I recently got one of those fancy smartwatches. You know the kind. It tracks my steps, tells me I’m sleeping like a teenager (a blatant lie), and occasionally congratulates me for standing up. It’s like having a tiny, optimistic fitness coach strapped to my wrist. But the other day, as it buzzed to tell me my heart rate was “excellent for my age bracket,” a thought crossed my mind: This little gizmo knows more about…