Interesting thread. My answer is yes and no. Pre-onset, I was sporty, more than most of my family and friends, doing lots of running, hiking, walking and aerobics. As a child I was very active with multiple sports going on at all times and a lot of outdoor time. But at times I also seemed to have limits that other sporty types didn’t have. As a teen I was a sprinter, but I literally couldn’t do the warm-up laps. Anything bursty – fine. Anything endurance-y – crumpled heap. I got injured a lot more than my peers and eventually had to just bow out of competitive sports.
In my twenties I learned what worked for me and what didn’t – I could jog regularly but needed to alternate a jog-day with a walk-or-hike-day to be able to sustain this, otherwise I would get colds +/- injuries; I couldn’t train for anything on other people’s terms, I needed to listen to my body rather than adhere to any kind of plan, e.g. others were able to train through a cold, whereas I learned from experience that I needed to stop anything beyond a light walk for at least 2 weeks with any infection, often longer. I exercised far more than most of my peers throughout my twenties, but these were the days before everyone was doing ironman/triathlon-type things. I did notice that I often became symptomatic with an infection the day after a run, so I seemed to be very susceptible to that little dip in immunity. A run seemed to be able to kind of put an infection through me, including the one that triggered my ME. Bit o' laziness would have helped me that night [she reflected from her hospital bed a few days later].
I think there’s likely to be a normal range of exercise tolerance, perhaps with a bell-curve distribution, and I was well within it, towards the upper end but not able for professional levels. Exercise was a big part of my life and I miss it a lot. I sometimes put myself to sleep by imagining going for a run, from putting on my runners…although it has crossed my mind that if the mind-body people are right, I’ll get PEM just from visualising it, ha ha. Hasn't happened yet. I still remember the thrill of winning sprints, God, that was great.
Edit Further to previous...I definitely had mild orthostatic intolerance from childhood, although I only realised what it was when it became a prominent part of my ME in my 30s. I think this is why tennis and most other ball sports made me feel awful - just vaguely nauseated and ugh-feeling, occasionally a bit lightheaded but generally just felt bad, even grumpy. All that standing still waiting for the other person to serve/take a penalty/whatever, and then bending down to pick up the ball. Not good. Makes so much sense now. Swimming was the same for me, I think because of indoor pools being heated (the air was the problem, not the water) and no fresh air. So running was a good find, as it builds up the muscles in your legs, which helps OI, and the colour of my face suggested plenty of blood was making it to my head. I wonder did any others have OI-type exercise issues as children?