The infraspinatus is a large, triangular muscle located on the back of the shoulder blade, playing a major role in complex shoulder joint movements. Understanding its function provides insight into the biomechanics of nearly all arm movements, from daily tasks to athletic feats. As a deep-lying muscle, its health is directly related to overall shoulder stability and the prevention of strain in the surrounding musculature. Proper function maintains comfortable, fluid motion in the upper extremity.
Anatomical Placement and Structure
The infraspinatus occupies the infraspinous fossa, the large concave area found on the posterior surface of the scapula (shoulder blade). The muscle originates from the medial two-thirds of this fossa. Its fibers converge laterally into a tendon that passes across the back of the shoulder joint capsule, inserting onto the middle facet of the greater tubercle of the humerus (the upper arm bone).
This anatomical arrangement places the infraspinatus as one of the four muscles that form the rotator cuff. Along with the supraspinatus, teres minor, and subscapularis, it creates a musculotendinous cuff surrounding the glenohumeral joint. The muscle receives its nerve supply from the suprascapular nerve. Its broad surface area makes it a powerful contributor to both movement and stability in the shoulder.
The Specific Actions of the Muscle
The primary action of the infraspinatus is the external, or lateral, rotation of the arm. This movement rotates the arm away from the body’s midline, such as when turning a doorknob or preparing to throw a ball. It works in partnership with the teres minor muscle, making them the main rotators responsible for outward movement. The infraspinatus’s large surface area allows it to generate substantial torque for this rotation.
The muscle’s second action is glenohumeral joint stabilization. The glenohumeral joint is inherently unstable because the head of the humerus sits in a shallow socket on the scapula. The infraspinatus tendon blends with the shoulder joint capsule, applying a compressive force that keeps the humeral head centered during dynamic movements. This stabilizing role is important during movements that involve lifting the arm or carrying weight.
When the larger deltoid muscle contracts to lift the arm, the infraspinatus helps to counteract the upward and forward pull of the humeral head. By pulling backward and downward, it prevents the humeral head from migrating upward and pinching surrounding tissues. This action is crucial for complex movements, such as the cocking phase of a throw or reaching behind the back.
Addressing Common Injuries and Weakness
Due to its constant involvement in stabilization and rotation, the infraspinatus is prone to injury from overuse, repetitive strain, and acute trauma. Common issues include tendinopathy (inflammation of the tendon tissue) and partial or full-thickness tears. These injuries often result from repetitive overhead activities in sports like baseball, swimming, or tennis, or from chronic poor posture.
A telltale sign of an issue is a dull, aching pain felt deep in the back of the shoulder, often worsened by attempting external rotation. Weakness can lead to difficulty performing simple tasks like brushing hair or reaching for a seatbelt. Pain may also increase at night, especially when lying on the affected side, and can sometimes be referred down the arm or into the upper back.
Maintaining the health of this muscle involves targeted strengthening and flexibility exercises. Strengthening often focuses on low-resistance external rotation movements, such as using a light resistance band or performing side-lying rotations. These exercises help build endurance and power, improving the muscle’s capacity to stabilize the joint. Addressing posture and avoiding repetitive overhead movements that cause pain are practical steps to prevent long-term strain and injury.