Motor neuron disease, also known as neurodegeneration, this disease is clinically known as amyotrophic lateral sclerosis (ALS), which causes muscle weakness and is often accompanied by significant wasting.
I have a friend with it. It makes him very tired, and restricts his movement. It is mostly stress-related, but is also compounded by him coping with the illness though various addictions which keep him unhealthy. I think that dealing with stress in healthy ways, and getting rid of harmful addictions would probably make a huge difference.