system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't I run at full speed for long durations?

I understand starting to sprint for a 5km run is not going to be efficient, but efficiency aside, why is there not a smooth curve of speed over time when you start running at full speed and you just get slower but you can run like that 'forever'?

For some reason I can hardly sprint for 50 meters and HAVE to stop running then. Maybe if a lion was chasing me I can do a bit better, but with a lion that's always just a bit slower than I am, I doubt I can sprint for even a kilometre if I really have to. What is the maximum distance/time that humans can sprint (without pacing)? Why is there a limit and can't we sprint forever albeit at a continuously reduced pace?

It seems like jogging may be the activity that mammals can do for long durations. Is sprinting something that is inherently 'damaging', like running nitro through your car all the time?
gpt
All muscle contractions require ATP.  When you sprint as hard as you can, or do any other maximum effort movement, you are depleting the current pool of ATP in your muscle cell very quickly, and far faster than you can replenish it.  As soon as the level of ATP in your cells begins to drop quickly, the other 2 sources of ATP (glycolysis and oxidative phosphorylation) will try to maintain it.  With enough of the proper training you may be able to smooth out the curve somewhat, in that you might be able to sprint all out and transition to a more sustainable pace without actually stopping.  

The transition between metabolic systems (ATP Phosphagen - Glycolysis - Oxidative Phosphorylation) is a continuum with overlapping energy sources at every intensity, so it is hard to say explicitly "what is the maximum time that humans can sprint" but power-duration curves for humans are sigmoidal with a small plateau at high intensity and low duration that represents sprinting, a steep drop of in the 3-5 minute range (glycolysis) and a then a long shallow plateau out to the longest reasonable durations.  Looking at those power duration curves you usually see that in untrained individuals the longest duration they can sustain using available ATP is <10 seconds, in average athletes it is closer to 20-30 seconds, and in world class athletes whose training is entirely based on extending their sprint endurance it reaches about 60 seconds.

Sprinting isn't exactly inherently damaging, it's unlikely that our inability to sprint is a protective mechanism, it's more likely that sprinting and recovering is energetically costly an inefficient, and it was never evolutionarily beneficial enough to develop that ability.  You might think there was a lot of evolutionary pressure to be able to sprint for long distances to avoid predators, but remember the old joke - "You don't have to to be able to outrun a lion, you just have to outrun the slowest person in your group".