After 30, muscle drops 3–8% a decade

At fifty-five he fell on the stairs last year. Nothing broke, but he needed someone's hand to get up. Muscle mass drops 3–8% per decade after thirty and faster after sixty, which takes balance, metabolism, and bone density with it — and eventually the strength to live alone. He started bodyweight squats at the park near his house. Ten was all he could do. Six months later the stairs were his again.