What is HIIT? High-intensity interval training, or HIIT, involves pushing yourself to near 100 percent effort on your chosen activity — running, for instance — in shorter intervals, broken up by active recovery. In a typical HIIT running workout, you’d perform a short warm-up and then move into an all-out sprint for 30 seconds, followed by a slow jog or walk for a full minute. After that, the cycle would be repeated a number of times, usually for a total duration of 20-25 minutes total — a much more efficient use of your workout time than the traditional endless treadmill.

But time efficiency isn’t the only benefit of HIIT. It activates more fast-twitch muscle fiber, it burns more stored glycogen (sugar stored in the muscle), and it uses more calories. For example, if you perform ten 30-second sprints at 95% effort, that’s a total of five minutes that you have run at that pace. In addition, you’re also working during your “rest” periods, even if at a slower pace, so the total impact of work performed is far greater than if you just jogged for a few miles at a slower pace. The bottom line is HIIT makes you leaner. Whether you’re a weekend warrior or an elite athlete, there’s nothing wrong with getting a little leaner — and HIIT is in a league of its own when it comes to burning fat.

How does HIIT work? HIIT significantly increases your body’s resting metabolic rate (RMR) following exercise, a term known as excess post-exercise oxygen consumption (EPOC). In other words, you continue to burn calories after working out, even when you’re home watching TV. Another way HIIT works to keep you lean is by it focusing on your body’s fast-twitch muscle fibers, which have the greatest potential for strength and growth. This means more muscle is grown through this kind of training. For competitors or those of you trying to increase muscle mass this is important. Keep in mind, extended distance running can lead to muscle loss, as the body eats through glycogen and fat stores and is then forced to utilize protein (muscle) as a fuel. It’s the difference between sprinter’s legs, which are generally thick and muscular, versus marathoner’s legs, which tend to be leaner.

A basic HIIT guideline for beginners Try this sample running workout:

IntensityDuration Walk 2 min. Slow jog 2 min. Sprint* 10 sec. Slow jog 1 min. Sprint 10 sec. Slow jog 1 min. Sprint 10 sec. Slow jog 1 min. Sprint 10 sec. Slow jog 1 min. Sprint 10 sec. Slow jog 1 min. Sprint 10 sec. Slow jog 1 min. Walk 2 min. TOTAL 13 min.

*For all of your sprints, aim for a speed that is anywhere from 75–95% of your max. As you progress, extend the duration of your sprints, eventually aiming for 30 seconds, or add additional intervals to your workout.

HIIT guideline for athletes: To get the most out of HIIT, try implementing 1–2 sessions per week to your usual workout schedule (see sample workout), substituting the duration with a 2:1 ratio; 2 minute slow jog, with a 1 minute sprint. These short-burst workouts train your body to perform better, look better, and last longer, helping you reach your goals more efficiently. * M&F*