If I started from a well rested state, I can do intense exercise for a short amount of time, without triggering PEM. I can also do other activities without significant problems. In fact that these activities are so unproblematic and rewarding is a problem, because it makes me want to do more. Doing more then leads to PEM. My PEM appears to result from the cumulative effect of activies and rest in the preceding days. Activity here means anything which involves more work than resting, even listening to music, reading or being around other people. Thinking of it as metabolic activity may be close to the truth.
Different parts of the body seem to be able to have their own PEM tresholds and generate somewhat different PEM profiles if stressed excessively.
Maybe PEM is how the body reacts when it cannot tolerate more than a certain amount of metabolic activity within a certain timeframe.
I now have better aerobic fitness and strength than some other healthy people, but have less endurance and begin feeling unwell after too much activity, and get PEM from too much cumulative activity. The main difference between me and a normal person appears to be that they can keep going for much longer and with no significant impairment. At least in my case, it is possible to improve fitness with the right approach, but this has little effect on disability. If someone can't be active for half a day on most days of the week then they can't earn enough money to sustain themselves, and won't fit into many existing systems built for normal people.