READ TIME: 20 Minutes
Don’t want to read? Listen to the article here.
Nothing makes me giggle more than when I scroll past a fitness influencer’s post of their “Killer HIIT” workout and see that it’s composed of exactly the opposite of high intensity movements. These people post workouts and unironically title them “Savage HIIT” after the deepest breath they took during the workout could barely fill the lungs of an asthmatic 7 year old. High intensity interval training (HIIT) should be high intensity. It’s in the got damn name. HIIT is a specific form of exercise training that has definitive characteristics which differentiate it from other forms of exercise. The benefits of HIIT training are widely touted and promoted in the fitness industry. However, very few of the workouts labelled as HIIT, are actually HIIT. That’s a little bit of a problem, no? These fitness influencers who post illegitimate HIIT workouts have the largest platforms in the history of the fitness industry. Some of them have millions of followers, who believe them to be an authority in the industry, and the fitspos are feeding them loads of uneducated crap. I want to show you what HIIT actually is, what HIIT can do for you, and how to spot a fake HIIT workout.
So then what is HIIT?
HIIT can be broadly defined as repeated bouts of exercise lasting between 10 seconds and 5 minutes completed at an intensity greater than the anaerobic threshold with rest periods between efforts¹. HIIT is different than traditional, continuous, submaximal cardio and it is different than interval training. High intensity means high intensity. The key here is that the exercise you are performing surpasses your anaerobic threshold. Your anaerobic threshold is the point at which your body cannot rely solely on aerobic metabolism to fuel your exercise. As exercise intensity increases, there exists a point where your body’s energy generating systems cannot keep up with the energy demand of that exercise. When this threshold is crossed, there is an exponential increase in lactate accumulation in the blood, ventilation, and carbon dioxide production².
To step it back even farther, your body uses three energy systems to generate energy. The ATP-PCr system, the glycolytic system, and the aerobic system. Your body burns both carbohydrates and fats in order to generate ATP, the energy currency of your body that fuels your movement. Your body can generate this energy through both aerobic and anaerobic metabolism. Aerobic metabolism occurs in the presence of oxygen. Anaerobic metabolism occurs when oxygen is not present in high enough concentrations to fuel the reactions of aerobic metabolism. Everyone who has taken a high school biology course has encountered these two pathways, you probably just didn’t care enough at the time to understand their significance and how it relates to your life. Aerobic metabolism is when carbohydrates or fats are broken down and sent through the Citric Acid Cycle (also called the Krebs Cycle) and the electron transport chain to generate energy. This process occurs inside the mitochondria of your cells, hence why the mitochondria is the powerhouse of the cell. These reactions require oxygen to be present in order to generate ATP. When exercise intensities increase and cross the aerobic threshold, the energy substrates (carbs and fats) are being broken down too fast for aerobic metabolism to keep up. This leads to a backup in the system and your body will have to rely on anaerobic metabolism to help generate energy without the aid of oxygen³. This is the point we must reach in order for an exercise to be considered as HIIT.
What’s the deal with HIIT anyway?
There are many benefits that come as a result to performing HIIT training. It’s intense exercise after all, of course it’s going to be good for you. The most popular selling point for HIIT training is that you can burn more fat using HIIT training than you would from traditional cardio. We are all looking for ways to lean up as easily as possible, but what does the literature say? HIIT training and regular training have been shown to lead to similar levels of weight loss when workload and energy expenditure are matched but when it comes to absolute fat mass reduction, HIIT training takes a slight lead⁴. It is theorized that the greater decreases in fat mass as a result of HIIT training come from greater increases in fat oxidation in the post exercise state as a result of glycogen depletion during the bout of exercise⁵.
With HIIT training you can receive similar benefits to traditional, lesser intense exercise modalities while exercising for a shorter period of time. When workloads or energy expenditure are matched, HIIT training can lead to an equal reduction in the amount of fat mass in up to 40% less time⁶. Some studies even suggest that HIIT can more effectively increase aerobic capacity than moderate intensity continuous exercise can, especially in people who are less fit⁷ ⁸.
A common point of concern lifters have with traditional cardio is that they are worried it will negatively impact the adaptations they are aiming to receive from their resistance training program⁹. It is a valid point, the pathways for adaptations to resistance training and aerobic exercise are different and high volume aerobic exercise can attenuate the adaptations from resistance training. HIIT training may be a more preferable exercise modality for people wishing to grow muscle as these negative effects aren’t seen like they are in high volume aerobic training.¹⁰ Thirty minutes or so of a light jog or an elliptical session a few times a week isn’t going to do anything to hurt you. However, if you’re doing hours of dedicated aerobic training per week it can begin to have an impact.
