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High Intensity Interval Training – The Ultimate Guide

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For years doctors and fitness professionals have been prescribing aerobic exercise to reduce the risk of cardiovascular disease and aid in fat loss, as well as to improve overall health parameters.
There’s an abundance of research supporting these claims and, for many individuals, forms of low intensity aerobic exercise is a great way to get fit and stay active [1].

However, the simple goal of getting and staying active probably doesn’t prioritize for many of the readers here. For those looking to take things to the next level and significantly improve their muscle mass or overall physique then aerobic training alone has some limitations.

First off, it tends to take a very long time where you could be focusing on building muscle; additionally, research has shown that, for those who are striving to maintain or increase lean mass, low intensity aerobic exercise may potentially conflict with these adaptations [2].

These issues have led researchers and strength / fitness professionals to search for an equally effective alternative to traditional aerobic training. The solution is known as HIIT, standing for High Intensity Interval Training which, in the last decade, has taken the health, sport and fitness world by storm!


Well, as HIIT has grown in popularity so has the research supporting it, with studies concluding that HIIT is either equally or even more effective than aerobic training for fat loss, improving VO2 max (main measure of fitness), sprinting performance and even several clinical markers related to cardiovascular disease.

Best of all? You can achieve this while working out for a 1/4 of the time [3]!

Here’s an ultimate guide to HIIT, what it is, how it works and how to perform it yourself.


What Is HIIT

Traditional aerobic training requires you to train at a low intensity for long periods of time. For example, 45 minutes of cycling at 50-60% VO2 Max. HIIT, on the other hand, involves alternating bouts of high intensity exercise with low intensity exercise or rest. For example, 4 x 30s sprints with 4 minutes’ rest [4].

HIIT training increases the amount of high intensity work performed throughout a training cycle, ultimately leading to improved adaptations to both your performance and your physique. Here’s the research and an overview of each specific benefit.


HIIT Research on Fat Loss

While most people perform steady state cardio for fat loss, the research shows HIIT may be a greater option.

One study analyzed the fat loss effects of endurance training (ET) compared to high intensity interval training (HIIT) on 27 healthy young men and women. These researchers found that HIIT training resulted in fat loss 9 fold greater compared to ET over the course of 20 weeks [5].

Another group of researchers recently compared the effects of HIIT & aerobic exercise on fat loss in young women following 15 weeks of training. The HIIT group performed 3 sessions per week consisting of 8 second sprints followed by 12 seconds of low intensity cycling. The aerobic group performed 40 minutes of cycling at 60% VO2 max.

The results demonstrated that the HIIT group lost an additional 5.5lbs of fat compared to the aerobic group performing more exercise and spending more time! [6]

Based on the research HIIT seems to be far superior for the amount of time you must commit, allowing you to spend more time on weight training or other exercise etc. These adaptations seem to occur as HIIT causes large spikes in fat burning hormones such as growth hormone along with elevating your metabolism for up to 48 hours after the workout [7].


HIIT Vo2 Max & Aerobic Fitness

Along with fat loss, several researchers have also noted improvements in VO2 Max and other markers of aerobic performance following HIIT training. In research terms, your VO2 max is a numerical calculation of your body’s ability to consume oxygen during exercise and is a key measurement in professional sport and health assessments.

A higher VO2 max is a good thing because it means that your body can take in more oxygen and deliver it to your muscles, enabling you to perform longer and faster at a given effort.

The effectiveness of HIIT training on VO2 max seems to depend on the duration of a training protocol. One group of researchers noted an increase in VO2 max of 13% following 2 weeks of HIIT training, while another group of researchers demonstrated increases in VO2 max by 41% following 8 weeks of training [8][9]. Both of these studies further demonstrate how HIIT can provide rapid rewards; for example, in the 8 week study the participants nearly doubled their VO2 max levels!

If you are an athlete then HIIT could be the novel stimulus you need to take your fitness to the next level. If you are focusing on gaining muscle mass, physique transformation or just want to lose fat then HIIT can still provide impressive fitness benefits with very little time commitment making it a key add-on for long-term health.


HIIT for Sports Performance

Although a lot of HIIT studies have been conducted on overweight or normal individuals, some studies have focused particularly on the effects of HIIT for athletes in sport-specific settings.

In one study, 24 hockey players were split into two groups and performed either steady state / aerobic exercise for 45-60 minutes at 65% of their VO2 max or 4 x 10-30 second sprints 2x a week. To put this into perspective, they compared 90 – 120 minutes of low intensive exercise vs only 2 to 4 minutes of HIIT per week!

Despite much less time and total exercise, the results demonstrated that the HIIT group increased peak power output by 11.7% compared to only 2.2% in the aerobic group. Also, specific to the sport of hockey, the HIIT group significantly decreased their ice sprint time. [10].

