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The High Intensity Cardio Debate

Studies break down HIIT effectiveness.

Interval training for muscle growth

The Heart of the Mattter

To be clear: There are Two forms of cardiovascular exercise. Aerobic training includes longer activities such as jogging, swimming, and cycling, and occurs at relatively low intensities (60–70% of your maximum heart rate, or approximately 120–150 beats per minute). Anaerobic training, meanwhile, includes lifting weights, sprinting, martial-arts training, and any other exercise characterized by short intervals of hard work followed by light activity or complete rest. (Here, you’re raising your heart rate above 150 beats per minute and sometimes as high as 90% of your maximum heart rate.)

Aerobic workouts are fueled by oxygen, which provides energy for a steady rate of activity but no explosive power. Anaerobic training runs on phosphocreatine (hence the popularity of the supplement creatine for refueling it) and carbohydrate, which can supply quick energy for intense activity but peters out fast. Aerobic workouts tend to be long (up to an hour or more), while anaerobic sessions can last just a few minutes. Because most sports and exercise habits fall under the umbrella of anaerobic, many believe that anaerobic cardio workouts are all most people need to be in shape.

The truth, however, is more complicated. While the aerobic system is less active during exercise lasting less than 60 seconds, it never shuts off completely, and its involvement increases rapidly as the activity goes on. Even during highly intense work lasting a minute (such as punching a heavy bag), the aerobic system provides nearly 50% of the total energy. According to Jamieson, after about 90 seconds, the aerobic system provides the majority of energy—even if you’re still working intensely.

In other words, the fitter your aerobic system, the better your anaerobic performance will be. “Lower-intensity work develops the vascular system—the blood vessels and supply network that deliver oxygen to the working muscles,” says Jamieson. “It improves recovery and work capacity, and helps you oxidize more fat. We can do more work without overtraining when we develop the aerobic system.”

Want to know why you breathe hard after running down the block to catch a bus? That’s your body trying to replace oxygen to refuel the aerobic system—even though your bus sprint itself was technically an anaerobic burst.

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