What will happen if you keep running? What will eventually cause you to stop?

Question

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:

1. Do the exercise.
2. 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.

Answer 1

It's been my understanding that a buildup of lactic acid (aka lactate) eventually causes the muscles to stop being able to process glucose. This article from Scientific American describes the processes the muscles use to convert to an anaerobic process as muscle performance outpaces oxygen supplies. The pertinent passage:

A side effect of high lactate levels is an increase in the acidity of the muscle cells, along with disruptions of other metabolites. The same metabolic pathways that permit the breakdown of glucose to energy perform poorly in this acidic environment.

Answer 2

If the runner can't drink then dehydration will be the first thing that takes him out. Water is the one essential thing he's going to lose rapidly through sweat, urine and respiration, and as he loses it he's going to lose electrolytes along with it.

The end result will be disabling muscle cramps, weakness, and exhaustion. If he somehow continues on despite these things, cardiac arrhythmias may follow due to electrolyte imbalances.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1318513/

Answer 3

Here's an unglamorous answer to the top level question.

I have asthma, and even when I'm very motivated (hourly bus nearing a bus stop, FedEx truck about to do last pick-up), in cold weather I literally run out of breath. It may take me a few blocks, depending on how cold, or if my lungs are already irritated (say an infection), but the bronchi squeeze tighter. It is a sharp diffuse pain, and I just can't breathe in enough to continue.

Your hero doesn't have asthma and is well-conditioned, but if they have to run beyond their cardiovascular limit, I assume something like that would still happen. And of course there's running out of water and electrolytes (since hard breathing, sweat makes you lose both, even when you're well-conditioned) and both are necessary for cellular function.