In the fitness world, the word “miracle” gets thrown around like a two-pound dumbbell. But when it comes to high-intensity interval training (HIIT), a very short workout, the benefits you’ve heard about are both legitimate and—we’ll say it—miraculous.
HIIT is a combination of brief, very-high intensity bursts of cardio exercise followed by equal or longer periods of rest. Think 30 seconds to a minute of sprinting, followed by a minute or two of walking or slow jogging. Repeat this cycle for just 10 minutes, and you’ll complete a HIIT workout.
“We now have more than 10 years of data showing HIIT yields pretty much the exact same health and fitness benefits as long-term aerobic exercise, and in some groups or populations, it works better than traditional aerobic exercise,” says Todd Astorino, a professor of kinesiology at California State University, San Marcos, who has published more than a dozen study papers on HIIT.
Whether your goal is to improve your fitness, lower your risk for cardiovascular disease, lose weight, strengthen skeletal muscle or help get your blood sugar under control, a few minutes of HIIT seem to be as effective as much longer periods of moderate-paced running, cycling, swimming or other forms of traditional cardio. For well-trained athletes, HIIT may be the best way to elevate your physical performance.
One small study of healthy but sedentary people found just one minute total of HIIT performed three days a week for six weeks was enough to significantly improve blood sugar scores and aerobic capacity, a measure of physical fitness. The study participants completed 10- to 20-second bouts of “all-out” cycling on a stationary bike, each broken up by a couple minutes of rest. The total workout time, start to finish, was 10 minutes.
Other research finds that HIIT may outperform traditional cardio when it comes to fat loss. A HIIT-induced surge in your body’s levels of growth hormones and other organic compounds “can increase fat burning and energy expenditure for hours after exercise,” says study author Stephen Boutcher, an associate professor of medical sciences at the University of New South Wales in Australia.
It doesn't just work for the young, fit and healthy. Among people with heart disease, HIIT improves cardiorespiratory fitness nearly twice as much as longer stretches of moderate-intensity running, cycling or other aerobic exercises, one review study concluded.
How can HIIT do so much good in so little time? During very intense exercise, “the heart cannot pump enough blood to satisfy all the muscles,” says Ulrik Wisløff, a HIIT researcher and head of the cardiac exercise research group at the Norwegian School of Science and Technology. This lacking oxygen delivery to the muscles starts a “cascade of molecular responses in most organs of the body” that produces a greater training response than more leisurely bouts of exercise, he says.
Exercise of any intensity switches on genes that increase the growth of mitochondria—the power generators of the cells—and triggers all of the other beneficial biological changes linked with physical fitness, says Astorino. “But to activate these genes with traditional cardiovascular exercises, you have to do fairly large or long bouts,” he says. With HIIT, it appears that even very short bouts of training can switch on those genes, so it's an efficient workout.
HIIT can work for a wide range of people. But how you should practice it depends on your fitness level.
The key to HIIT is pushing your heart rate up above 80% of its maximum, Astorino says. “Subtract your age from 220 to estimate your maximal heart rate,” he says. (A heart rate monitor can provide an accurate assessment. But if you’re really sucking wind after pushing yourself, you've probably hit your target, Astorino says.)
If you’re fit, try sprint interval training. After walking or slow jogging for a few minutes to warm up, sprint as hard as you can for 30 seconds, then recover for four minutes by walking or jogging slowly. Complete four to six sets of this sprinting-recovery program. (For an even faster version, keep the warmup, then complete three sets of 20-second sprints, each separated by two minutes of recovery, Astorino says.)
If you’re overweight or obese and you haven’t exercised in months, sprinting isn’t necessary (or safe for your joints). Instead, 30 seconds to four minutes of brisk walking on an inclined treadmill or hill should be enough to push your heart rate up into the HIIT zone, Astorino says.
You can also practice these programs with a stationary bike, rowing machine or in the pool. Any form of cardio can push your heart into the HIIT zone, Wisløff says. (This fact sheet from the American College of Sports Medicine offers more in-depth details on how to design a HIIT program.)
And yes, HIIT is safe. Wisløff and colleagues analyzed nearly 50,000 hours of HIIT data collected from cardiovascular disease patients in Norway. In seven years of data, he turned up just two instances of (non-fatal) cardiac arrest.
He says people with unstable angina or serious heart issues should speak with their doctor first. But, in general, “it’s much more dangerous not to perform HIIT than to perform it,” he says.