It's also consistent with muscles whose mitochondria are defective, which produce abnormal amounts of reactive oxygen species due to an uncoupled electron transport chain, whose fibre type has shifted to rapidly fatiguable glycolytic predominant as a compensation, and which results in increased muscle cell necrosis/repair when over-challenged.
In the scenario where pwME say their muscles can't perform, but that if absolutely necessary they can push through at cost of much worse PEM, I think what's happening might be relying on two things: the muscle fibres that are still relatively uncompromised but that will now be further added to the damaged quota; using inefficient (rescue?) metabolic pathways that will produce more and more ROS.
If those things are true, then a pwME won't know that's the case (and indeed we don't know this mechanism is correct scientifically) but the pwME will learn the symptoms, including the associated lag and will try and judge what they can and can't do in order to live their life to the fullest with the least symptoms. And it's only a best-guess with inherent variability. Eg I got it wrong on Saturday and paid for it that night and Sunday.
To throw in fatiguability, there is a specific 'judder/three tugs' that eg my legs sometimes do when they are 'done' - it was particularly obvious when I once went to the gym and a personal trainer was stretching them out further into the session, and it freaked her out but I just calmly said 'yeah that's me/cfs'
In my arms with fatiguability I just become unable to lift them straight after doign something I shouldn't have and can't lift a drink to my mouth as my arm shakes and then just can't get there. They are so exhausted I have to put pillows under them to support then about 3-4hrs later they slowly become usable.
And after these, but also just days and times when I've probably been doing more but in that mindset where I'm not noticing, then I will get visible muscle 'twitches'. These aren't predictable to certain muscles but might be on the back of one arm or one of my thighs and I can sit there and watch like it's a little alien.
It doesn't quite happen regularly enough that I've assumed it is 'something else', and isn't a deficiency thing as it definitely seems to relate to exhaustion and usage of muscles.
A total random one (but closer to the 'normal person' analogy as it is twitching eye) I once had an eyebrow stuck raised for 2 weeks, but it felt to me like the eyelid itself wouldn't stop twitching, not that you could see
that in the mirror. I was utterly exhausted at that point from general environment and commitments. But recent eye tests have shown that my eye muscles get tired quickly and at different rates etc. leading to unfortunate symptoms when tired.
Of course the other main thing with my muscles is exactly the same ache as I'd have got if I'd done extreme exercise when I was well, but even deeper and more than I could have achieved then, just after doing something really small. Like these days having my legs down for an hour, in the past driving for 45mins. It sounds ridiculous but it stops you walking and standing-balance easily sometimes (I think I also tend to have accomanpying stiff ankles and sore around feet, but you can only identify easily the thing hurting most) because not the pain but they can't stretch etc.