you posted a link to the Russian fasting research abstracts, but are you aware that the core of the method, which is followed to a tee in those clinics, is exactly what I described: purge at the onset of a fast, daily enemas, phys. activity and absolutely no bed rest.
Your assertion that phys. activity during a fast will waste muscle quicker -- on what do you base this? This is speculation on your part, isn't it? The reality is the other way around. While some muscle is always lost on a long fast, the losses are minimal when those muscles are used. Look up Cori cycle.
If you fasted regularly yourself, you would know how absolutely essential physical activity is and what a difference it makes.
There are many reasons to fast regularly, too many to cover in a post. In a word, this is the best method to keep young, healthy and active into old age.