SHOULD YOU USE A HEART RATE MONITOR? These days there’s a profusion of technology available that tracks your health, from counting steps to counting calories to letting you know how well you slept. But the most valuable fitness device is a heart monitor, which tells you how many times a minute your heart beats. I have a Fitbit Charge 2 wristband that continually monitors my heart rate. Most of the time I don’t much care what it reads, but when I jog I refer to it all the time to make sure I’m exercising at the right intensity. Using it,...