Muscle resistance and nutrition is what it takes to create and maintain muscles. If you have a concern you could do light resistance just to keep the statu quo, but I wouldn't worry about it unless you are going to do the Master-Cleanse for longer than 10 days. I would think you would lose little or none in such a short period of time and if you did, you could gain it back when you complete the cleanse. If you are still concerned or are planning to do the cleanse for a long period you might want to look into doing a juice "fast" instead.