I think one of the first things you’ll be interested to hear is modern definitions of ME/CFS don’t note fatigue as the most disabling symptom but a disease process called post-exertional malaise, wherein a person’s entire disease can worsen (sometimes permanently) based on the amount of energy they use.

It’s really fascinating, A recent study by Wust et al. found muscle clotting and abnormalities following exertion in people with ME/CFS.

Also people with ME are the only known disease group to preform markeldy worse in specific markers on a 2 day CPET. Which would serve as a biomarker if it were not for the fact doing the test can worsen people’s disease permanently.

I highly recommend this summarising article on the issue: nationalgeographic.com/…/long-covid-patients-exer…