High-intensity interval training (HIIT) is an enhanced form of interval training, an exercise strategy alternating periods of short intense anaerobic exercise with less-intense recovery periods. HIIT is a form of cardiovascular exercise. Usual HIIT sessions may vary from 4–30 minutes. These short, intense workouts provide improved athletic capacity and condition, improved glucose metabolism, and improved fat burning. HIIT workouts are slightly more complex. Perform your activity as hard as you can (90 to 100 percent of maximal capacity) for a brief, set time period (usually two minutes or less), then back off for a predetermined rest interval (usually three minutes or less), and repeat the cycle four times or more. HIIT, by contrast, is anaerobic: The work intervals don’t rely exclusively on oxygen, and are fueled mostly by stored carbohydrates. (Counterintuitively, HIIT makes you breathe harder, and burns more fat, than steady-state cardio. More on that in a moment.)
Low Intensity Steady State (LISS). It is the term coined for low intensity cardio such as walking or jogging without putting much stress on the body. Steady-state cardio workouts are as simple as they come. Perform your activity at a steady, challenging-but-manageable pace (60 to 70 percent of maximal capacity) for 20 minutes or more, aiming for a heart rate of 120 to 150 beats per minute. Steady-state cardio is aerobic: It requires oxygen and is fueled mostly by stored fat.
Both HITT and LISS are needed. Knowing which one to do depends on your goals and heart rate. Experienced exercisers seeking general fitness should take this simple test: Sit quietly, find your pulse, and count your heartbeats for one minute. If your resting heart rate is below 60, feel free to experiment with HIIT. If it’s above 65, you need steady-state cardio training. Drop other cardio activities and follow the recommendations for beginners.
Once your aerobic system is up to snuff, dial back the steady-state training and switch to HIIT. Make sure, however, that your resting heart rate stays below 65 beats per minute. If it shoots above 65, return to aerobic work and limit HIIT.
When you exercise at a high intensity (while interval training, for example), your heart often beats so fast that the left ventricle — which stores oxygenated blood momentarily before pumping it out — can’t refill completely between contractions. At a slightly lower intensity (and, thus, a lower heart rate), the left ventricle fills completely before it contracts, which causes it to grow in capacity — and thus pump more blood with each contraction — over time. This triggers your heart rate to drop substantially, both at rest and during exercise.
That’s a good thing. A lower heart rate isn’t just an indication of a healthy and high-functioning cardiovascular system. It’s also indicative of high “parasympathetic tone” in the nervous system — an enhanced ability to relax, focus, and recover from stress, including intense exercise.