Everything You Need to Know About HIIT

 It’s been a while since high-intensity interval training (HIIT) stormed onto the fitness scene and began its dogged takeover of the cardio universe. One study at a time, HIIT knocked steady-state cardio off its long-held throne, becoming the most relied-upon method for burning unwanted body fat in gyms everywhere.

But taking it to the max, as any HIIT workout requires, is serious business. When dialing up the intensity, it’s important to properly prepare for the fat-fighting agenda you’ve so brazenly mapped out and to perform it with the same purpose-driven enthusiasm you pour into your sets at the squat rack.

Much has been made about the effectiveness of intervals over traditional cardio, but there’s been less attention paid to the mechanics of HIIT. How much is too much? How do I gauge intensity? What work-to-rest ratio is most effective? For as much research as the lab coats have managed to drop in our lap, they’ve still left us with questions.

Justin Grinnell, C.S.C.S., a performance coach and owner of State of Fitness in Michigan, knows that connecting the dots on HIIT will make it an even more efficient protocol for torching fat.

“HIIT is a method of conditioning that uses alternating periods of work and rest,” says Grinnell. “It’s great because it can be done by using various modalities, such as an Airdyne bike, a treadmill, a Concept2 rower, a heavy bag, a StepMill, or by just hitting the pavement and sprinting. This type of training is not only being used by athletes to improve conditioning but also by trainers and their clients as one of the best methods for fat loss and conditioning. With our busy lifestyles, who has time to do 40–60 minutes of aerobic training? The scientific data now show that less is better when it comes to fat loss—you just have to know how to do it right.”

HIIT is hard; and while we have no intention of making it easier, these insights can help make it much more effective.

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Young boxer performing a HIIT workout by Punching a heavy bag with red boxing gloves


Diversity of Activity

HIIT really describes a protocol, a workout prescription more about application than exercise selection. Working in short, intense bursts of activity holds greater benefit than droning about at a slow pace doing anything else—and that holds true whether you’re rowing, boxing, jumping rope, or swimming. Most of the research, however, has been done on sprinting, which, judging by treadmill row at rush hour, is the most relatable method of cardio for most gym-goers.

So while many of the same benefits can be had adding HIIT stylings to your next workout of choice, sprinting is a sure bet for max results.

“Sprinting workouts aren’t effective just for fat loss but also for muscle growth,” Grinnell says. “Elite sprinters are generally very muscular and lean because the conditions required for muscle growth are all present during sprinting— general overload, volume fatigue, and high concentric contractions.”

And it just so happens that most guys would rather look like a gold medal sprinter than an emaciated distance runner.

Bottom Line

Choose the HIIT activity that you’ll do consistently. Ideally, you should shoot for movements that are full body in nature, such as sprinting (where most of the research has been done), swimming, or rowing.

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Female resting during a HIIT workout by running and looking at her workout interval time

Pheelings media

Setting Your Intervals

So how long do you have to perform each high-intensity interval to reap the benefits? Grinnell advises, “The most recent study, out of Australia, reported that a group of females who followed a 20-minute HIIT program consisting of eight-second sprints followed by 12 seconds of rest lost six times more body fat than a group that followed a 40-minute cardio program performed at a constant intensity of 60% of their max heart rate.” That’s three eight-second sprints a minute for 20 minutes. Very intense, but at least it’s over quickly.

A 1996 study by Japanese scientist Izumi Tabata gave rise to the now ubiquitous 20-on, 10-off Tabata protocol, which boosted conditioning in elite athletes. And University of Western Ontario researchers found that subjects who did four to six 30-second sprints with four minutes’ rest after each lost twice the fat of a steady-state group. Simple logic applies that the shorter the rest periods, the more intense the workout is.

The only catch to all this intensity is that it’s hard on the body. “You must change your workload frequently to avoid overtraining and burning out your central nervous system,” Grinnell says. “Sprinting is very demanding on both the brain and the body, so proper rest is important to maximize performance and results. The longer you sprint, the longer you need to rest.”

Bottom Line

The duration of your work and recovery bouts is important, but the name of the game is exertion. Keeping your sprints eight to 30 seconds long is adequate for stimulating fat loss. Recovery between intervals can vary but should be shortened as you become more acclimated to the demands of sprinting, to keep your body guessing (and leaning out).

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Muscular man doing a HIIT workout by sprinting on a running track

Jesus Cervantes

Workout Volume

No one can keep up a sprinter’s pace for very long periods of time, but intermittent bouts of recovery allow you to sprint multiple times in a given workout.

