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HIIT Science and Guidelines

HIIT Science and Guidelines

Take the confusion out of HIIT

Written By Jim Stoppani, PhD

Updated January 26, 2020

In case you haven't heard, there's a new type of cardio training that has become very popular, called high-intensity interval training (HIIT).

In a nutshell, this form of cardio involves intervals of high-intensity exercise (such as running at a very fast pace) interspersed with intervals of low-intensity (walking at a slow pace) or complete rest. This is in sharp contrast to the typical continuous steady-state (slow and steady) cardio that most people do at a moderate intensity, such as walking at a fast pace or jogging for 30 to 60 minutes minutes.

But whether you have just learned about HIIT or have been using it for a few years, the concept of HIIT is not new at all.

HIIT was mainly developed by track coaches decades ago to better prepare runners. Then it was known as Fartlek training. And you can see why that name hasn't caught on. Fartlek is actually Swedish for speed (fart) and play (lek), so it means speedplay, which is essentially what HIIT is. Today HIIT has crossed over to the fitness industry due to beneficial results that have been established through both anecdotal reports and published research studies. In fact, studies comparing HIIT to continuous steady-state exercise have shown that HIIT is far superior for fat loss, despite the fact that it requires much less time.

One of the first studies to discover that HIIT was more effective for fat loss was a 1994 study by researchers at Laval University (Ste-Foy, Quebec, Canada). They reported that young men and women who followed a 15-week HIIT program lost signify more body fat than those following a 20-week continuous steady-state endurance program, despite the fact that the steady-state program burned about 15,000 calories more than the HIIT program. A 2001 study from East Tennessee State University demonstrated similar findings with subjects who followed an 8-week HIIT program (subjects dropped 2% in percent body fat) as compared to those who followed a continuous steady-state program (subjects had no drop in percent body fat) on a treadmill.

A study from Australia reported that females following a 20 min HIIT program that consisted of 8 second sprints followed by 12 seconds of rest, lost 6 times more body fat than the group who followed a 40-minute cardio program performed at a constant intensity of 60% of their maximum heart rate. Researchers from the University of Western Ontario had male and female subjects follow one of two cardio programs for six weeks. One group of subjects ran slow and steady for 30-60 minutes three times per week. The other group did four to six 30-second sprints with a 4-minute rest period between sprints three times per week. That's basically HIIT with an extended rest period between the high-intensity exercise. They reported in a 2011 issue of the journal Medicine and Science in Sports and Exercise that the group doing the sprint intervals lost more than twice as much body fat as the slow and steady group despite the fact that they only did 2-3 minutes of total cardio exercise per day and just 6-9 minutes per week! The sprint interval group also gained over one pound of muscle.

The major reason that HIIT works so well to drop body fat to a greater degree than continuous steady-state cardio appears to be due to the greater increase in resting metabolism following HIIT. A 1996 study from Baylor College of Medicine (Houston, TX) reported that subjects who followed a HIIT workout on a stationary cycle burned significantly more calories during the 24 hours following the workout than those who cycled at a moderate steady-state intensity. The 2001 East Tennessee State University study mentioned above also found that subjects following the HIIT program burned almost 100 more calories per day during the 24 hours after exercise. A study presented at the 2007 annual meeting of the American College of Sports Medicine by Florida State University (Tallahassee) researchers reported that subjects who performed HIIT burned almost 10% more calories during the 24 hours following exercise as compared to those whole performed continuous steady-state exercise, despite the fact that the total calories burned during the workouts were the same.

In addition to the increase in resting metabolism, research confirms that HIIT is effective at enhancing the metabolic machinery in muscle cells that promote fat burning and blunt fat production. The Laval University study that found a decrease in body fat with HIIT discovered that the HIIT subjects' muscle fibers had significantly higher markers for fat oxidation (fat burning) than those in the continuous steady-state exercise group. A study published in a 2007 issue of the Journal of Applied Physiology reported that young females who performed seven HIIT workouts over a two-week period experienced a 30% increase in both fat oxidation and levels of muscle enzymes that enhance fat oxidation. A study from the Norwegian University of Science and Technology (Trondheim), reported that subjects with metabolic syndrome who followed a 16-week HIIT program had a 100% greater decrease in content of the fat-producing enzyme fatty acid synthase as compared to subjects who followed continuous moderate-intensity exercise.

