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Either increased adenosine production or decreased adenosine reuptake in fascia seems to be a plausible mechanism for the fatigue we observe in ME/CFS after sport. Adenosine is anti-inflammatory so there's reason for why the body might upregulate it for example when a COVID/lyme infection produces a lot of inflammation. It would explain the symptom of CFS.
Someone should run the experiment of whether adenosine in fascial tissue is increased in those patients after exercise.