The infraspinatus is a thick, triangular muscle positioned on the back of the shoulder blade, forming a significant part of the four-muscle group known as the rotator cuff. This muscle group operates as a dynamic anchor, surrounding the shoulder joint to maintain the precise alignment of the upper arm bone within the shoulder socket. Its continuous, coordinated effort ensures the shoulder remains stable during the wide range of motions the arm performs daily.
Anatomical Position of the Infraspinatus
The infraspinatus muscle occupies the majority of the infraspinous fossa, the large, shallow depression found on the posterior surface of the shoulder blade (scapula). It originates from the medial two-thirds of this bony surface, lying directly beneath the ridge known as the spine of the scapula. The muscle fibers converge laterally, forming a tendon that crosses behind the shoulder joint.
This tendon ultimately inserts onto the greater tubercle of the humerus, a bony prominence located on the upper, outer part of the arm bone. This attachment point is positioned on the middle facet, distinguishing its connection from other nearby rotator cuff muscles.
Primary Role in Shoulder Movement
The principal function of the infraspinatus is to produce external rotation of the humerus, which involves turning the arm outward away from the body. This action is used in everyday movements, such as opening a door or throwing an object. The infraspinatus acts synergistically with the smaller teres minor muscle to execute this rotational movement.
The infraspinatus also performs a major role as a dynamic stabilizer of the glenohumeral joint. By applying a compressive force, the muscle helps to center the head of the humerus within the shallow shoulder socket during all arm movements. This stabilizing action prevents the upper arm bone from migrating upward during overhead activities, allowing the larger deltoid muscle to function efficiently.
Common Sources of Infraspinatus Pain
Pain often arises from overuse or repetitive stress, especially in activities involving frequent overhead reaching or throwing motions, which can lead to tendinopathy. Tendinopathy can manifest as tendinitis (inflammation of the tendon) or tendinosis (microscopic tears and tissue degeneration). These conditions frequently cause a dull ache deep in the back of the shoulder.
A partial or full-thickness tear of the infraspinatus tendon is another common source of pain, often resulting from acute trauma or gradual wear and tear. Impingement syndrome occurs when the tendon gets compressed by surrounding structures, particularly when the arm is lifted. Symptoms include pain that worsens at night or when sleeping on the affected side, alongside noticeable weakness when attempting to rotate the arm outward against resistance.
Simple Ways to Maintain Muscle Health
Targeted strengthening of the infraspinatus focuses on external rotation exercises to improve its endurance and resilience. A effective, low-impact method is the use of a light resistance band while keeping the elbow bent at a 90-degree angle and tucked close to the side. The movement involves slowly rotating the forearm outward against the band’s resistance.
Sidelying external rotations with a light dumbbell (one to three pounds) is another excellent way to isolate the muscle without excessive strain. Maintaining good posture is also important, as a slumped or rounded-shoulder position can narrow the space in the shoulder joint and compress the tendon. Avoiding repetitive, high-force overhead movements that strain the muscle can help prevent future irritation.