If you’re looking to improve performance in team sports HIIT perfectly matches the stop-start activity and required energy systems. Just 2 short HIIT sessions per week can likely have a noticeable effect for an athlete who is currently just using conventional training.


HIIT for Clinical and Health

HIIT isn’t only great for your physique or sports performance; it can actually improve your health to the same extent as regular aerobic training.

One recent meta-analysis (review of all research) was conducted in order to draw conclusions on HIIT and its effects on various different health parameters. These researchers analyzed 7 studies with a total of 182 subjects.

After the researchers had assessed all the current studies they summarized that HIIT resulted in [11]:

  • Improved artery blood flow,
  • Improved cardiovascular fitness,
  • Reduced oxidative stress,
  • Reduced inflammation,
  • Improved insulin sensitivity.


How Does HIIT It Work?

As with most adaptations within the body it is likely that there’s not just one factor responsible for all these great benefits. Based on the research, there are multiple unique factors that contribute to drastic fat loss, improved performance and the health benefits that HIIT provides.


Short-Term Responses

  • Large increases in Heart rate which is directly associated with the intensity of the exercise. By stressing the heart and general cardiovascular system it’s no surprise that we see large boosts in energy expenditure [12].
  • Increases in the hormone Epinephrine and other catecholamines which can increase lipolysis (the breakdown of fat) and aid in weight loss [13]
  • Spikes in growth hormone which has multiple effects within the body including increasing rates of protein synthesis as well as fat breakdown. Growth hormone has been shown to increase 10x higher following HIIT training compared to aerobic training; however, it should be noted that the short-term rises in growth hormone levels is being debated. [14][15] If interested, read: Post Workout Anabolic Hormone Elevations – A Complete Guide
  • Increases in excess post-exercise oxygen consumption (EPOC), also known as oxygen debt, which is the metabolic process that causes an ‘after burn’ or boost in your metabolism and energy expenditure. However, it should also be noted that EPOC may only be responsible for 6-15% of the total exercise cost. Therefore, it is likely that the majority of fat loss occurs during the actual exercise itself [16].


Long-term Adaptations

  • Chronically HIIT sessions may result in increased ability to breakdown and burn fat, often referred to as your ‘metabolic flexibility’. HIIT may also aid in weight loss by decreasing appetite after the workout which in turn helps reduce total daily calorie intake [17][18][19].
  • Regular HIIT training also significantly lowers insulin resistance which increases your ability to utilize carbohydrates for fuel, a key factor in health and also important for physique enhancement and performance [20].
  • Improved VO2 max and fitness, which is a key measurement of cardiovascular fitness and a general marker of health [8][9].


Equipment for HIIT

Another great aspect of HIIT training is that it doesn’t need fancy or expensive equipment, it can be performed at home, in the gym or outside. Here are the main pieces of equipment you could use:

  • Track, treadmill or just running outside on an even and safe surface
  • Prowlers
  • Spin bikes or an outdoor bike
  • Elliptical / X trainer
  • Hill sprints
  • Battle ropes
  • Row machine
  • Swimming pool


How to Apply

As you probably could have guessed from the research presented earlier, there are multiple uses and methods for HIIT.

Deciding which method is right for you will depend on your primary goal and current regime. Below I describe some examples of protocols that have been used in previous research and are proven to help with fat loss or improve fitness.

Example 1 Short Duration Short Rest Intervals (Beginners)
8s sprints x 12s rest – Repeat for 30-60 rounds
15s sprints x 15s rest- Repeat for 20-30 rounds

Example 2 Long Duration Long Rest (Moderate)
4min high intensity x 2 min rest- Repeat for 4 rounds
4min high intensity x 4 min rest- Repeat for 4 rounds

Example 3 Short Duration Long Rest (Advanced)
6s sprints x 2 min rest – Repeat for 10 – 20 rounds
10s sprints x 3 min rest- Repeat for 6 – 10 rounds
15s sprints x 3 min rest- Repeat for 4-6 rounds
30s sprints x 3 min rest- Repeat for 4-6 rounds



In conclusion HIIT may be the most time effective way to train to reach your fitness goals. If you are short of time and just want to reap the rewards of a healthy active lifestyle then why train for 2-5 hours of cardio per week when you could get away with less than 30 minutes with HIIT?

Dozens of studies have now proven that HIIT increases fat loss, VO2 Max, sprint performance, as well as general markers of health.

If you are currently body building or just doing weight training, try adding in 2 or 3 HIIT workouts per week at the end of your workouts, or, as separate sessions to reap some major rewards!

Remember, a little pain for a lot of gain!



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About the author

About Rudy Mawer
Rudy Mawer

Rudy Mawer is human performance researcher and a certified Sports Nutritionist from the International Society of Sports Nutrition (ISSN). He has a first class bachelor's degree in Exercise, Nutrition and Health and a Master's degree in Exercise and Nutrition Science. Rudy has worked as a sports nutritionist and trainer for 7 years, and has helped...[Continue]

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