What level of exercise to prevent/mitigate muscle loss on a 21 day fast?

From what I've read, you want to do weight lifting but not hard enough that it'l break down the muscle fibers, just enough to engage the muscles. How does this translate exactly into a workout? Would one set of bodyweight stuff (pushups, squats, pull ups/chin ups) suffice?

Answer

I just worked out yesterday. I did a full body weight training session. I did 3 different lifts for each muscle group, at a lower weight, less reps, and only 3 sets. Example: lat pull down 50lbs 3x10Bent over row 50lbs 3x12

Take what you would normally do and reduce the weight by 5-10lbs, reduce your sets by 2-3, reduce your reps by 3-5 that way you are engaging the muscles but not straining them. With as little as what I did yesterday I still felt a little sore today when I woke up, but it was the good kind of sore. Most important thing is to just listen to your body, if something hurts stop.