I live in a ski town. Every day, particularly in winter, provides opportunity for expending effort. I don't have kids, but my friends do, and a decent amount of my efforts at least amount to teaching them how to ski and shoe and whatnot (and generally mucking around).

Given two tribes, one where their elderly are contained at home, and another where they're wandering and learning and teaching, I guess I could see a hypothetical fitness function.

I can't imagine it's too strong, though. A simpler, more-mechanical explanation may make more sense: when you stress bones, you set off all kinds of repair processes, many of which have systemic effects.