After several miles of running and sprinting in intervals, the runner’s muscles ache and feel fatigued. which is the best explanation for what has occurred?

After miles of running and sprinting in intervals, the runner's muscle ache and feel fatigue due to anaerobic respiration taken place in the muscle cells.Anaerobic respiration in animal cell gives Carbon Dioxide and lactic acid as products. Due to the formation of lactic acid after running, muscles feel fatigue and ache.
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When your muscles don't have enough oxygen for the exercise they're doing they have to break down carbohydrates into something called lactic acid. Lactic acid also increase the acidity of the muscle cells, so it makes your muscles sore. It may seem counterproductive for your body to produce something that would make your muscles not function as well, but without the lactic acid permanent damage could could happen because your muscles wouldn't be getting the oxygen they need.

The answer would be dogs.