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.)