Orthopedics focuses on the musculoskeletal system. Learn about the diagnosis, treatment, and rehabilitation of bone, joint, ligament, and muscle conditions.
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Shoulder impingement is a mechanical problem. It occurs when the moving parts of the shoulder do not fit together smoothly. While it is easy to blame a single day of heavy lifting, the reality is usually a combination of anatomy, aging, and daily habits. The subacromial space is small—often only a centimeter or so high. Any factor that narrows this space or increases the volume of the tissues inside it will lead to pinching.
Understanding the root cause of your impingement is critical. If it is caused by a bone spur, no amount of rest will make it disappear. If it is caused by poor posture, surgery won’t help unless you fix how you sit and stand. This section explores the structural and functional causes of impingement, from the shape of your bones to the way you throw a ball.
As we age, the ligament that connects the front of the shoulder blade (coracoid) to the top (acromion) can calcify. This turns the soft ligament into bone, creating a bone spur on the underside of the acromion.
These spurs act like stalactites hanging from the roof of a cave. Every time you lift your arm, the rotator cuff tendon scrapes against this jagged edge. This condition is a primary cause of impingement in people over 40. The spur physically reduces the clearance space for the tendon, making wear and tear inevitable unless the mechanics are changed or the spur is removed.
The shape of your acromion bone is something you are born with or develop during growth. Doctors classify acromions into three types. Type I is flat and offers plenty of room. Type II is curved, dipping down slightly. Type III is hooked, with a sharp beak pointing down.
People with Type II and Type III acromions are genetically predisposed to impingement. The curved or hooked shape naturally encroaches on the rotator cuff. While you can’t change your bone shape without surgery, knowing you have a “hooked” acromion explains why you might be more prone to shoulder pain than your friends.
The rotator cuff has a vital job: it depresses the humeral head. When the large deltoid muscle lifts the arm, it naturally wants to pull the arm bone up into the roof. The rotator cuff fights this, pulling the ball down and into the socket.
If the rotator cuff is weak or tired, it fails to pull the ball down. The strong deltoid wins the tug-of-war, pulling the arm bone violently upward. This causes the ball to crash into the acromion, pinching the bursa in between. This functional imbalance is a leading cause of impingement in weekend warriors who play sports without proper conditioning.
When muscles are fatigued, their timing is thrown off. In a pitcher or swimmer, the rotator cuff muscles often tire out before the large power muscles.
Once the stabilizers are exhausted, the mechanics of the shoulder joint fall apart. The ball migrates upward, and impingement begins. This phenomenon is why injury often happens at the end of a game or practice.
In older adults or sedentary individuals, the rotator cuff muscles can shrink (atrophy) from lack of use. Weak muscles cannot hold the ball centered in the socket.
This defect allows the humeral head to ride up higher than normal, causing it to rub against the acromion during even simple movements like reaching for a cup.
The shoulder blade (scapula) is the foundation of the shoulder. When you lift your arm, the shoulder blade must rotate upward to clear the way. Imagine opening a drawbridge; the acromion is the bridge that must lift up to let the arm pass.
Scapular dyskinesis means abnormal movement of the shoulder blade. If the muscles around the scapula are weak or tight, the blade doesn’t rotate enough. The “drawbridge” stays down. When you lift your arm, the humerus hits the acromion because the scapula didn’t move out of the way. This kind of injury is very common in office workers with poor posture (“slumped” shoulders).
Tendons are analogous to ropes; a single abrasion against a rock typically does not cause significant damage. If you rub it a thousand times, it frays. Repetitive overhead activities cause cumulative micro-damage to the tendon fibers.
Swimmers, for example, perform thousands of strokes per week. Each stroke involves a moment of mild compression. Over time, the compression leads to swelling (edema) in the tendon. A swollen tendon is thicker, which means it takes up more room in the already tight subacromial space, leading to more rubbing. This cycle of swelling and rubbing is the hallmark of overuse impingement.
The joint capsule is the watertight sac surrounding the shoulder. In many people, specifically throwers or those with poor posture, the back (posterior) part of this capsule becomes tight and thick.
A tight posterior capsule acts like a wall pushing the ball of the shoulder forward and upward. This shift in position forces the ball into the acromion during movement. Stretching this tight capsule is a key component of non-surgical treatment to restore normal joint mechanics.
In athletes who throw, the posterior capsule thickens as a natural adaptation to stop the arm from flying off during the follow-through.
However, if it gets too thick, it pushes the humeral head forward. This alters the center of rotation and leads to internal impingement, where the undersurface of the tendon gets pinched.
Sitting with rounded shoulders for years allows the posterior capsule to shorten adaptively. This tightness limits internal rotation.
When you try to lift your arm, the tight capsule forces the ball to rise up, causing impingement against the roof of the shoulder.
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Yes. Slouching rolls the shoulders forward and tilts the shoulder blade down. This lowers the acromion roof, drastically reducing the space for the tendons. Simply sitting up straight opens the space and can relieve pain immediately.
Not necessarily. Many people have bone spurs without pain. Surgery is only needed if the spur is tearing the tendon or causing severe pain that doesn’t respond to therapy.
Yes. Exercises like upright rows and behind-the-neck presses place the shoulder in an internally rotated and elevated position—the exact position of impingement. Modifying these exercises can protect the shoulder.
While sleeping on the arm doesn’t grow a bone spur, it can compress the bursa and limit blood flow to the tendon for hours. This aggravates existing inflammation and is often why the shoulder hurts most in the morning.
The shape of your acromion bone is genetic. If your parents had hooked acromions and needed shoulder surgery, you are more likely to have a similar bone structure.
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