I was getting bad muscle cramps at one time. Coincidently, a relative was taking up jogging at the time and spoke about having to warm muscles up before exercising. I thought that because I did not move very much when I did move it was like a normal person going straight from walking to running without the warm up exercises.
Nowadays, I try not to make sudden movements so that the muscle fibres get a chance to start working and get lined up with the blood flowing and the linkages changing gradually.
I may have the science all wrong, but I get much less cramp and if I feel it starting I can usually fend it off. A slow stretch doesn't cause the cramp I always used to get either.
Permanently spasmed muscles sounds more like a chemical problem though. There are many genetic diseases that affect mineral movement across membranes but they are often missed.