It can be easier to think of the process as overdoing things so your anaerobic threshold drops and lowers how much energy you can produce.
I strongly disagree. My physically-induced PEM seemed to be due to t-cell activation from muscle damage (consistent 24 hr delay). My PEM did not affect my physical energy production/accessibility. Even my more severe PEM episodes wouldn't have reduced my body's ability to bike 40 km; the neurological symptoms simply made me not feel up to doing such activities. I consider reduced physical ability to be a downstream effect of ME's neurological dysfunctions, not an actual reduction in ATP production. There might be some measurable reduction due to changes in hormone production and various other neurologically-controlled physical and chemical aspects of the body, but I do not consider ME to be a loss of actual chemical energy.
Pacing didn't work for me, since my ME wasn't triggered by overall exertion; it was--as best I could figure out--triggered by damage to muscle cells by using them in ways that they weren't used to working. Biking and hiking were not triggers (unless well beyond my usual distances), but even a minute of using my muscles in unusual ways would trigger PEM 24 hrs later. Washing a window or a ceiling (arms up) would definitely trigger PEM, as did climbing a ladder. I expect it would have taken less than a minute of swordfighting lessons or a dancercise class to trigger PEM.
So no, PEM is not directly about energy overuse, and it isn't, AFAIK, a reduction in available ATP (there's no clear clinical evidence of that). For the more common form of ME, overexertion probably alters some chemical ratio, which alters something else, and so on, until it changes whatever mechanism is ME's core dysfunction. I believe that's neurological, or neuroimmunological. That affects function in various parts of the brain and nervous system, which possibly many steps later results in the feeling of a lack of energy and/or an inability to make those muscles work correctly.
Hmmm, do the severe ME patients show a reliable deficiency of ATP? If the reason they can't go for a run is that they are lacking in chemical energy, that should show up most strongly in those patients. My guess is that the differences between them and a healthy person would more likely show up on brain scans or nerve impulse measurements. Maybe it's the part of the brain that converts the 'I will now get up and go for a run' signal into nerve impulses. Maybe it's the nerves themselves. Maybe it also involves the feedback signals and processing. I think it would be easy for some small changes in just a few brain cells (hard to notice even with today's technology) to reduce a person's physical ability, but difficult for there to be a physical reason that hasn't been noticed.
I still think a valuable experiment would be to have the muscles of a PWME (typical type, not my weird type) activated by electrodes, and measure the physical output. Is it less than the healthy control? That experiment should answer the question of whether ME's symptoms are physical or neurological.
