Delayed PEM is common but not a definite thing. My onset of PEM depends on how much I exerted and what kinds of exertion. If I did much, my onset is 1 to 3 hours! Perhaps less. Edit: I should add I have Fibro too, that may account for differences.
It is more about the duration of PEM. In the primer for practitioners, the M.E. experts write "Recovery is delayed, taking more than 24 hours".
Also our type of PEM is not only physical pains, but we have intensified cognitive symptoms and other symptoms. I believe many people assume PEM is feeling sore or tired, when it is many more symptoms (at least for those meeting the ICC definition of M.E.)
I agree. My PEM has always been an exacerbation of all my me/cfs symptoms. For me PEM is not a different symptom, it's a flare of all of them. It's like I have an IV of poison running slowly, then with over-exertion/stress, it's opened up to a faster rate. How long of a delay before I react to that increase, has changed over time.
True, that not everyone has a lengthy delay in the PEM response. But it seems the majority at some point do experience this, whatever that length of delay may be. I've gone from 24-48 hour delays for years at a time, to currently being 0-8 hours (last 3 years). But the length of my recovery to that PEM has also changed. It did take weeks to months, it now takes 1-8 hours. I have less of a delay, but I also have a much shorter recovery time.
Our diagnostic tools do list prolonged recovery from PEM as a likely symptom of me/cfs, and rightfully so, but they also mention delayed responses as significant for these purposes. It's just my opinion, but I think the length of recovery following PEM would be common to more conditions that would a delayed reaction to the overexertion.
Anyhow, I have great respect and appreciation for the our diagnostic tools. I still favor the CCD and all it's listed diagnostic criteria, since it did end a painful decade long search for a diagnosis (it fits me to a t), and it led me to a great me/cfs doc (also a co-author of the CCD). For me, whatever he says goes.....no argument, regardless of my opinion on these issues.