My stiffness is the only thing that other people can see. (Apart from the Ache, which my wife can see in my eyes.)
I long believed if the Ache is in all muscles and the pains in all big joints, then the stiffness when I move after staying in a position for >10' is (in) the muscles & joints. I've studied it, tho, by feeling them & moving at the same time. And I've come to realize it is not in the muscles (or joints) themselves, it's mainly the tendons of the muscles, above and below the big joints and in the (mainly lower) back and backside. My TCM/acupressurist is trying to work on this, but no success as yet. She also regularly finds stiff and other problems in my muscles, which I don't all directly notice, but some as a muscular pain. When she makes them supple, I feel better, probably need less energy to move. But it doesn't really make the stiffness better, at least not for long. But it's important for her to know that it's more the tendons to find something that may help.
This fits to the alternative diagnosis of my rheum. "polyinsertionstendomyopathy", which just means: pain in all the ends of the tendons of the muscles (pathy in poly insert(ion)s of tendo of the myo). Which is for me by far not an acceptable alternative, because it only describes the stiffness of fibro, nothing else.
So: Are you (and all these "people") sure it's your muscles? Just asking - but of course I'm still looking for a possible solution myself. If anyone will find it, then my acupressurist, because she looks & thinks closely and has lots of ideas and doesn't give up easily. Not too hopeful, tho. I can't remember having seen any studies which explain this and it doesn't fit well into most theories either. I keep getting my pains down; my Ache & feverishness I get down by pacing, up by overdoing; but stiffness and sleep are "stiff ones"... (You've perhaps seen the "similar threads" on this, and written one yourself which I've made suggestions to and you've "liked": so I hope this helps a little bit too - the theory behind what you could try?)