...Or actually, just neutral, with respect to injury. Here's the USATF page linking to the study and here's the NYT article discussing the results.
I hate stretching and I'm very inflexible, so I like these kinds of results (which is why I gleefully posted similar findings before). But note a) the endpoint is injury, not performance: did the stretching runners go faster and/or longer? And b) If you're more flexible, do you perform better or get injured less? Which is what I think we care about; really stretching for a certain number of weeks is just a proxy endpoint assumed to correlate with flexibility.
I would like nothing better than for stretching and for that matter flexibility to have nothing to do with performance, but so far as I know these are still open questions. There may be studies that an exercise physiologist or related specialist could tell us. Hey, my Wyoming geography request got answered quick-like so why not ask about this too?
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