If you mean why you get sore more easily when you haven’t worked out for a while then it’s because your tolerance to lactic acid has decreased. It’s one of the longer term effects of exercise, but it also works the other way round, if you haven’t worked out, you’ll become less resistant. This will also cause you to fatigue more easily but will allow the lactic acid to damage the muscles, which is part of what makes you sore.
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