What makes the body/muscles tired out?

I’m just curious of a biology scenario. Anyway, when someone exercise what makes the muscle or body lose energy or quit? Like is there some kind of compound, body/neurology chemical, or substances behind it? Like for me example when I run or do exercises I feel like I can do way more but my muscle can’t go on no... show more I’m just curious of a biology scenario. Anyway, when someone exercise what makes the muscle or body lose energy or quit? Like is there some kind of compound, body/neurology chemical, or substances behind it? Like for me example when I run or do exercises I feel like I can do way more but my muscle can’t go on no more since — no expert here — I guess that is my limit to my muscle capacity?

Thanks!
3 answers 3