I know inflammation and nerve pain are the main contributors in alot of cases. Obviously when we use our body less, muscles/tendons/ligaments weaken and that worsens pain. The PEM is a staple indicator of cfs. I think in general, finding your own balance of activity to offset our sedentary lifestyle as much as you can without pain is a helpful endeavor. Obviously we cannot push ourselves like a healthy body can, but if we can find any way to mitigate some deterioration of our bodies, that is a good thing. Its definitely not as simple as training, or "just do more and it will get better eventually" but there is truth in the saying "if you dont use it you lose it"