You're not going to gain mass during a flare, best you can do is slow the weight loss. Exercising on prednisone is a good idea, reduces the bone wasting to have some light exercise. We also have higher blot clotting risks during flares and staying active and moving is highly recommended.
You will findout that you heal slower on pred, suppose you had a moderate workout one day, you'll wakeup the next morning feeling like you were hit by a train and had pushed yourself to your limits. You are still sore and achy and do not feel refreshed. So, most certainly lower your weight during lifts, and do less sets while on pred. We had a guy powerlifting on pred, and he tore a ligament off of the bone (painful). Prednisone weakens ligaments, so do not push yourself to the limits while on it. It did heal but was painful and a slow process.
I worked out during flares, the gym had a bathroom a short jog across the room, with multiple toilets. I ran over and used them a number of times. Certainly, some excercises can be uncomfortable during a flare, anything putting pressure on your lower abdomen, like ab crunches.
It might be slightly different in my case because I take DHEA and pregnenolone while I'm on prednisone. These are hormones that go offline while taking pred, which are responsible for wound healing and muscle development. I find that even if I sit around and do nothing, if I take DHEA I conserve muscle mass. But I'm still losing weight, which is why I need the gym.
I'm not concerned about
ligaments breaking or anything as I don't power lift, but that's a really good caution to remember, thank you. I seem to recall that prednisone hinders collagen production, so that would explain the tendency for more soft tissue injuries!
Either way, I'm going to start at a reduced weight.
The weird thing is that I'm losing most of my mass from my mid-section, like around my waist, while my arms and chest still look relatively muscular. It's giving me a very weird shape.