HIIT can be intense—but the breaks can make you fitter too if you follow these guidelines for ramping up your downtime
Interval training helps you blast fat and boost your fitness—and it also gets you in and out of the gym in time to watch The Big Bang Theory. (Those are just two of the Benefits of High-Intensity Interval Training (HIIT).) And while you probably know that working harder through the tougher portions of the workout (the "work") can help you hit your goals, varying the intensity and time of the easier parts (the "rest period") is another tool in your get-fit arsenal.
To understand why that is, you first have to understand what's going on in your body during the intense parts of a HIIT workout: Those tough work periods are actually changing the chemical composition of your muscles, making them more powerful and giving them more endurance, says Yuri Feito, Ph.D., an assistant professor of exercise science at Kennesaw State University in Kennesaw, Georgia. When you push hard, you burn through your stores of ATP (the fuel your body makes from food), and you train your body to use more fat and your heart to be more powerful.
During the rest period? Your body works to restore itself to a neutral state, replenishing everything you've utilized. Your ATP stores get topped off, you can catch your breath, and your aerobic metabolism takes over, also building your endurance, he says. Basically, your body works really hard to get itself back to normal.
But Laura Cozik, a coach at New York City treadmill studio Mile High Run Club (try their Exclusive Treadmill Workout!) uses a different technique in her endurance-building interval classes. She encourages runners—especially those who aren't beginners—to resist the urge to walk during the breaks, and instead jog or run slowly.
Why? If you're not walking the rest periods, she explains, it'll force you to keep the work periods more manageable so you can last through a tough workout. "And lots of physiological changes happen at that recovery pace," she says. "Your lung capacity improves, you burn fat, and your oxygen transport becomes more efficient."
Basically, you're becoming fitter during every part of the workout—not just the hard parts. Plus, you get more comfortable with the feeling of being, well, uncomfortable, says Cozik. "When you keep the run going, even when you think you can't, you get a sense of accomplishment and empowerment, and you become stronger mentally and physically," she says. Where that'll come in handy: The next time you hit a tough stretch in a race, you'll be used to running through it...not used to hitting the brakes. (Inspired? Check out the .)
One exception? When it comes to building speed, you will want to incorporate those "hit it and quit it" workouts where you sprint as fast as you can and then walk, says Cozik. These will help your muscles adapt to working at a higher intensity, making them more powerful so you can go faster. The bottom line: Mixing these workouts in with endurance-focused intervals and steady state training will build up what Cozik calls your "aerobic engine" so you can go longer and faster. A win-win!