MS patients, due to the very nature of their illness, have areas of the body where the muscles simply don't work properly.
Good point Harry,
This is a lesson I had to learn the hard way. Before diagnosis I still exercised to the max, taking a morning power walk besides doing other exercises as well. I had spasticity with pain and spasms in my legs but I had no idea what it was at the time. I didn't even imagine anything at all was wrong with me.
I still sometimes hurt myself by overdoing it. Spasticity and heat intolerance are both problems for me. It's not easy to accept slowing down when you're a former overachiever.
Now, I'm at the point of having to psych myself up to even exercise at all, but I know that if I want to keep my mobility, I have to keep dragging myself into the TV room and doing it (since I can't exercise outside anymore.)