I've been able to make a lot of observations in the past year because I went from mild to severe state in only 3 monthes then slowly back to moderate state ( this was caused by targeted aerobic exercise, before I knew better than to do it ) I learned how to completely avoid major crashes ( typically recognisable because I lose the ability to eat for a while ), and by christmas I got to the point of quite good stability, as such I find it easy to notice any changes from that. Heres what I noticed - there is the main type of PEM that you typically hear about, thats the one that causes muscle aches, slight headaches, loss of motivation and or concentration / memory, tends to make you particularly sleepy, can also get sweating or increased orthostatic intolerance etc, this happens so frequently that you sort of get used to it and typically its not highly disruptive, this form tends to dissipate after 2 to 5 days. The second type feels very different and its more subtle, if you didn't know what to look for you might confuse it as being part of the normal PEM or fatigue, this one seems to accrue over a longer period and seems to be linked to at least a medium term worsening of the disease. I started to notice it recently because I tried to do this thing called NoFap which for older generations it means no masturbation, its based on a psychological theory of how modern lifestyle is at odds with evolved stimuli responses that were based on tribal hunter gatherer era humanity, so this changed my energy expenditure from evenly spread out to asymmetrical, I started going for say 5 to 7 days without it which would cause a lot of recovery but then I would do on average 1 hour per day for 3 days in a row due to something they term the 'chaser effect', first occasion I did this I got quite a lot of PEM after the 3rd time but recovered reasonably well. Did the same exact thing a week later when I felt like it but this time after the 3rd day I noticed something was different, there seemed to be some sort of underlying damage to the body which was evident alongside the normal PEM, its hard to explain but its sort of a feeling like an inability to recover. I forgot the exact sequence of events but what happened after that was at some point I was feeling generally normal, no PEM or something, functioning well, but then I noticed my fingers suddenly started to lock up, this was a symptom I used to get really badly around 6 monthes ago, could barely use a keyboard and mouse at times, but its since faded into almost non existence, so it was quite odd for it to suddenly flare up like that and I realised it was a warning sign, next morning I decided to go into rest mode aka move all my stuff back to the house from my room in the garden, which was good because when I did that I noticed my walking speed was very slow which sometimes happens, I walk at like 1/2 speed and it feels like a lot of effort. Started to feel better after a couple of days of rest though, but then I did something inadvisable that took a bit longer than I expected, although I thought I could probably manage it, again got moderate PEM, but theres an even stronger sense of this secondary PEM or underlying damage, most notably because on the first day of recovery I was clicking my mouse rapidly for 3 hours or something which is not unusual, then what quickly happened was my forearm muscles started to sort of become sensitive as if hit by a hammer all the way along, this has happened before though when I was less healthy, at the time I thought it was permanent as it took weeks / monthes to return to normal, the arm was initially almost unusable and there seemed to be nerve control problems. I'm on the 3rd day of recovery now and the time its taking is out of line with the amount of PEM that I got, symptoms also differing slightly in general, hard to describe specifically how though apart from the muscle thing which is really obvious. A prominent thing though is a sort of waves of feverish heat that are moving around the skin, particularly the legs, I get temperature spikes / sweating normally but this feels different in nature, it normally feels more natural. So basically there is a sort of secondary subsurface form of PEM which is harder to notice, but its very important in how the body is going to react to exertion, it can be tracked and compensated for though you just have to be more careful than I have been.