Supraspinatus and infraspinatus tendinosis is a chronic degenerative condition affecting two of the four rotator cuff tendons in your shoulder. Unlike tendinitis, which involves active inflammation from a sudden injury, tendinosis describes a breakdown in the tendon’s internal structure that develops gradually over weeks to months. The distinction matters because the two conditions require different treatment approaches.
What These Tendons Do
The supraspinatus and infraspinatus are two of the four muscles that make up the rotator cuff, a group of tendons that stabilize and move your shoulder joint. Each has a distinct job. The supraspinatus handles the first 0 to 15 degrees of lifting your arm out to the side. It’s the only rotator cuff muscle that doesn’t rotate the arm at all. Once you pass that initial range, the larger deltoid muscle takes over. The infraspinatus is a powerful external rotator, responsible for turning your arm outward, like the motion of pulling a door open or winding up for a throw.
Because these two tendons sit on top of and behind the shoulder joint, they’re especially vulnerable to compression against the bony arch above them during overhead movements. This mechanical reality is one reason they’re the most commonly affected rotator cuff tendons.
Tendinosis vs. Tendinitis
These terms sound nearly identical but describe very different problems at the tissue level. Tendinitis is inflammation caused by sudden overload, where the tendon is stretched or pulled too hard, too fast, causing micro-tears and an inflammatory response. It’s an acute injury.
Tendinosis, by contrast, is a degenerative process. Under a microscope, the changes are distinct: healthy tendons are made mostly of mature, well-organized collagen fibers that line up parallel to each other to bear load. In tendinosis, those mature fibers are replaced by immature, disorganized collagen. The fibers lose their alignment, sometimes failing to link together at all. New blood vessels grow haphazardly into the tissue, and the material between cells increases. Critically, inflammatory cells are rarely present. This is why anti-inflammatory medications alone don’t resolve tendinosis. The problem isn’t inflammation; it’s structural deterioration.
Histological studies show that when inflammation does appear alongside tendinosis, it’s typically a secondary event, not the primary driver. Scar tissue and calcification show up only some of the time. This means many people with tendinosis have pain and dysfunction without any of the classic signs of an “injury” on imaging.
What Causes It
Tendinosis develops through repeated micro-trauma to the tendon that outpaces the body’s ability to repair itself. Each small insult causes minor damage. When there isn’t enough recovery time between loading events, the tendon remodels poorly, substituting weaker collagen for the strong, organized fibers it needs.
Several factors accelerate this process. Overhead and repetitive shoulder movements, common in painters, swimmers, tennis players, and warehouse workers, place repeated stress on the supraspinatus and infraspinatus tendons. Age plays a significant role as well. The prevalence of rotator cuff injuries rises substantially with age, driven partly by the accumulation of sugar-based cross-links between collagen fibers (a normal part of aging) that makes tendons stiffer and less adaptable. Research suggests that age alone may not be the dominant factor, though. Overuse, accumulated tissue damage over a lifetime, and underlying chronic diseases like diabetes likely combine to make rotator cuff tendons more vulnerable in older adults.
How It Feels
The hallmark of supraspinatus and infraspinatus tendinosis is a dull, deep ache in the shoulder rather than a sharp, localized pain. It tends to worsen at night, particularly if you sleep on the affected side. Many people first notice it when reaching overhead, combing their hair, or reaching behind their back to tuck in a shirt or fasten a bra.
Arm weakness often accompanies the pain, especially with movements specific to the affected tendon. Supraspinatus tendinosis makes it harder to initiate lifting your arm to the side. Infraspinatus tendinosis weakens your ability to rotate your arm outward, which you might notice when opening a heavy door or decelerating your arm after a throw. Over time, without intervention, the shoulder can lose range of motion permanently.
How It’s Diagnosed
Diagnosis typically involves a combination of physical examination and imaging. During an exam, your clinician may use the Empty Can test (also called the Jobe test) to assess the supraspinatus. You hold your arm out at about 90 degrees, angled slightly forward, with your thumb pointing down as if pouring out a can. The examiner pushes down on your arm while you resist. Inability to hold against that pressure, or significant pain during it, suggests supraspinatus involvement. Infraspinatus testing uses different maneuvers focused on resisted external rotation and observing whether the arm drops when held in a rotated position.
MRI is the standard imaging tool for confirming tendinosis. On MRI, tendinosis shows up as changes in signal intensity within the tendon and altered tendon shape. Importantly, the tendon remains intact, which distinguishes tendinosis from a partial or full-thickness tear. Severity grading on MRI is still somewhat subjective, based primarily on how much the tendon’s appearance and signal have changed from normal.
Risk of Progression to a Tear
Tendinosis is a progressive, degenerative process. The weakened, disorganized collagen in a tendinotic tendon is more vulnerable to tearing than healthy tissue. When micro-trauma continues and healing remains insufficient, the result can be a partial tear that may eventually become a full-thickness tear.
Not all tendinosis progresses this way. Several factors influence whether a partial tear worsens. Larger tears are more likely to deteriorate further, while small tears may remain stable. Tears that are actively enlarging are about five times more likely to become symptomatic than stable ones. Age matters significantly here: people over 60 are more likely to develop tears that progress, while younger individuals appear better able to adapt to stress even when a tear is present. The location of the tear within the tendon also affects the odds of progression.
Treatment Through Loading
Because tendinosis is a problem of tissue quality rather than inflammation, treatment centers on stimulating the tendon to produce better-organized collagen. The most studied approach is eccentric exercise, where you slowly lower a weight through the muscle’s range of motion rather than lifting it. This type of controlled loading appears to promote collagen remodeling in ways that rest alone does not.
Research on eccentric training for rotator cuff tendinosis has tested several protocols. One study had patients on a surgical waiting list perform eccentric exercises for the supraspinatus and deltoid muscles for 12 weeks: 3 sets of 15 repetitions, twice daily, every day. All nine subjects showed meaningful clinical improvement. Another protocol used two main eccentric exercises for the supraspinatus and infraspinatus performed in a side-lying position with dumbbells at the same frequency. A shorter trial used eccentric training at moderate speed for shoulder abductors over 6 weeks, with 3 sets of 10 repetitions twice a week.
The honest reality is that no single “best” protocol has been established. Different studies use different loads, repetitions, and durations, and the underlying biology of tendon remodeling is still not fully understood. What the evidence does consistently show is that progressive, controlled loading helps, while complete rest does not. Most rehabilitation programs span 6 to 12 weeks at minimum.
What Recovery Looks Like
Tendon tissue remodels slowly. Even after surgical repair, collagen alignment and organization don’t begin appearing until around 8 weeks. The fibers that anchor tendon to bone don’t show up in meaningful numbers until 12 weeks. For non-surgical tendinosis, these timelines offer a useful frame of reference: expect meaningful improvement over months, not weeks.
Early rehabilitation focuses on reducing pain and gradually restoring range of motion. Stretching protocols often use 3 repetitions of 30-second holds with 30 seconds of rest between them, performed two to three times per week. As pain allows, eccentric strengthening is introduced and progressively loaded. The key principle is that some discomfort during exercise is expected and even part of the stimulus for remodeling, but sharp or worsening pain signals that the load is too aggressive. Managing expectations around this timeline is one of the most important parts of recovery. People who understand that tendon healing takes 3 to 6 months are far more likely to stay consistent with their program than those expecting a quick fix.