I cannot recall the site but I was doing my usual trawl through the internet for various medical conditions (I should have been a doctor but I'm not brainy enough and like people too much.) I read a article about why muscles become tight and painful. The theory was that with any metabolic condition enough ATP does not enter the muscle; the muscles contract but do not release. If this lack of ATP continues over a long time the muscles lose their ability to expand, remaining fixed and tight.
This sounded logical to me. If I get to find it I'll post the link.