent the upper arm bone from gliding too far forward or upward. This stabilizing function is necessary to maintain proper joint alignment.
Common Causes of Injury
Injury to the subscapularis tendon often occurs due to two main mechanisms: acute trauma or chronic overuse. Acute tears frequently result from a forceful event, such as falling onto an outstretched arm or a sudden, unexpected external rotation of the shoulder. This rapid, forced stretching can cause the tendon to tear where it connects to the humerus.
Chronic injuries, known as tendinopathy, are more common in individuals who engage in repetitive overhead or forceful internal rotation movements. Athletes in sports like baseball pitching, swimming, tennis, and weightlifting place repeated stress on the muscle, leading to gradual wear and degeneration of the tendon over time. This continuous strain can cause the tendon to become inflamed and partially torn.
Shoulder impingement can also affect the subscapularis, as the muscle’s tendon passes underneath the coracoid process, a bony projection of the shoulder blade. If the muscle or tendon becomes irritated, it can lead to thickening, which then causes it to be pinched during certain movements. Furthermore, a complete tear of the subscapularis can cause the long head of the biceps tendon, which runs nearby, to become unstable and slip out of its groove, often resulting in additional pain and weakness.