Not an expert but I would not think you would suffer much muscle loss in a short (less than 1 week) fast. It takes longer than that for your metabolism to transition into starvation mode. Maybe there's some risk if your body fat is already very low.
It looks like you can lost a fair amount of muscle mass, and that you lose the most muscle mass early, so 1 21 day fast would be better for you than 3 7 day.
