I've done both with no trouble
When I was doing a water fast, I was walking 15 kms a day with occasional weak moments...did 11 days and 13 days. The key to strength was to rest when needed and frequently eliminate via enema or equivalent...I don't do swfs. On my 11 day fast I was extremely weak, but had decided that I didn't feel like doing the daily enemas...this was my mistake. Once I did a colonic, I found that there was a lot of waste in my bowels that had been sitting there for a while and I was reabsorbing the toxins! Litterally 30 minutes after the
colonic started I felt strong again.
MC is definitely a lot easier. I was running through my entire 29 day fast, save the final week, when I decided to take it easy.
As long as you are moving you should be fine. There could be a sensation of muscle loss, but is it really that? Or could it also be possible that there is a lot of metabolic waste that has been eliminated from the muscular tissue, thereby giving it a more deflated look? Who knows. But it all comes back.
Any rapid weight loss will give you the saggy skin look, but it all tightens up with activity and good healthy eating, post fast.
Good luck!