Losing Muscle Mass when fasting?

Hi all! I'm interested in fasting but am a competitive athlete and am concerned about my body burning up it's muscle...

I've read that on a fast it's typical to lose 1lb per day but I know that the body can only burn about 2lb's per week of fat..

Is there a length of fast that would be more appropriate for someone like me? Should one excercise a lot, would this keep the body from burning muscle?

thanks!