At what point does exertion cause Bad Things?

Question

(This is closely related to this question, but I didn't want to hijack it because it was asked by someone else...)

Let's say that I have some "technomagical" device that triggers "supercharged" metabolism, as in this related question. How much power and/or energy could this process produce before doing serious harm to the body producing it due to either exhausting something that can't be quickly replaced or the body's inability to process the generated waste products? (To clarify, when I say "metabolism", I'm talking about the cellular-level processes by which the body produces energy to generate heat or allow muscles to do work, i.e. the ATP cycle. I'm not interested in higher-level processes such as digestion, unless those would be the limiting factor.)

Yes, I realize there are a lot of factors in play. Some sort of chart or guidelines for making an educated guess at how time and energy relate would be most helpful, but I am especially interested in very short terms, e.g. at what level would a few seconds to a minute of this "supercharged metabolism" cause damage from which a person couldn't recover just by resting immediately afterwards? (Information such as whether they would need to very quickly drink a liter of water to prevent follow-on problems from dehydration is also interesting/useful.) Note that I don't actually care what factor specifically (except possibly, as noted, to the extent that the person would need to know in order to take action afterwards) causes the problem.

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While a general answer would be preferred, perhaps a more concrete example will help. In my other question, LSerni postulates a peak metabolic output of ~24kW (maybe more). If we hypothesize a body that is metabolizing at this rate and ignore the heat (assume some hand-waving mechanism of dealing with that and only that), how long could someone (assume a body mass of 75-100kg) sustain this level of output before something else causes Very Bad Things to happen?

Answer 1

It's complicated

Humans evolved to exert a lot of energy over a long period of time. In a sprint, a cheetah can run massively faster than a human. But I've never seen a cheetah run a marathon. With the increasingly popularity of ultramarathons in recent decades, scientists have started to examine the limits of human endurance. This story about a man who runs the equivalent of a marathon every day gives a good overview. The ATP cycle isn't necessarily the limiting factor here.

If you want the science, check out this abstract from, appropriately enough, Science:

The limits on maximum sustained energy expenditure are unclear but are of interest because they constrain reproduction, thermoregulation, and physical activity. Here, we show that sustained expenditure in humans, measured as maximum sustained metabolic scope (SusMS), is a function of event duration. We compiled measurements of total energy expenditure (TEE) and basal metabolic rate (BMR) from human endurance events and added new data from adults running ~250 km/week for 20 weeks in a transcontinental race. For events lasting 0.5 to 250+ days, SusMS decreases curvilinearly with event duration, plateauing below 3× BMR. This relationship differs from that of shorter events (e.g., marathons). Incorporating data from overfeeding studies, we find evidence for an alimentary energy supply limit in humans of ~2.5× BMR; greater expenditure requires drawing down the body’s energy stores. Transcontinental race data suggest that humans can partially reduce TEE during long events to extend endurance.

There are other factors at play when pushing the limits of the human body, from breathing muscles to "the complex interplay between cerebral adaptations/alterations and hormonal influences" (source). Every human body is going to react differently to being pushed beyond its normal limits. I suggest using a little hand waving to set whatever limits make the most sense for your story.