I have been thinking about why it's very important that PEM is defined specifically as the usually delayed, and usually lasting more than a day, significant worsening of a person's MECFS that is triggered by exertion.
I think it's vital it doesn't get conflated with all other worsening of symptoms that happens during and immediately after every exertion that can be relatively minor and short lived if we are able to rest enough. I say relatively, because the point is that PEM by definition is being much more ill than our usual.
If we muddle together all symptoms after exertion and call it all PEM, how can we explain to carers, family, and clinicians that something I did today has any relation to how I feel tomorrow?
They see us apparently coping with an outing, and don't see the consequences for us that confine us to bed for the next week. They don't understand the need to stop doing something we're apparently coping well with, or understand the cumulative effect of having a shower on the same day we have a visitor, when we might usually manage one but not both without horrible consequences. They don't understand that someone with very severe ME can be made even worse for weeks or longer by apparently trivial actions that barely register to others as exertion.
When we were new to ME/CFS, it surely helped us when we learned to recognise the triggers that were likely to lead to waking up a day or two later badly crashed. It gives us at least a fighting chance of some level of control. I can't remember whether the term PEM existed when I was diagnosed with ME in 1990, but I soon figured out for myself that if I did a bit more than usual, and pushed through the usual worsening pain, nausea, weakness etc, and kept going with activity for too long, I'd end up having to take sick leave for a week or two.
And how would researchers learn whether there are different things going on with our biochemistry if they don't test before and for some days after an exercise challenge?
I was trying to think of parallels with other conditions, where fluctuations in symptoms are related to identifiable factors.
I remember back to my teens and early adult life having what seemed like random mood swings, with every now and then sinking into a black cloud of weepy depression for a few days. I somehow didn't make the association with my menstrual cycle. Once I found out about premenstrual syndrome, it all suddenly made sense. It wasn't me being pathetic and psychologically inadequate, it was caused by my hormones. Once you know the trigger, it makes management possible.
Similarly with migraines. I have gradually recognised triggers, so have more chance of avoiding them.