The Nike commercials used to say "just do it." The imperative of our bodies is to use our muscles or lose them. It is so much harder when you are sick but you have to do what you can. I have polymyositis and lost 40 lbs of muscle in a week and also suffered severe disuse atrophy and cellular death which resulted in being a quadriplegic. 5 years later I am downhill skiing and playing golf again only because of lots of PT/OT and then working out on my own for 5 years. The hardest thing I have ever done but so glad I did.
Therapy can be very modest efforts but they pay off in the long run and it is best to start with your doctor and a physical therapists.