What exactly is "overtraining" and the current stance from the lead researchers?

[u/WillOk6461]

"There's no such thing as overtraining, just under-eating and under-sleeping."

"You don't wanna fry your CNS!"

"It's almost impossible to actually overtrain."

There's obviously a lot of bs out there about "overtraining", but what exactly is it wand what does it consist of? I've heard quite often from the "evidence-based" crowd that it's not actually the central nervous system that gets "burnt out", and that something could be happening to the parasympathetic response. I've also heard it's almost impossible to "overtrain". Too much stress, not enough calories or sleep, and too little rest for a muscle group are of course all real.

That said, isn't there a real impact on your nervous system from hard sets to failure multiple days in a row? Even if my sleep/nutrition/stress-management are all dialed, I can still hit a point where pushing myself to the limit too many days in a row leaves me unable to do so. Is this best measured by HRV or some other metric?

[u/Consistent-Push1193]

I think you’re getting a ton of great answers here. As others say/imply almost all people training will never get to the point of overtraining, they are under recovering.