I've had ME/CFS for about 14 years. If I really pace myself, I am relatively stable albeit limited in what I can do. But I try to get daily exercise walking my dog, shopping etc. If I push myself too hard, by being more physically active, I do not build bigger and stronger muscles. The reverse happens! Within a period of about 4 day or less, my calf muscles go from being firm and smooth to being visibly shrunken, and to feeling mushy and soft to the touch. I get excrutiating nerve pain in my feet and calves, and my legs go so weak I literally cannot go up stairs. Since I have some cardiac issues (likely also due to CFS) I assume that this muscle wasting is due to lower oxygen levels in my extremities, and poorer blood flow. But maybe its metabolic? I don't know. But I would be very interested to know if anyone else has this symptom, if their docs have suggested a reason for it, and how they deal with it. It's so frustrating! The only thing I am able to do is complete bed rest followed by a vry lowered activity level and it takes me ages to rebuild the muscle and my strength.