Shoulder impingement happens when the tendons and fluid-filled cushion (bursa) inside your shoulder get pinched in a narrow gap between bones every time you raise your arm. The causes fall into two broad categories: structural problems that physically shrink that gap, and functional problems where muscle imbalances or poor movement patterns let the bones drift into the wrong position. Most cases involve some combination of both.
The Space That Gets Too Tight
Your shoulder has a small corridor called the subacromial space. The bottom boundary is the ball of your upper arm bone. The top boundary is a bony shelf called the acromion, along with a tough ligament that connects it to another bony point nearby. Packed into this corridor are the supraspinatus tendon (the most commonly affected rotator cuff tendon), the long head of the biceps tendon, the joint capsule, and the subacromial bursa, a fluid sac that helps everything glide smoothly.
When you lift your arm, these soft tissues need to slide freely through that corridor. If anything narrows the space from above, or if anything swells the tissues from below, they get compressed against the bony roof on every repetition. That compression is impingement, and over time it irritates the bursa, damages the tendon fibers, and produces the characteristic pain with overhead reaching.
Structural Causes: When the Space Is Physically Smaller
Clinicians call this “primary impingement” because the problem starts with the anatomy itself. Several structural changes can narrow the subacromial space from the top down:
- Acromion shape. People are born with different acromion shapes, classified as flat (Type I), curved (Type II), or hooked (Type III). A hooked acromion was originally thought to strongly predict impingement, since nearly 70% of full-thickness rotator cuff tears occurred under hooked acromions. Later research found conflicting results, and the link between acromion shape and impingement specifically (as opposed to full tears) is less clear than once believed. Still, a more downward-sloping acromion does reduce the available corridor.
- Bone spurs. Over time, bony growths can develop on the underside of the acromion or along the acromioclavicular joint (the small joint on top of the shoulder where the collarbone meets the acromion). These spurs eat into the subacromial space from above.
- Fracture malunion. If the greater tuberosity of the upper arm bone heals in a slightly elevated position after a fracture, the bone itself pushes upward into the space.
Primary impingement can also come from below when the soft tissues themselves swell and take up more room than usual. Two common examples are subacromial bursitis, where the bursa becomes inflamed and thickens, and calcific tendinitis, where calcium deposits form within the rotator cuff tendon. These deposits trigger inflammation and physically rub against the acromion during arm movement.
Functional Causes: When Movement Goes Wrong
Secondary impingement has nothing to do with bone shape or spurs. Instead, the muscles that control your shoulder blade and rotator cuff aren’t doing their job properly, which lets the ball of the upper arm bone drift upward or forward during movement. That migration compresses the same soft tissues against the same bony roof, producing the same pain pattern, but the root cause is a movement problem rather than a structural one.
The shoulder blade plays a critical role here. Normally, as you raise your arm, the shoulder blade rotates upward and tilts backward to keep the acromion out of the way. When this coordination breaks down (a pattern called scapular dyskinesis), the shoulder blade may sit too far forward at rest, rotate too far inward during elevation, or fail to tilt upward enough. All of these patterns bring the acromion closer to the humeral head and shrink the subacromial corridor during the exact movements that matter most.
Rotator cuff weakness amplifies the problem. The rotator cuff’s primary job isn’t to move your arm; it’s to hold the ball of the upper arm bone centered in the socket. When the cuff muscles are weak or fatigued, the larger deltoid muscle overpowers them and pulls the ball upward during lifting. That upward migration narrows the subacromial space from below. This is why impingement often worsens as you fatigue during repetitive overhead tasks, even if the first few repetitions feel fine.
Who Is Most at Risk
Repetitive overhead work is the clearest occupational risk factor. A study of over 1,500 workers found that slaughterhouse employees, who perform forceful, repetitive shoulder-intensive tasks, had more than five times the prevalence of shoulder impingement compared to workers in less physically demanding roles. Former slaughterhouse workers had nearly eight times the prevalence, and the risk increased with cumulative years of exposure. Painters, electricians, carpenters, warehouse workers, and anyone who spends hours with arms at or above shoulder height faces similar mechanical demands.
Athletes in overhead sports are another high-risk group, particularly swimmers, baseball and softball players, volleyball players, and tennis players. The combination of high-volume repetitions and the extreme positions these sports demand accelerates both tendon wear and the kind of muscle fatigue that leads to secondary impingement.
Age matters too, though perhaps not in the way you’d expect. The tendons of the rotator cuff gradually lose blood supply and elasticity starting around age 40, making them more vulnerable to compression injuries. But younger athletes and manual laborers develop impingement through overuse, so this is not exclusively a condition of aging.
How It Typically Feels
The hallmark symptom is pain in the front or side of the shoulder when you raise your arm, especially between about 60 and 120 degrees of elevation. This middle range is where the subacromial space is at its tightest. You might notice it reaching into an overhead cabinet, putting on a seatbelt, or sleeping on the affected side. The pain often starts as a mild ache after activity and, if ignored, progresses to sharper pain during activity and eventually at rest.
Weakness is common but can be misleading. In many cases the muscles are capable of producing force, but pain inhibits the effort. True weakness, where you can’t hold your arm up even when you push through pain, may indicate a rotator cuff tear and warrants closer evaluation.
How It’s Diagnosed
Diagnosis is primarily clinical. Your provider will likely perform a few provocative tests: raising your arm in specific positions to reproduce the pinching sensation. Two of the most widely used tests have pooled sensitivity around 74% to 78%, meaning they correctly identify impingement roughly three-quarters of the time. Their specificity is lower (around 57% to 58%), so a positive test doesn’t guarantee impingement, and the overall clinical picture matters more than any single maneuver.
Current clinical guidelines recommend against ordering imaging as a first step for typical shoulder pain presentations. X-rays and MRI are useful when symptoms don’t improve with initial treatment or when a tear is suspected, but they aren’t needed to start a rehabilitation program.
Treatment and Recovery
The most important finding in the impingement treatment literature is how consistently non-surgical approaches match surgical ones. A large overview of systematic reviews of randomized controlled trials found no clinically important or statistically significant difference between supervised exercise programs and arthroscopic subacromial decompression surgery at any time point: short-term, mid-term, or long-term. Both exercise and surgery were significantly better than placebo at reducing pain and improving function, but neither was superior to the other. Home-based strengthening exercise produced similar outcomes to surgery plus post-surgical rehabilitation.
Because of this evidence, current guidelines recommend an active rehabilitation exercise program as the initial treatment. This typically includes motor control exercises to retrain shoulder blade movement, resistance training to strengthen the rotator cuff and scapular stabilizers, and gradual progression of load. Over-the-counter anti-inflammatory medications or acetaminophen can help manage pain in the short term while you build strength.
Most people begin noticing meaningful improvement after six to eight weeks of consistent rehabilitation. Full recovery ranges from a few weeks for mild cases to six months for more persistent ones. The wide range reflects both the severity of the initial problem and how well the underlying cause (structural versus functional) responds to exercise. Cases driven primarily by movement dysfunction and muscle weakness tend to respond well to rehab. Cases with large bone spurs or significant calcification may take longer or eventually benefit from a procedure, but even then, a rehabilitation program is the recommended starting point.