The Subscapularis muscle is a large, triangular muscle that forms part of the rotator cuff, a group of muscles and tendons that provide stability to the shoulder. The Subscapularis is important for shoulder joint movement and stability, playing a crucial role in internal rotation of the arm at the shoulder.
Location
The Subscapularis muscle is located in the anterior (front) part of the shoulder blade, filling the subscapular fossa. It is situated beneath the scapula, between the scapula and the rib cage, making it the lesser visible member of the rotator cuff from the outside. The muscle fibers run laterally from the inner surface of the scapula to the lesser tubercle of the humerus, which is the bone of the upper arm.
Anatomy
Origin
The Subscapularis originates from the subscapular fossa, which is the concave surface on the costal (anterior) side of the scapula (shoulder blade). This broad origin covers almost the entire area of the subscapular fossa, giving the muscle a large and flat appearance at its beginning.
Insertion
The muscle fibers converge into a smaller, more compact tendon as they move laterally from the scapula. The Subscapularis inserts onto the lesser tubercle of the humerus and the front of the humeral neck. This insertion point is significant as it positions the Subscapularis in a place where it can exert powerful forces on the humerus.
Muscle Fibers and Structure
The Subscapularis muscle is composed of thick, fleshy fibers that are organized in a somewhat segmented fashion, with the fibers often described as being arranged in two to three tendon-like layers. This multi-layered structure allows different parts of the muscle to be engaged selectively, providing a range of mechanical advantages and motion capabilities.
Innervation
The Subscapularis receives its nerve supply primarily from the upper and lower subscapular nerves, which stem from the posterior cord of the brachial plexus. These nerves enter the muscle at different points, ensuring that all regions of the muscle can be effectively activated and controlled.
Blood Supply
The blood supply to the Subscapularis comes from the subscapular artery and its branches, including the circumflex scapular artery. These vessels provide the necessary nutrients and oxygen required for the muscle’s function and repair.
Bursae
Adjacent to the Subscapularis are several bursae, which are fluid-filled sacs that reduce friction between the muscle and other structures, such as the scapula and rib cage. The most notable is the subscapular bursa, which facilitates the smooth movement of the muscle against the bone surfaces.
Relationship with Other Structures
The Subscapularis is the largest of the rotator cuff muscles and lies beneath the scapula, making it the most anteriorly positioned rotator cuff muscle. It is situated in close proximity to other structures in the shoulder, including the capsule of the shoulder joint, which it helps to reinforce. Additionally, its position and size make it integral to the overall stability and mobility of the shoulder joint.
Function
The Subscapularis muscle, as a critical component of the rotator cuff, performs several important functions that are vital for the movement and stability of the shoulder joint:
Internal Rotation of the Humerus
The primary function of the Subscapularis is to mediate internal rotation of the humerus. When the muscle contracts, it pulls the humerus inward, turning the arm so that the palm of the hand moves towards the body’s midline. This action is essential for numerous daily activities such as reaching for a seatbelt, typing, or swinging a bat.
Stabilization of the Glenohumeral (Shoulder) Joint
The Subscapularis plays a crucial role in stabilizing the shoulder joint. By holding the head of the humerus firmly within the glenoid fossa of the scapula, it prevents dislocation and excessive movement that could lead to injury. This stabilizing effect is particularly important during arm lifting and rotation movements.
Adduction of the Humerus
In addition to internal rotation, the Subscapularis assists in adduction of the humerus, which is the action of drawing the arm toward the body’s midline. This function is vital for actions such as pulling an object toward oneself or performing a rowing motion.
Prevention of Anterior Dislocation of the Shoulder
The Subscapularis muscle acts as a front shield for the shoulder joint, providing a physical barrier against the anterior (forward) dislocation of the humerus. By maintaining tension in the front part of the shoulder, the Subscapularis contributes to the joint’s overall integrity and resilience against injury.
Assisting in Arm Abduction
When the arm is lifted sideways away from the body, the Subscapularis works in conjunction with other rotator cuff muscles to lift the humerus. Initially, it helps in the first 15 degrees of arm abduction. Beyond this initial phase, it acts to stabilize the shoulder joint, allowing other muscles, primarily the deltoid, to continue the movement efficiently.
Facilitating Respiration
Indirectly, the Subscapularis, through its connection to the scapula and its role in shoulder mechanics, can influence thoracic movement and stability, thereby playing a supporting role in the respiratory process, particularly during deep breaths or forceful exhalation.
Clinical Significance
- Rotator Cuff Injuries: The Subscapularis, as part of the rotator cuff, is susceptible to tears and tendinitis, common conditions in athletes or individuals performing repetitive overhead motions. Its injury can lead to pain, weakness, and reduced range of motion in the shoulder, significantly impacting daily activities and athletic performance.
- Shoulder Instability: Dysfunction or weakness in the Subscapularis can contribute to instability in the shoulder joint, including partial or complete dislocations. Its strength and integrity are crucial for maintaining the humeral head within the glenoid fossa, especially during dynamic shoulder movements.
- Frozen Shoulder and Stiffness: The Subscapularis can be involved in conditions like frozen shoulder (adhesive capsulitis), where its restricted movement contributes to pain and limited shoulder mobility. Addressing the muscle’s flexibility and strength is often a key component of rehabilitation.
- Surgical Repair and Rehabilitation: In cases of rotator cuff surgery, understanding the anatomy and function of the Subscapularis is vital for surgical repair and postoperative rehabilitation. Techniques aimed at restoring the muscle’s function can significantly influence recovery outcomes and shoulder functionality post-surgery.
- Postural Considerations: Due to its location and function, the Subscapularis is also relevant in addressing postural issues. Abnormalities in its function can contribute to anterior shoulder pain and poor posture, often seen with prolonged sitting or computer use.
- Diagnostic Imaging and Evaluation: The Subscapularis is a focal point in diagnostic imaging studies, such as MRI or ultrasound, used to evaluate shoulder pain and injuries. Its condition can provide valuable information regarding the overall health of the rotator cuff and the shoulder joint.
