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When we exercise for a relatively extended period of time, lactic acid is produced as a product of anaerobic respiration, which itself inhibits further muscle contractions. However, why did we develop this? In a chase, for example, endurance could be key, and inhibiting muscles doesn't seem like too much of a favorable characteristic.

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    $\begingroup$ In an evolutionary context "why questions" are very hard to answer. $\endgroup$
    – Chris
    Commented Jun 10 at 7:29
  • $\begingroup$ Well, we are endurance hunters, running down any prey over long distances - the opposite of a cheeter's strategy in the hunt. We can also sprint, but for short distances. Everything is a tradeoff. $\endgroup$ Commented Jun 10 at 7:32
  • $\begingroup$ But it's not like we're the only ones with the lactic acid system, which just makes it weirder; why is this such an advantageous quality? $\endgroup$ Commented Jun 10 at 7:53
  • $\begingroup$ Oops, I meant of course "cheetah". I'm not sure what answer you're looking for here. As you point out, we're at no particular disadvantage because of it as the rest of the animal ecosystem has the same thing going on. The origins go way back to single celled ancestors (possibly), and it's not as if we can give definitive answers about the exact evolutionary path right at the beginning there. Are you looking for a biochemistry/thermodynamics answer that points out the pathways and their origins at a primitive level? What's your expectation and current understanding of this? $\endgroup$ Commented Jun 10 at 8:24
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    $\begingroup$ To a man with a hammer, everything looks like a nail. But the purpose of the anaerobic production of lactic acid (and indeed of ethanol by yeast) is not to inhibit exercise, it is to regenerate NAD+ from the NADH produced in glycolysis. This is biochemistry not exercise physiology and you can read about it in my answer to the following question: biology.stackexchange.com/questions/54186/…. Incidentally, as a new user, welcome to SE Biology, but please finish reading the Tour to learn about this site. $\endgroup$
    – David
    Commented Jun 10 at 9:11

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