Can tracking heart rate variability make you fitter?

THE human heart beats like a metronome, right? All evenly spaced thumps and regular rhythms? Wrong.

When you're at your healthiest, your heart is actually more like your six-Scotches-deep uncle whaling on the family drum set: The... beats... are spaced... unevenly.

It's only when your body is stressed - either for a short-term reason (like a bad night of sleep, a cold, or an insane deadline) or a long-term one (like you're a smoker, or have a chronically stressful job) - that the beats are more sober and steady. (We know: We were surprised, too.)

This fluctuation is called heart rate variability, or HRV for short. And some experts say that you can give yourself an athletic edge by tracking and responding to changes in your HRV - especially now that wearable devices such as the Apple Watch, the Garmin Fenix, and Morpheus (an app-…