Back in 1990, Dowsett and Ramsay’s study “Myalgic encephalomyelitis–a persistent enteroviral infection?” had this section on why exercise was harmful for people with ME: (reference numbering has been removed)

Exercise related morbidity in ME

Coxsackie viruses are characteristically myotropic and enteroviral genomic sequences have been detected in muscle biopsies from patients with ME. Exercise related abnormalities of function have been demonstrated by nuclear magnetic resonance [20] and single fibre electro-myography including a failure to coordinate oxidative metabolism with anaerobic glycolysis causing abnormally early intracellular acidosis, consistent with the early fatiguability and the slow recovery from exercise in ME.

Coxsackie viruses can initiate non-cytolytic persistent infection in human cells. Animal models demonstrate similar enteroviral persistence in neurological disease, myopericarditis and the deleterious effect of forced exercise on persistently infected muscles. These studies elucidate the exercise-related morbidity and the chronic relapsing nature of ME.

[From the Conclusions section:]

ME predominantly affects the most socially and economically active section of society. Misinterpretation of this common illness as psychogenic delays the early recognition mandatory for modification of life style which may avoid progression to chronic disability.

In a 1992 interview “Conversation Piece. Interview by P.D. Welsby,” Dr. Elizabeth Dowsett said this about exercise and treatment of ME:

“The medical management of ME differs greatly from that of other comparatively short-lived post infectious debility, such as may follow influenza, in that the patient with ME has a 30% chance of cardiac and other systemic complications and must modify their lifestyle to a reduced capacity for as long as it takes the illness to stablize. Early recognition and sensible advice to avoid mental or physical over-exertion, can do much to avert a prolonged chronic illness.”