Why is parenting so physically exhausting?

I work out, I'm in decent shape, but being a dad is destroying my body in new ways. My back hurts from picking up toddlers, my knees are shot from crawling around, and I can't remember the last time I felt rested. And it's not even like one hard thing, it's 8 hours of constant low level physical activity with no actual break. Just venting here because my wife said I sound like her dad complaining about his back and now I'm having an identity crisis about being old.