February 09, 2015 2 min read

You’ve probably seen the colorful heart rate posters at gyms, but how do you really know if you’re in the fat-burning zone or cardio zone. And what is your body doing when you’re there?

Heart rate monitors -- either the kind you strap on, or the sensors on cardio equipment -- can be a great tool for keeping you honest about the intensity of your workout and therefore achieving your goals. Before you begin heart rate training, you must determine your individual targets. An online calculator can help get you started. 


Here’s what happens in your body while you’re working up a sweat:

60-70 percent of your maximum heart rate

It feels like you’re working at a comfortable pace that can be maintained. You’re not short of breath and can carry on a conversation. In this zone, you’re building endurance. The body is using fat as fuel, strengthening the cardiovascular system and improving muscles’ ability to use oxygen.

70-80 percent MHR 

When your breath starts getting deeper, you can say only a few words at a time, and work up a light sweat, you’ve entered the cardio zone. Just as it sounds, you’re strengthening your cardiovascular system. Your body is training to move oxygen to muscles and carbon dioxide away, leaving you stronger and fitter. This is also where muscles really start strengthening.

80-90 percent MHR 

This range takes you into the anaerobic zone. It’s exhaustive. Your muscles are working to fatigue and breathing is heavy. This intensity is only sustained 30 seconds to a couple minutes at a time (though is often repeated after a recovery period in a workout). In this zone, your body is breaking down glycogen -- or sugar -- for quick energy.This boosts your body’s lactic acid levels, which turns fat into fuel and develops muscles, improving performance over time.

90-100 percent MHR 

This is not the zone for beginners to train in. Here, in the “red line zone,” you develop speed and gain muscle mass while primarily fueling your body with glycogen rather than oxygen. Since your body is using reserved energy, this intensity cannot be maintained long. Short bursts of training at this intensity develops your fast twitch muscles fibers, which improves speed. In this zone, your body becomes more efficient and your heart stronger.

To begin working in the heart zones, try adding intervals of higher intensity work than you’re used to. For instance, if you are a walker, add 60 seconds of jogging then recover, allowing your heart rate to drop back to the 70 percent MHR. Then add another jogging interval. If you have more energy, consider adding an all-out sprint, going as fast as you can, then recover again.

Pushing your limits can help you build endurance and help your body run efficiently. Always check with a doctor before beginning a new exercise program.

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