One of the most popular exercise fads these days is high-intensity interval training, also known as HIIT. Its proponents promise strong, toned muscles and significantly decreased body fat. Like any form of exercise, it has benefits and dangers. Here is a bit more about why this extreme way of working out is getting so much publicity.

What Is HIIT, Exactly?

High-intensity interval training is exercising in alternating periods of nearly full effort and moderate effort or rest. Short bursts of intense activity like jumping, sprinting, or hopping are combined with periods of walking or moderate jogging.

HIIT is usually used for aerobic training, such as running, using an elliptical machine, or plyometric jumping. It can also be adapted to anaerobic training, such as weight lifting.

In a HIIT routine one is attempting to work at about 80-95 percent of maximum aerobic capacity, called the submaximal level. Some athletes use sprint interval training techniques (SIT), where they work at 100 percent of their capacity during the intense intervals.

How Does HIIT Work?

HIIT allows you to work at a very high intensity while maintaining a safe form. The rest periods let muscles recover just enough that you can resume in a form that will not cause injury, but the rest breaks are too short for the burning to slow down. Thus you burn more calories and fat in 30 minutes of HITT than 30 minutes of jogging, even though you are only working very hard for maybe 10-18 minutes of that.

Interval training tricks the body into burning more fat than steady-state cardio. Pushing your body in this way uses up a lot of oxygen, which is not completely restored during the short rest intervals. This means the metabolism and circulation fire up to get more oxygen through the body.

It also significantly increases heart rate in a way that is more manageable than just running at full speed for long stretches. This helps improve heart health and increases endurance.

What Are the Benefits of HIIT?

HIIT is a very effective way to boost the metabolism. The oxygen-debt caused by the intervals can take a while to be refilled after the workout. This means the metabolism and circulation keep working harder for hours after the routine.

A strong HIIT session can keep the metabolism boosted for 24 hours after the workout is over. This means you keep burning fat and calories, even in your sleep.

This phenomenon is called excess-post exercise oxygen consumption. For a little word humor you could say that an epic workout makes for a new epoch in life.

HIIT also increases aerobic capacity, also because of the increased oxygen use. The more oxygen you use in a workout, the greater your endurance and capacity for future exercise.

Intense circuits help create strong, lean muscle. They stimulate hormones that build muscle, such as IGF-1 and growth hormone.

They also increase what is called the lactate threshold. Usually muscle soreness after exercise is caused by a build-up of lactic acid, a by-product of intense muscular effort. Your ability to metabolize lactic acid before it causes soreness and tightness in increased by HIIT.

And HIIT can help with weight loss because it improves insulin sensitivity. This means that your (now very hungry) muscles will draw glucose (sugar) out of your bloodstream to replenish them, rather than that glucose being stored as fat.

A Typical HIIT Routine

You always want to warm-up for at least 5 minutes before any work out, and cool down and stretch afterwards to prevent injury and improve recovery.

HIIT routines are all about ratios. If you are a beginner, start with a 1 to 2 work-to-rest ratio. For example, if you work hard (80-95 percent of your full capacity) for 30 seconds, you would rest (walk or jog in place) for 60 seconds. If you are in excellent shape, you can also use a 1 to 2 work-to-rest ratio.

The work section is the intense part of the training, where you are running, jumping, pushing, swimming, or doing jump-squats (yes, those are as unpleasant as they sound) at your full capacity. Generally the work portions are 20 to 40 seconds, though they can go up to 120 seconds, depending on your fitness level and the type of exercise.

The rest portions are what make this type of intense training possible. You walk or jog at a relatively gentle pace for anywhere from 10 to 60 seconds, again depending on your activity level.

You can perform anywhere from 5 to 20 sets of exercises. Usually 20 to 28 minutes of cardio HIIT that targets every part of the body, plus the warm-up and cool down, makes for a good effective workout.

If you are using HIIT for strength training, you would do the same thing, just working through different muscle groups. Instead of doing a set of bicep curls, for example, then just waiting for your arms to recover, you would do squats or deadlifts while your arms are resting, then switch back to the bicep curls.

A note of warning: Any type of high intensity exercise can be dangerous for your joints, and HIIT may not be appropriate if you have a heart condition. Speak with your health care provider before trying a new form of exercise. Let your provider know that you are planning to try HIIT so he or she can be sure your heart is up to it.

Always give your body at least one rest day between HIIT sessions, and most trainers recommend no more that three HIIT sessions per week. Because form is so important in HIIT, you may want to work with a personal trainer at first to make sure learn correct alignment.

HIIT is a popular form of exercise that can increase endurance, strength, tone, and metabolism. With proper form it can be a powerful way to get a lot of results from short workout sessions.