A more specific form of my question would be, "Barring mental barriers, what will lead to collapse after extended cardiovascular exercise?" I was considering asking this in the worldbuilding SE, but I figured that I would get more detailed or realistic answers here.
When you do cardiovascular exercise for an extended period of time, the typical process (pretty much as roughly speaking as possible) is:
- Do the exercise.
- Make the conscious decision to stop doing the exercise once you're sufficiently exhausted or once you've exercised for a sufficiently long time.
You can make the argument that there are some exercise routines that swap step 2 for "Get to the point where you can't do the exercise anymore and stop there," in which case the decision to stop wouldn't exactly be a conscious decision. I suspect, however, that even with an exercise routine like that, if someone pointed a gun to your head and told you to keep exercising, then you could probably keep going. Those mental barriers are just too strong: at some point, your brain will say "No more!" before your body does and you will stop prematurely.
In stories that I write, I occasionally run into the scenario of characters in a "run for your life" sort of situation, where something is perpetually on their tail that they need to escape from. They can't slow down or stop: they have to go as fast as they can for as long as they can in order to escape danger. In this sense, it's like the hypothetical I described: a gun is basically being pointed to their heads and they're being forced to keep running. At this point, mental barriers are thrown out of the window in order to prevent premature death, so they're not going to get in the way.
But something will eventually. At some point, the hero will collapse, and I'm interested in knowing what's going on in the body when that point finally happens. Do the muscles cease their functionality due to a deficiency of ATP? Do you briefly black out or pass out due to lack of oxygen (or rather, a surplus of CO2) being distributed through the body? Or is it still going to be a mental barrier that inevitably causes collapse (in other words, will something kick in the brain before other parts of your body fail that will force you to stop running regardless of external stimuli)?
We can assume that prior to the extended sprint, the character is well-rested and hydrated. If different cardiovascular exercises are expected to yield different results, we can limit things to just running, since that's what I'm primarily interested in. Also, since many people can jog pretty much indefinitely provided the jog is slow enough, we can assume that the character is continuously exercising at the peak of his or her capabilities.