A gung-ho young athlete who is improperly trained can screw himself up with too much anaerobic exercise, and then his/her body will just refuse to keep going - for up to 3 weeks. That is called "over-training syndrome."
Professional and collegiate trainers keep close tabs on their athletes because of this.
For some reason our bodies shift into anaerobic metabolism (generally anything that sends our heart rates over 100) too soon. In my case, just walking does it when I'm sick. So you could say that our bodies are responding to "normal" activities as if we were athletes pushing too hard, that is, to a certain degree we are perpetually in the midst of "overtraining syndrome."
They use the VO2 MAX test (or CPET - Cardio-Pulmonary Exercise Testing) to measure this.
... The amazing thing Staci Stevens and Chris Snell found was that high-functioning patients may score the same as deconditioned controls (the afore-mentioned couch potatoes) in one day of exercise - but on the SECOND day, the controls' scores don't change, whereas the patients' scores plummet IN HALF.
Which makes sense if you have a good understanding of this disease. But is really quite an astonishing finding for outsiders.
... CDC's explanation for not doing the two-day test is that it would be an imposition for patients. But both Staci and Chris found that while the deconditioned controls could get whiney about having to do the test, patients with ME/CFS (Canadian) would walk on hot coals if it would move the science of this disease further along.
Christopher R. Snell, Staci R. Stevens, Todd E. Davenport, and J. Mark Van Ness. "Discriminative Validity of Metabolic and Workload Measurements to Identify Individuals With Chronic Fatigue Syndrome." Physical Therapy (2013). Click HERE for the abstract.
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Yet another bit of proof that this is physical, not psychological, which the powers that be don't want to become common knowledge. If more CFS patients get this testing, then SSDI has to pay more Disability claims, because the patients now have objective medical evidence of impairment. And private disability insurance, which often contains a limitation of 2 years for mental illness, while physical illness is paid to age 65, would also have to stop categorizing CFS as psychiatric in order to save money.