Infraspinatus.

Lateral rotator of the arm at the shoulder joint.

Nerve supply: C5 and 6

Skin: dorsal rami of T1 to T6.

So I was just wondering what to say about this muscle, and I came across a great page called "Meet Mr. Infraspinatus":

"... One way that the infraspinatus, in particular, can be injured is in the slowing down movement just following a throwing action...Another thing people usually notice when infraspinatus is the main culprit is that they can't sleep on the sore side because it compresses the shoulder joint, causing further discomfort. They also can't lie on the other shoulder because as they fall asleep, the injured shoulder moves forward, stretching and aggravating the condition and waking them up. If you are counting, you know that such a person has now run out of shoulders to sleep on. In fact, some people who struggle with this situation even end up propping themselves up in armchairs to get some semblance of a comfortable night's rest." - from

https://www.holmescountyshopper.com/local%20news/2012/12/13/meet-mr-infraspinatus-the-rotator-cuff-muscle

This is a page by David Pratt, Millersburg, US. He gives excellent advice including

- trying a pillow in front of you when you lie down with the bad shoulder uppermost, so that the shoulder and arm is not dragging down.

- visiting a therapist for soft tissue work to reduce inflammation etc. My thoughts entirely!