'Zone Zero' Cardio Isn't What It Sounds Like

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Cardio zones aren't real—not in the way your fitness app makes them sound, anyway. Yes, heart rate zones are a way to describe how hard you're working during a cardio workout, like running or cycling. But the cardio zones everyone's always buzzing about are just a way of bucketing exercise intensity into digestible categories. They're a useful shorthand, but there are no physiological lines in the sand where your body suddenly switches from "zone 2" to "zone 3." It's a spectrum...