I forget what it's called -- it's the part of my body that starts from the head and goes down to the toes?
Fortunately, I'm such a creature of habit, that if I don't work out I feel even worse. So, as long as I can get to the gym my "auto-pilot" kicks in. Of course, the prospect at having eye candy to look at as I work out (oh, don't worry -- I don't oogle, I just catch a glimpse here and there).
But, seriously, it's probably my back. It generally responds well, but once in a while I'll pull something (usually the same spot) and I spend the next few days wincing when I move a certain way or as I roll over in bed. I just never know when it's going to happen, so I don't know what to do to prevent it.