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If lifting your arm overhead causes pain, pinching, or stiffness, the issue may not just be tight muscles or weakness. One often-overlooked factor is the shape of your acromion.
The acromion is a bony projection of the shoulder blade that forms the “roof” over the rotator cuff. Its shape can influence how much space the shoulder structures have to move—especially during overhead motion.
The 4 Types of Acromion
Type I – Flat
A flat acromion typically provides the most space for the rotator cuff tendons. People with this type often have smoother overhead movement and a lower risk of impingement.
Type II – Curved
This is the most common acromion shape. The curve slightly reduces the space under the acromion and may increase the risk of shoulder irritation, especially with repetitive overhead activity.
Type III – Hooked
A hooked acromion significantly narrows the subacromial space. This type is frequently associated with shoulder impingement, rotator cuff irritation, and pain with lifting or reaching overhead.
Type IV – Convex Upward
This less common type curves upward rather than downward. While it may allow more space in some positions, it can still affect shoulder mechanics depending on movement patterns and muscle balance.
Why Acromion Shape Matters
When you lift your arm overhead, structures like the rotator cuff tendons and bursa pass beneath the acromion. Certain acromion shapes—especially curved or hooked types—can increase compression in this space, leading to:
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Pain with overhead lifting
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Shoulder pinching or catching
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Weakness or limited range of motion
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Rotator cuff irritation over time
How Is Acromion Type Diagnosed?
Acromion type cannot be determined through a physical exam alone. Imaging, typically an MRI, is used to identify acromion shape and evaluate the surrounding soft tissues.
What Can Be Done?
Even if your acromion shape contributes to symptoms, pain is not inevitable. Treatment often focuses on:
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Improving shoulder and thoracic mobility
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Strengthening the rotator cuff and scapular muscles
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Correcting movement mechanics
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Reducing inflammation and tissue irritation
The shoulder works as part of a chain, so addressing posture, upper back mobility, and shoulder stability is key.
Final Thoughts
If you’re struggling with overhead activities—whether it’s lifting weights, reaching into cabinets, or sports—it’s important to understand why your shoulder hurts. Acromion anatomy is one piece of the puzzle, but proper evaluation and individualized treatment make all the difference.
If shoulder pain is limiting your daily life or workouts, consider having it evaluated so you can get answers—and a plan.
Stay moving.
Dr. Steve Muscari
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