I have a friend who has had fibro & CFS for years - she used to bike a lot before she was too sick. I asked her recently if she thought it contributed to her health decline, and she said she thinks she would have gotten sick much earlier if she had not been trying to bike for as long as she could.
That line of thinking definitely inspires me to keep trying to move while I still can, esp. since I seem to be able to build up very slowly now that my meds seem to be adjusted right for me. It's a difficult balance to find - to do all that you can and yet still get the rest that you need.
The real trick, imo, is to stay as active as possible while still staying well under your AT for routine activities, and near your measured AT only briefly for more strenuous activity. It is impossible for some PWME to stay under their AT during normal activities of daily living. Exercise for the sake of exercise seems foolish under those circumstances, although the most basic mobility maintenance activities (range of motion, stretching) are probably valuable for the bedbound, if possible without going over the AT.
My sense is "appropriate exercise" is highly dependent on the level of functioning. Those who are working full-time are probably able to walk or bike for the sake of exercise. Those who are bedbound and housebound probably get as much "exercise" as their bodies can tolerate just getting through the day. To encourage all PWME to exercise or increase activity ignores the reality of the variations in the illness. Too many mild patients, who don't realize the degree of disability of moderate-severe patients, insist that exercise for the sake of exercise is valuable for ME because it worked for them. The same is true of some fibro-only patients who have been told they have "CFS". These people don't understand how much more physically capable their bodies are than those of a large portion of the ME population.
We need some kind of objective measure to know whether our activity/exercise is harmful or helpful. AT monitoring is probably the best we have at the moment.
For quite a few years I was unable to walk across the room without going over my AT. I was at 95+% of my AT just sitting in a chair. A nice little walk around the block would have been foolish under those circumstances. Now that I'm better, I can be on my feet around the house a lot more. I can take a shower without going over my AT. I can walk up a flight of stairs without going over my AT. I can walk from my house to the garage without going over my AT. None of this happened because I exercised. It's the result of pacing to stay below my AT, lots of rest, and appropriate medical treatment for pathogens and OI. I increase my activity as my condition improves, not the other way around.
I still can't walk 100yds or so without PEMing myself, so I can't shop without a cart or wheelchair. I can't go for a walk around the block. However, now that I can shower, cook, and do light cleaning without going over my AT, I can afford to do the kind of exercise Connie Sol at INIM as recommended for me -- supine exercises with light weights, about 6 different exercises at about 6 reps. No more than 4-5 mins at a time, none of it aerobic. This is the reality of "exercise" for the moderate-severe patient.