Stand up.
Put one hand behind your back. Slide it up between your shoulder blades. Now switch sides.
For most people, one side goes further than the other. The side that doesn't move is the side with a locked rotator cuff. Probably the side that aches when you sleep on it.
That asymmetry is information. It usually points to a specific muscle. Almost never the one you'd guess.
This is part of a series on referred pain. Quick refresher in case you're landing here cold: a tight muscle doesn't always hurt where the muscle is. The shoulder might have the most counterintuitive patterns in the body. Pain at the front of the shoulder almost always comes from muscles on the back of the shoulder. Pain reaching behind your back almost always comes from a muscle you can't reach at all.
The shoulder is the most mobile joint in the body. It pays for that mobility with stability. Four small muscles, the rotator cuff, do most of that stabilizing work. When one of them locks, the entire shoulder gets stiff, and the pain shows up in places that aren't where the locked muscle lives.
Four to know:
1. Infraspinatus. Back of the shoulder blade. Probably the most common shoulder-pain referrer in the body. The pain shows up at the front of the shoulder and down the front of the arm. People walk in pointing at the front of their shoulder. The muscle that needs work is behind it.
2. Subscapularis. Tucked under the shoulder blade, against the ribs. Hard to reach by yourself. When this one locks, the reach-behind-the-back test fails. Refers pain to the back of the shoulder and an oddly specific band around the wrist.
3. Supraspinatus. Top of the shoulder blade. Initiates lifting the arm out to the side. When loaded, the deep ache shows up on the outer shoulder, the exact place people press on and say "right here."
4. Teres minor. Small. The single sore dot at the back of the shoulder that won't let go.
A note on what I look for. The reach-behind-the-back test is one of the cleanest assessments in shoulder work. If the asymmetry is more than a few inches, the work is usually subscapularis-heavy. If the pain is in the front but the test is roughly even, it's usually infraspinatus. The pattern tells you where to start.
Two things to try this week.
Cross-body stretch. Bring one arm across the chest, pull the elbow toward the opposite shoulder. Hold thirty to sixty seconds each side. The stretch should be felt in the back of the shoulder, not in the elbow.
Wall slides, slowly. Stand with your back flush against a wall. Goalpost arms, elbows bent ninety degrees, backs of hands and elbows touching the wall. Slide arms up the wall as far as they go without losing contact. Five slow reps. The point where contact breaks is where the cuff is stuck.
When to bring in a professional.
A bodyworker, if the referral patterns above match and there's no specific injury.
A physical therapist, if range is significantly limited (especially overhead or behind your back), if there's weakness, or if it started with a specific event.
A doctor, if the pain came on suddenly after a fall, if there's significant weakness lifting the arm, or if numbness or tingling runs down the arm. Cuff tears and impingement show clearly on imaging.
A stuck shoulder isn't a broken shoulder. It's an asymmetry the body has been protecting. The reach-behind-the-back test shows you what's protected. The bodywork lets the body know it's safe to stop.