“The hormonal effect of sprinting is so dramatic that you don’t need endless sprints to increase fat loss,” says Grinnell. “The quality of sprints is much more important. HIIT requires great focus and energy demands. So if you’re able to perform at a high intensity for an extended period of time—much longer than 20 minutes—then you may not be doing it right.”

As to frequency, Grinnell says, “Two to three days per week is best for improving body composition and heart health.”

Bottom Line

The number of sprints performed is secondary to the quality of sprints performed. To make sure you’re able to maintain top speed throughout the session, perform as many sprints in the eight- to 30–second range as you can within 20 minutes.

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Muscle building foods and healthy ingredients next to a protein shaker and supplements

George Rudy

Food, Fuel & Fat

Research exists on the effectiveness of training with an empty stomach, but this comes with some caveats for HIIT. “A study from Northumbria University [in England] showed that people can burn up to 20% more body fat by exercising in the morning on an empty stomach,” Grinnell points out. “However, I don’t think it’s optimal if you’re trying to preserve or build muscle tissue while burning body fat. To enhance the fat-burning and muscle-building effects of HIIT, consume 10 grams of BCAAs or 30–50 grams of whey protein 30–90 minutes before.”

This, Grinnell says, will play into HIIT’s most cherished payoff— muscle preservation while still allowing you to dig into stored fat. But at all times, even in the presence of branched-chain amino acids, you’ll want to be properly fueled for the work ahead, which may mean starting your prep well in advance.

“The day before a HIIT or sprinting workout, it would also be wise to have a higher-carb day to store extra glycogen for more intense work,” says Grinnell. “This is particularly important if you’re combining a sprint workout with weightlifting.”

Bottom Line

Go early—but don’t go empty. HIIT cardio before your first meal of the day can help you to burn more fat, but make sure you take 10 grams of BCAA s or 30–50 grams of whey 30–90 minutes before training.

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Protein powder scooper filled with chocolate whey supplement powder


Supplement Upgrade

If you’re dedicated to high-intensity interval training, then you’re all about performance. So it makes sense to power up with proper supplements—those that go beyond your typical fat burner—to fuel that performance. If you’re in the middle of a 20-second sprint and you hit the wall 10 seconds in, you’re depriving yourself of those extra few seconds in high gear. Luckily, there’s a supp for that.

“There’s no doubt that proper supplementation can help any type of exercise, but especially HIIT,” says Grinnell. “When working out at a high intensity, it can be hard to sustain an intense output for long. Beta-alanine has been shown to enhance muscular endurance. Many people report being able to perform one or two additional reps in the gym when training in sets of 8–15 reps. It can also improve moderate to high-intensity cardiovascular exercise performance, like rowing and sprinting, and offer protection from exercise-induced lactic acid production.”

But that’s not the only useful supplement. “Creatine improves power output and is often used by athletes to increase high-intensity exercise capacity and lean body mass,” Grinnell says. “Beta-alanine and creatine are must-haves for interval trainers— but the latter may be more vital.

“Creatine is also helpful for replenishing the phosphagen, or ATP/CP, system during intense exercise,” Grinnell adds. “So if you’re feeling fatigued during a sprint workout, supplementing creatine can help you keep your pace high. If the phosphagen system gets enough creatine in it, you have a better chance of sustaining max exertion during sprints and recovering properly between them.”

Bottom Line

Taking three to five grams of creatine and two to three grams of beta-alanine 30–60 minutes before and immediately after sprint workouts will help you sprint faster for more total sprints in a given session.

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Physically fit man using HIIT to stay in shape


The Fast Track Workout

#1: Track Option
Day Sprints Distance Work:Rest Ratio
1 10 20 yds 1:1*
2 6 40 yds 1:2**
3 3 100 yds 1:3***

*Rest as long as it takes to do one sprint.
**Rest twice as long as it takes to do one sprint.
***Rest three times as long as it takes to do one sprint.

#2: Time Option
Day Sprints Time Recovery
1 10 10 sec 10 sec
2 6 15 sec 30 sec
3 3 25 sec 75 sec

Note: For either of these first two HIIT workouts, add one sprint each day over the course of four weeks to continue upping the ante.

#3: Distance Option
Sprints Distance Recovery
5 100 yds 2-3 min

Perform this sprint workout two to three times per week on nonconsecutive days. Add one sprint and cut 10 seconds off your recovery time each week until you reach eight sprints total.

#4: 30-30 Option
Sprints Time Recovery
6-8 30 sec 30 sec

“The most effective method I’ve found is the 30-30 method,” says Grinnell. “It’s just enough sprint time to sustain high intensity, and just enough rest not to break down too much, too fast.”

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