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edit: And another thing I want to add. I don't know about sprinting more than once a week-- even if that is the only HIIT you do.

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why's that?

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I don't have a good reason other than I've been told that over the years-- that sprints should be held to a small part of overall running training. I think part of it is risk of injury, and the other part (wildly speculating) could be akin to why plyos should be incorporated so judiciously-- you can't train explosiveness when fatigue is present, and such training is much harder on the system than one may feel. But then again. I might just be repeating the equivalent of a wive's tale.