"Mysterious"

"Overtraining syndrome"

I don't see why it's so mysterious, fatigue accumulates until something tips you over the limit

Do we know the biological mechanism to cause OTS, though? That would open up avenues for treatment and more precise prevention besides "rest more" (not that that's a bad preventative).

Yes, a lot of them. Non exaustive list:

- bone microfractures

- nervous system depletion

- tendon "microlesions" (not so sure about this term though)

- immune system blunting due to stress

- raised cortisol levels

- (possible) nutritional deficiencies given the increased exertion (or just metabolic bottlenecks)

What's fatigue? Is there a fatigue molecule?

I'd say ask ChatGPT because the list is loooong

Lactic acid?