Gittit,
One could put it in context and correlate it with all the other studies that the Vitamin D Council accumulated on studies that showed improved performance with better levels of Vitamin D.
Girls who spend more time outside also are likely less depressed, which could be because they are getting more Vitamin D and they are also more active.
It does all fit together, hard to split apart, but muscles may simply function better with better levels of Vitamin D...
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