Hypertonic muscles, they're called. A month or two ago I was standing in my living room, minding my own business, when my knee began to really hurt. Couldn't think why. Figured an injury from earlier had decided to ache. Not much sleep that night.
Next morning, while having a tentative poke around said knee, I discovered the muscles down each edge of the knee felt like metal cable, and the kneecap (patella) wasn't where it usually is, and wouldn't budge. I realized at that late point what had happened: the muscle spasm had dislocated the kneecap. I really, really wish I'd worked it out earlier.
Some aggressive self-massage (assisted by a wooden mallet) encouraged the tortured muscles to relax enough I could nudge the knee home. Instant 70-80% reduction in pain. It took a few weeks for the tortured tissues to heal up. No marathons for Ms MS - unless you count ADLs? (Activities of Daily Living... Like showering, cooking & housework generally now I come to think of it.)
My medical team prescribed a foam roller (DIY personal masseur)& to keep up regular massage. And lots of gentle stretching once the muscles are out of spasm. My Chinese masseur panicked when he first laid hands on me, tsking & tutting anxiously until I told him I have MS, "Oh, oh right, that's okay then", he responded. It's ok to be a total crock if there's a reason for one's crock-ness.