The whole causation / correlation thing is a big part of this, I think. Many will likely read this and think that there's brain power in your legs, when in fact

The whole causation / correlation thing is a big part of this, I think. Many will likely read this and think that there's brain power in your legs, when in fact it's most likely saying that leg health -> physical activity in general -> brain health. I think the writing about the study should be more clear about this, as I'm sure the original work is.