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.

  • 3
    Does the runner get food and water?
    – Carey Gregory
    Commented Nov 14, 2017 at 23:36
  • I wasn't really expecting dehydration to play a factor before something else did, but the answer would be no. Let's assume 20 degrees C and standard pressure, though. Commented Nov 15, 2017 at 23:05
  • Mitochondria can alternatively use ketones to synthesize ATP, which will not produce lactic acid. Additionally, breaking down stored fats for energy conversion releases water. Perhaps this can be used to gain an advantage over the pursuer. Commented May 12, 2018 at 13:58

3 Answers 3


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.

  • 1
    Here is a paper about the physiology of an ultramarathoner: ncbi.nlm.nih.gov/pubmed/23393784 This one only ran for 6 hours, some run for 24 or 48! Commented Nov 15, 2017 at 14:21
  • So basically, it's the anaerobic processes of running that will lead to collapse, and not the aerobic processes? Your muscles eventually die in the same way they would after perpetually lifting weights? Commented Nov 15, 2017 at 23:09
  • It's not so much the running, as the muscles requiring more oxygen than the circulatory system can deliver, which leads them to convert to an anaerobic process for energy. That in turn eventually leads to an excess amount of lactic acid that interferes with that anaerobic process.
    – BillDOe
    Commented Nov 16, 2017 at 20:48

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.


  • Sorry if I wasn't clear: we're assuming that the runner can't afford to pace himself. He must continually go as fast as he can because he's, say, being chased by a pursuer. Commented Nov 17, 2017 at 1:51
  • @ArbitraryRenaissance Yeah, you need to provide necessary details with hypothetical questions. So with that constraint it becomes a simple question that could have been answered in the fitness exchange.
    – Carey Gregory
    Commented Nov 17, 2017 at 5:11
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    Other than invoking lactic acid by referring to the incorrect accepted answer, this is the best answer. The alternative to pacing yourself is getting caught by whoever is chasing you, not organ failure or death.
    – De Novo
    Commented Jan 6, 2020 at 4:20
  • @DeNovo Thanks! Fixed.
    – Carey Gregory
    Commented Jan 6, 2020 at 5:10

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.

  • 1
    Thank you for your answer! However, we are not a discussion forum, and expect our answers to be backed up with verifiable resources, not simply assumptions about what might happen.
    – JohnP
    Commented Jan 6, 2020 at 14:08

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