I have problems walking. I can walk normally for about a block, but then my calves and quads start to burn like they used to back pre-fibro when I was in athletic training and I'd be doing weightlifting workouts to "total muscle failure" (the kind where you do reps at a lower weight but do them until you literally can't do another lift and your muscles are shaking). Now with the fibro, I can push myself through the burn of my second city block, but by the third, I can feel the burning muscles start to cramp. If I keep trying to walk without resting, I end up looking like Frankenstein, I think.
It's the same with any muscle group I use--I can blow dry half my hair before my arm muscles start to burn. If I keep going, the burn turns to cramps. I can smile at a party or at work for a few minutes, but then my face muscles start to burn. When I rest any of these muscles, they like to keep going with rolling cramps (fasciculations) that make it look like some alien creature is crawling around under my skin. It's kind of amusing to watch, in a gotta-laugh-or-I'd-cry kind of way.