Coming back to this, I find I want to question some assumptions and add some points:
But in terms of definition I suspect we have to just say that it is a worsening or expanding of symptoms that are not just normal immediate tiredness or delayed muscle soreness following exertion.
If you experience a worsening of orthostatic intolerance with nausea and headache while you are still involved in some exertion does that have to be excluded from the topic of PEM? If you experience these things the next morning for just one day is that to be excluded?
My point, and this is emphatically not based only on my own experience, is that the 'usual' increases in symptoms with any activity for pwME is at a certain level, and the 'unusual' increase in symptoms in PEM is disproportionately much greater, given the small increase in activity.
So of course the type and severity of symptoms when ME/CFS is at its current non PEM level for a pwME is abnormal for a healthy person, and needs to be researched, and the pwME needs to adjust their lifestyle, sometimes drastically, to cope with it.
But the sudden step change from the pwME's current usual symptom severity to a much greater symptom severity and debility is way out of proportion to the slight change in their activity in the preceding days.
That needs to be named, described, investigated and coped with by pwME in a different way to coping with their usual symptoms and capacity.
We need to recogise PEM/crashes as something different from the rest of ME/CFS:
Say someone with ME/CFS does Z amount of exertion one day, and gets their usual symptoms at level Y during and immediately afterwards, and this their usual uncrashed state. If they can do about the same level of activity Z, with the same symptom level Y on subsequent days, then we say they are not crashed, or not in PEM, and their current exertion capacity is greater than or equal to Z.
Say one day they do Z+0.1Z amount of exertion one day, they may experience a proportionate Y+0.1Y level of symptoms that day.
Say they wake up the next morning and for the following several days they suddenly increase to 10Y level of severity with their usual symptoms plus 10y level of new symptoms and have 0 activity capacity to do any of their usual activities. Then they are experiencing PEM.
It's not only about delay, it's about a disproportionate response to a small increase in usual activity.
I also think that there is a problem using personal experience to generalise for a group of people with an illness. Each person may have a different pattern of timing and symptoms with the PEM concept still being useful. Syndrome patterns really need to be judged by independent observers (in practice doctors) who have taken histories from many people in thrown words. And even then they get things wrong. Over the years the validity of clinical symptoms and signs has been questioned and quite often found unreliable, as for the so-called 'trigger points' of fibromyalgia.
We can only go on what recent and current clinicians have gleaned from their patients, and follow their current definitions of PEM. Of course there are variations between patients. If I dare to mention my own case again, my experience of ME/CFS is different from my daughters, just as our experiences of migraines are different from each others. And expanding to all the thousands of others' descriptions I have read here and elsewhere and the consclusions drawn from the many thousands collated into current PEM definitions. They are all about observation and patients experiences, not making any claims to cause or biology.
So no, I don't agree with this:
But in terms of definition I suspect we have to just say that it is a worsening or expanding of symptoms that are not just normal immediate tiredness or delayed muscle soreness following exertion.