One of the things that you can do when you are wanting to exercise and look great is overtraining. Overtraining is when you don't let your muscles recover before you stress them again.
I am sure you can overtrain aerobically, like the 700 pool laps the girl did in Mallrats, but that's not where I have my problem
However, it is easy to overtrain when you are doing strength training, even for relative newbies like me.
According to a study by some Canadian physiologists, it takes at least 36 hours for smaller muscles to recover to pre-workout levels. It may take longer for larger muscles to recover.
Body for life recommends three weight training days a week. These are in rotation. First week: upper, lower, upper. Second week: lower, upper, lower. And so on.
When I was doing this, I gained about four pounds of muscle. It was over a 12 week period, so I wasn't very impressed originally. But, you know, every pound of muscle you put on increases your metabolism. I should have just been thrilled to be going in the right direction.
But I wasn't. So I figured I could exercise every other day and still not stress my muscles. (That's longer than the 36 hours from the Canadian study.) I will tell you that my arms felt fine when I did this. However, my legs got so bad that I couldn't even do one squat or lunge set. I should have seen that as a sign that I was overworking them. Which I kind of did. But instead of going to weight lifting less often, I would keep doing them or I would wait one extra day for the lower body exercise.
I didn't gain any muscle doing it that way.
So the next challenge I changed up my routine. Which you need to do every so often or your muscles become conditioned to doing what you're doing and they don't continue to grow. But I was still training 5 days a week. Again I didn't gain any muscle.
I've dropped back down to the BFL recommendation of three times a week, and I am doing great again. Gained one pound of muscle in the last two weeks.