It's fairly easy, actually. Strengthening your muscles in life necessarily strengthens and thickens your bones. They're the support structure, they have to be reinforced to accomodate the greater loads and stresses stronger muscles put on your skeleton.
Modern humans (assuming developed nations) have more pooling at the extremes than ancient peoples; more sedentary and weak people, but also more highly-active and strong people. People in the middle are probably weaker because they're not engaged in manual labour every day.
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