“The best health metric is the one that changes what you do in a way that improves your health,” said Dr. Ami Bhatt of the American College of Cardiology. | Yagi Studio/Getty Images As I am typing this, a device rests on my wrist that purports to unlock a trove of real-time information about my body’s performance. I can click a button and check my heart rate and review how much it’s varied over the course of the day. It can tell me how many steps I’ve taken, how many minutes I’ve been “active”