Shoulder impingement happens when the tendons and fluid-filled cushion (bursa) at the top of your shoulder get pinched in a narrow gap every time you raise your arm. The gap between the top of your upper arm bone and the bony roof of your shoulder is only about 1 to 1.5 centimeters wide. When that space shrinks even slightly, the soft tissues inside get compressed, causing pain and inflammation that can worsen over time if left unaddressed.
What Happens Inside the Shoulder
Your shoulder has a small corridor called the subacromial space. The floor of this corridor is the rounded head of your upper arm bone. The ceiling is formed by a bony shelf called the acromion, along with a tough ligament that connects it to another nearby bone. Packed into that narrow corridor are the rotator cuff tendons (four muscles that stabilize and move your shoulder), the biceps tendon, and a fluid-filled sac called the bursa that reduces friction.
Even in a healthy shoulder, raising your arm overhead temporarily narrows this space and increases pressure on the tissues inside. That’s normal. The problem starts when something causes the space to stay narrow or become narrower than usual. When that happens, every time you lift your arm, the tendons and bursa get squeezed against the bony ceiling. Over time, this repeated compression leads to irritation, swelling, and eventually damage to the tendons.
Why It Develops
The causes fall into two broad categories. The first is structural: some people are born with a more hooked or curved acromion bone that leaves less room for the tendons. A classification system describes three acromion shapes (flat, curved, and hooked). Research has found that nearly 70% of full-thickness rotator cuff tears occurred in people with the hooked type, though the direct link between bone shape and impingement symptoms is less clear-cut.
The second category is movement-related. Weakness in the rotator cuff muscles allows the head of the upper arm bone to drift upward during arm elevation, eating into that already tight space. Poor posture, repetitive overhead motions (swimming, painting, throwing), and abnormal shoulder blade movement can all contribute the same way. Most cases involve some combination of both structural and movement factors.
How It Feels
The hallmark symptom is pain when raising your arm between roughly 70 and 120 degrees, a range sometimes called the “painful arc.” You might feel fine with your arm at your side and fine once your arm is fully overhead, but that middle zone is where the pinching is worst. Reaching behind your back, pulling a seatbelt across your body, or doing any forceful overhead movement typically reproduces the pain.
Night pain is extremely common. Lying on the affected shoulder compresses those already irritated structures, which is why many people first notice the problem because it keeps waking them up. The pain is usually felt on the outer part of the upper arm rather than deep inside the joint, which can be confusing since the actual problem is at the top of the shoulder.
How It Progresses
Shoulder impingement tends to follow a pattern if it isn’t managed. In the earliest phase, the tendons swell and the bursa becomes inflamed. This is most common in younger athletes who do a lot of overhead activity, and it’s typically reversible with rest and rehabilitation.
If compression continues, the tendons begin to thicken and develop scar tissue. Small partial tears may appear. This stage involves more persistent pain and stiffness that doesn’t resolve as quickly between flare-ups.
In the most advanced phase, which is more common in people over 40, the rotator cuff tendons develop partial or complete tears. Bony spurs may form on the acromion, further narrowing the space. Pain and loss of function at this stage are significant, and conservative treatment alone may not be enough.
How It’s Diagnosed
Diagnosis is primarily based on a physical exam. Two commonly used tests involve your doctor positioning your arm in specific ways to recreate the pinching inside the shoulder. Both of these provocative tests have a sensitivity of about 79%, meaning they correctly identify impingement in roughly 4 out of 5 people who have it. Their specificity is lower (53% to 59%), so a positive test doesn’t guarantee impingement is the sole problem. Imaging with X-rays or MRI is often used to check for bone spurs, acromion shape, and the condition of the rotator cuff tendons.
Exercise as the Core Treatment
Exercise is the foundation of treatment. A large meta-analysis of randomized trials found that exercise was significantly better than doing nothing for both pain reduction and functional improvement. More importantly, targeted shoulder exercises outperformed generic exercise programs. That means a routine specifically designed to strengthen the rotator cuff and correct shoulder blade mechanics works better than general upper body activity.
A typical rehabilitation program focuses on three things: strengthening the rotator cuff muscles so they hold the arm bone down and centered during movement, improving shoulder blade control so the acromion doesn’t tilt into the tendons, and stretching the back of the shoulder capsule if it’s tight. With daily exercises, many people notice meaningful pain improvement within about two weeks, though full recovery takes longer depending on severity.
Specific exercises generally progress from isometric holds (contracting muscles without moving the joint), to resistance band work, to functional overhead movements as strength and pain allow. The goal is to widen the subacromial space dynamically by training the muscles to keep the bones in better alignment during movement.
Injections and Their Limits
Corticosteroid injections into the subacromial space can provide meaningful short-term relief. The benefit typically lasts anywhere from 3 to 38 weeks. For rotator cuff-related pain, injections have been shown to help for up to 9 months. However, the longer-term picture is less encouraging. A two-year follow-up study found no lasting difference between injection and physical therapy, and up to half of patients experienced recurring symptoms. Injections work best as a tool to reduce pain enough that you can participate in a rehabilitation program, not as a standalone fix.
When Surgery Becomes an Option
Surgery is considered when several months of consistent physical therapy and other conservative measures haven’t provided adequate relief. The most common procedure is arthroscopic subacromial decompression, where a surgeon shaves away a small amount of bone from the underside of the acromion to create more room for the tendons. It’s done through small incisions with a camera.
Outcomes are generally favorable. Roughly 90% of patients who undergo the procedure report improved symptoms, better range of motion, and increased strength. High satisfaction rates have been documented lasting at least six years after surgery, and the procedure appears effective regardless of age or how long symptoms have been present. The most common complication is post-surgical stiffness, which typically resolves on its own or with physical therapy.
One important trade-off: patients who manage their symptoms with conservative care tend to return to work significantly sooner than those who have surgery. This makes a strong case for exhausting rehabilitation options before pursuing an operation.
Sleeping and Daily Adjustments
Since lying on the affected shoulder is one of the most common pain triggers, adjusting your sleep position can make a noticeable difference. If you sleep on your back, place a pillow under the affected arm with your elbow bent and hand resting on your stomach. This lifts the arm slightly and reduces pressure on the joint. If you’re a side sleeper, switch to the unaffected side and stack one or two folded pillows in front of your chest to support the painful arm, keeping it elevated rather than letting gravity pull it downward.
During the day, keep your work surface at a height that doesn’t require you to repeatedly reach above shoulder level. If you work at a desk, your elbows should rest near your sides at roughly 90 degrees. Avoid carrying heavy bags with a strap over the affected shoulder, and when reaching for objects overhead, use a step stool to bring yourself closer rather than stretching at the edge of your range.