And yet another way that HIIT appears to work has to do with getting the fat to where it will be burned away for good. A new study published in the American Journal of Physiology sheds some light on another way that HIIT burns more body fat. Researchers reported that 6 weeks of HIIT increased the amount of special proteins in muscle that are responsible for carrying fat into the mitochondria (where fat is burned away for fuel) by up to 50%. Having more of these proteins in muscle means that more fat can be burned up for fuel during workouts and when resting.

While many bodybuilders and trainers argue that going slower and longer with cardio is best to burn fat and protect muscle mass, the opposite appears to be true. HIIT, done at a higher intensity for a shorter period of time will not only help you maintain your muscle, but can actually help you build muscle mass. When you train at a slow and steady pace for a longer period of time, you are training your muscle fibers to be more aerobic and have greater endurance. Do you know how muscle fibers adapt to becoming more aerobic and gaining greater endurance? By becoming smaller and weaker! The smaller a muscle fiber is, the less time it takes for nutrients to travel within the muscle fiber. This speeds up the rate that the nutrients can be burned for fuel. But even if you think of this from a common sense perspective, it makes perfect sense.

Stating that slow and steady cardio for longer periods of time is best for maintaining muscle mass is similar to saying that curling 5 pound dumbbells for 30 minutes straight will build more muscle than curling 40 pound dumbbells for sets of 10 reps with two minutes of rest between sets. See, the higher intensity workout clearly builds muscle better. In fact, when you think about it, weightlifting is actually a form, of HIIT. One study in the Journal of the International Society of Sports Nutrition reported that male subjects following a six-week HIIT program (done for 15 minutes per day at a ratio of 2:1 for exercise to rest, 3 days per week) while supplementing with beta-alanine gained over 2 pounds of muscle despite the fact that they never lifted weights during the program.

In another study, New Zealand researchers had competitive cyclists complete four weeks of HIIT training involving 30-second sprints on a stationary cycle separated by 30 seconds of rest. One group sprinted with high resistance on the pedals, making it harder to pedal, while the other group used a lighter resistance, which was easier to peddle. Both groups peddled as fast as they could during the 30-second sprints. They found that the men peddling at the highest resistance increased their testosterone levels by almost 100%, while the group peddling at a lighter resistance only increased test levels by about 60%. Since testosterone is critical for boosting muscle size and strength, this means that doing HIIT with greater resistance can aid muscle growth and strength.

HIIT also helps you to maintain your sanity by getting you done with cardio quicker. I can't think of anything more monotonous than being stuck on a treadmill, stairmaster, stationary cycle, or elliptical machine for a good 30 minutes straight! With HIIT the intensity bursts may be more grueling, but they are short and challenging. That makes the workout more "fun" and makes it over quicker.

Another benefit of HIIT is that you can do it almost anywhere with any piece of equipment: or without any equipment at all! Although it can be, HIIT does not have to be done on the typical cardio equipment in your gym. The possibilities are virtually limitless. You can use it with a jump rope, with weights, with strength bands, with your body weight.

So consider doing less slow and long workouts in the cardio area and do more HIIT. The benefits will be maximal fat loss due to a ramping up your resting metabolism and fat burning enzymes, while building muscle, all in a minimal amount of time.

Beginner To Advanced 8-Week HIIT Program

The following HIIT program is a sample to take someone from an HIIT beginner to an HIIT stud in eight weeks. The suggested time of each phase is just that, suggested. It is not carved in stone. If you feel you need to spend more than two weeks at a particular phase before moving up, then do so. Or if a phase seems too easy and you want to jump right up to the next phase, then do so. It starts with a work:rest ratio of 1:4 in phase 1 for a total workout time of just under 15 minutes. Then in phase 2, it bumps up the amount of time in the "work" phase to bring the ratio up to 1:2 and the total workout time to 17 minutes. In phase 3, the rest ratio is cut in half to bring the ratio up to 1:1 and the total workout time goes up to 18.5 minutes. And finally in Phase 4, the rest ratio is cut in half again to get the ratio up to 2:1 and the total time at 20 minutes, which puts you in the advanced ranks for HIIT. You can do these workouts using anything, such as a jump rope, or simply doing jumping jacks, or sprinting, or working on a stationary cycle. Use your imagination and just follow the work to rest intervals as indicated.

Phase 1 (1:4) – weeks 1-2

Time Activity

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

1 min. Rest or Low-Intensity Exercise

15 sec. High-Intensity Exercise

Total Time: 14 minutes

Phase 2 (1:2) – weeks 3-4

Time Activity