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Of course, this is correlational. There could be a 3rd variable, like "overall being healthy" which causes both.

I was surprised to find this article because I believed it was an established fact, causation mechanism (hypertrophy due to inactivity) included.

Maybe I have mistaken it for other effects, like some people my age awakening three times a night. More good news, anyway.

they claim to have controlled for that. you may share my skepticism of the accuracy of such a claim.