The Chief Medical Officer's working group report states: "One of the most common and characteristic complaints of adults, particularly in the early stages of the illness, is of intolerance to both physical and mental exertion with delayed impact. So perhaps the key pointer to a diagnosis of CFS/ME is the way in which the symptoms behave after increased activity."
All 3 of the strata criteria for the PACE Trial include fatigue and the CDC criteria specifies fatigue, ''is not substantially alleviated by rest" and "post-exertional malaise lasting more than 24 hours", the latter being one of the multi-choice factors. There appears to be no specific mention of the delayed affects of effort which mean that people with ME/CFS might still be experiencing worsening of symptoms hours later and often cannot repeat an exercise the following day.
The ME Association and The Young ME Sufferers Trust wrote (2008. http://tinyurl.com/yc63ome):
"When patients are made worse by GET (which is based on the flawed concept of deconditioning) this is not merely due to a problem with the way in which the therapy is delivered. Post-exertional malaise is a key diagnostic criterion for ME/CFS. Put simply, the illness worsens as a result of physical and mental effort. Advocating progressive exertion is to show a worrying lack of knowledge about the nature of the illness itself."
The MEA's view is supported by the research of Paul et al who tested delayed recovery from exercise in CFS. They concluded, "These findings support the clinical complaint of delayed recovery after exercise in patients with CFS." (http://www.ncbi.nlm.nih.gov/pubmed/10209352)
Cort Johnson wrote about the Pacific Fatigue Lab findings when they had people with ME/CFS repeat an exercise test the following day (http://aboutmecfs.org/News/PRJan09Pacific.aspx):
"Their results are both profound and disturbing. About half of the ME/CFS patients they've tested do, in fact, 'fail' or significantly under perform in the first single exercise test - they cannot generate normal amounts of energy even when they're 'rested'. It's the rest of the patients that are so intriguing, though. When you give these patients a second test a day later many of them will fail as well--and fail spectacularly."
"The amount of impairment the Lab see's can be astonishing - some patients suffer as much as a 50% drop in their ability to produce energy the next day. Ms. Stevens spoke of a twenty-something man whose next day exercise tests were worse that those of a normal 85 year old. In a hospital setting his cardiopulmonary exercise profile would suggest he had heart failure."
If the PACE Trial is not studying the delayed effects of effort which can be objectively measured and which seems to be quite specific to ME/CFS, it is a lost opportunity to gain information about the suitability of the therapies for particular patients according to their physiological response to activity.