Muscle Minute TUESDAY - The Acromioclavicular Joint

Shoulder joint anatomy illustration with muscles and ligaments.

The acromioclavicular joint (AC) is something we rarely consider unless it is injured in a traumatic incident. But understanding its function and movement during arm elevation will open up solutions to treating shoulder impingement. Let’s get started.

Try this test…

Lie flat on your back with no pillow under your head. Locking your elbow straight, lift your arm up over your head as far as you can.

Do you feel a stretch in your armpit area or a pinch at the top of your shoulder? If you feel a pinch, the problem could be in improper movement of your AC joint.

How does the AC joint contribute to arm elevation?

Illustration of shoulder anatomy and ligaments.

When you lift your arm, the scapula upwardly rotates along with the humerus. This is known as scapulohumeral rhythm. From 0˚-90˚, the scapula upwardly rotates about an axis at the sternoclavicular joint. But from 90˚-180˚, the scapula continues its upward rotation with its axis at the AC joint. This occurs due to the tension placed on the coraclavicular ligaments (circled in red) that pull the clavicle into a posterior rotation along its longitudinal axis. It’s like the clavicle goes “belly-up”, allowing the scapula an additional 30˚ of upward rotation.

That was a lot of words but here’s the point: the clavicle has to go “belly-up” in order for you to get your arm over your head and that movement happens at the AC joint.

So what’s the problem?

If at rest, your scapula is already positioned in upward rotation (a common problem in people with shoulder/neck pain), the coracoclavicular ligaments are already on slight tension, placing the AC joint in an abnormal position at rest. As the arm is elevated, the AC joint “bottoms out” or runs out of room to move and this creates a pinch at the top of the shoulder.

How do I know this is the case?

If you get a pinch at the top of your shoulder, check for trigger points in the upper trapezius and infraspinatus. Also, roll the skin around your AC joint (fascial release). Are any or all of these painful? Here’s a quick video to remind you how to do these things.

Fix what you find and re-test arm elevation. You SHOULD feel a stretch in your shoulder instead of a pinch a the top.

Why does this matter?

Shoulder impingement is often blamed on the rotator cuff, but the secret to clearing impingement at the top of the shoulder lies in understanding the role of the AC joint in arm elevation. It’s a tiny joint that has big impact. Check it out.

Because nobody has time to be in pain.

Until next time…