Shoulder pain in swimmers usually starts in the mid-back

Raise your arm overhead. Roughly a third of that motion happens at the shoulder joint itself. The rest comes from the shoulder blade rotating upward across the back of the ribcage — and the ribcage has to be able to extend and rotate for it to do that.

Which means your overhead range depends on the mobility of your mid-back: a region that spends most of the modern day flexed forward over a desk or a steering wheel. If that describes your working hours, the desk posture article is the other half of this story.

Stiffen the mid-back and the shoulder blade can't rotate as far. But the arm still has to get overhead — thousands of times per session. The remaining range gets taken from the shoulder joint, which reaches the end of its own range early and starts compressing the structures inside it. The rotator cuff is what gets irritated. Mid-back stiffness is often what put it in that position.

Lie on your back, knees bent, low back flat to the floor. Keep it flat — this is the whole test. Now reach both arms overhead toward the floor behind you.

If your arms reach the floor with the low back still flat, your overhead range is genuinely available. If your ribs lift and your back arches to get there, you don't have the range — you have a substitution. And that substitution is exactly what you're doing in the water, thirty times a minute.

What to work on. Mid-back extension and rotation first: rotation drills on your side, extension over a foam roller at the mid-back, and an overhead reach that lengthens one side of the body at a time so the shoulder blade integrates with the ribcage rather than working in isolation. Then load it — overhead work in the new range, starting light, because range without strength doesn't hold. Then technique: a crossover entry, a dropped elbow in the catch, or breathing exclusively to one side will keep reproducing the problem no matter what your mid-back does. Worth getting a set filmed.