The supraspinatus is a small muscle in the upper back, extending from the shoulder blade to the upper arm bone. It is one of four muscles comprising the rotator cuff, which stabilizes the shoulder joint and enables its wide range of motion. Despite its modest size, the supraspinatus is the most frequently injured rotator cuff muscle, with tears and inflammation being common. Understanding its unique characteristics helps explain its susceptibility to injury.
Its Unique Anatomical Placement
The supraspinatus tendon travels through a narrow passageway in the shoulder known as the subacromial space. This space is situated between the humeral head and the acromion, a bony projection from the shoulder blade. The subacromial bursa cushions the tendon within this space, reducing friction during movement.
When the arm is raised, especially during overhead motions, this subacromial space naturally narrows. This compression can lead to impingement, where the tendon gets pinched between the humerus and the acromion. Repeated compression causes tendinitis and damages the tendon. Variations in acromion shape can further reduce this space, increasing impingement likelihood.
Demanding Functional Role and Repetitive Strain
The supraspinatus initiates arm abduction, the motion of lifting the arm away from the body. It is active during the first 0-15 degrees of this lifting motion, after which the deltoid muscle takes over as the primary abductor. The supraspinatus also helps stabilize the humeral head within the glenoid cavity, the shoulder socket.
These functions mean the supraspinatus is consistently under stress, especially during overhead arm movements. Sports like swimming, throwing, and tennis, or occupations like painting and carpentry, involve frequent, repetitive overhead actions. Each time the arm is raised, the supraspinatus tendon experiences microtrauma. Over time, this cumulative stress and strain can lead to wear and tear, causing the tendon to fray, develop tendinitis, and lead to partial or full-thickness tears.
Limited Blood Flow and Age-Related Wear
The supraspinatus tendon has an area, often called the “critical zone,” located 8 to 15 millimeters from its humeral attachment. Historically, this zone was believed to have reduced blood supply, hindering its ability to repair after minor injuries or daily wear. Despite some newer findings suggesting increased vascularity in symptomatic cases, it remains a region prone to degeneration.
As individuals age, tendon tissue undergoes degenerative changes, making it weaker and less elastic. Studies show a significant reduction in supraspinatus vascularity, especially after age 55, slowing natural healing. This age-related weakening, combined with reduced healing capacity, makes the supraspinatus more susceptible to tears, even from minor stresses that would not affect a younger tendon. By age 80, many adults show evidence of rotator cuff tears due to cumulative degeneration.