In this era of fitness trackers, we have easy access to our heart rate at any given moment. Every so often, a number catches your eye as it flashes on your Garmin or Apple Watch while you're sitting ...
But what is zone 2 cardio, exactly? How can you make sure you're doing it right? And is it really so bad if your heart rate drifts into zone 3? What is zone 2 training? Zone 2 training is cardio ...