You may have heard the term afterburn which in the scientific world is known as excess post exercise oxygen consumption (EPOC). EPOC is a measurable increase in metabolic rate in the period following a bout of exercise. It is dependent on the intensity of that exercise, the greater the intensity the greater level of EPOC¹¹. While you may burn more calories in the post exercise period following a HIIT session than you would with a traditional cardio session, it is only a small amount.
HIIT or sHIIT?
When deciding to do a HIIT workout, it is important to choose a modality that allows you to achieve the proper intensity. As stated earlier, we must cross the anaerobic threshold to really receive the benefits of HIIT training. Interval training is often mistaken for HIIT training. It should also be noted that sprint interval training (SIT, a more intense version) is different from HIIT. Putting a few different exercises together into a circuit where you move from one exercise to the next is not necessarily HIIT training. If you would like to do interval style HIIT training you need to make sure the movements you choose allow you to reach the correct intensity. This means that the transition time between movements as well as the turnover rate between repetitions both need to be quick enough to maintain intensity. There is some evidence showing that running/sprinting movements lead to better performance in HIIT style training. My research group even observed the same phenomenon when we were studying a similar research question back when I was in school.
A rule of thumb for choosing exercises to include in a HIIT workout is to pick full body movements and ones that involve a large amount of musculature. It is my belief that the best HIIT movements are the ones where the speed of the movement is not slowed by the rate that you can cycle through the reps. Sprinting, cycle sprints, airbike sprints, sled pushes, and other similar movements are my favorites for HIIT training because they allow for the truest form of high intensity training. You could always perform the movement faster whereas with something like a kettlebell swing, you are limited by the speed at which gravity pulls the kettlebell back down to Earth.
It is rare that anything in fitness or nutrition has a definitive answer for what is always best practice. There are so many individual variables and caveats for this to happen. Because I know how people just want a straightforward answer I will do my best to rank different movements based on their value in HIIT style workouts.
The Cream of the Crop
- Cycle Sprints
- Airbike Sprints
- Sled Pushes
- Row Sprints
- Ski Sprints
- Kettlebell Swings
- Snatch Variations
- Clean Variations
- Air/Jumping Squats
- Jumping lunges
- Med Ball Slams
- Mountain Climbers
- Push Press
- GHD Situps
- Skipping Rope
Ehh, maybe but probably not
- Jumping jacks
In what world is that high intensity, babe?
- Curls into Shoulder Press
- Any kind of isometric hold
- Lateral Raises
- Pretty much all of the classic “ab” exercises
At the end of the day it is about how much intensity you are able to put into the movement. Remember intensity is a function of the oxygen cost of the workout. High intensity exercise makes you breathe, it burns your lungs, and resting between efforts is a must. You don’t want the rate limiter of your exercise to be because you can’t perform any more reps due to lack of strength. For example, if you can only do 10 air squats before your legs won’t let you do another rep then this isn’t a great exercise to choose for HIIT training. It is going to be nearly impossible for you to cross the anaerobic threshold, hence why the exercises listed above in the top category are the best. The rate limiter in all of them (apart from maybe burpees in a small number of people) is your ability to deliver oxygen to the working tissues.
Programming HIIT Workouts
Before you perform any workout/workout program you should be asking yourself, “What is the goal of this exercise?” If you are doing HIIT training for sport, you will want to specify the training to match the goals of your sport. If you are doing HIIT for general exercise then you are going to have much more freedom in your design. There are lots of considerations to make when programming HIIT workouts into your overall training program. How much fatigue does it add? How long will it take to recover from? How often should I do it? In a perfect world you would account for all of these variables and include HIIT training into your training block in an intelligent way. How many of you are going to do that? Less than 1%. When most of you do HIIT training it is thrown at the end of a workout because you “want to get some cardio in” so here is what I would consider.
What intensity do you want to work at? You can decide on intensities ranging from 85%-120% of your VO2max. Most of you won’t know what you VO2max is or what it feels like to actually work at those specific intensities. A good estimate would be to use a perceived exertion (RPE) scale from 1-10 where 1 is laying in your bed and 10 is sprinting as fast as you possibly can, you should be working somewhere between a 7-10. Higher intensities will mean that the duration of your exercise will be shorter and vice versa for lower intensities. If you decide to work at RPE 10, your exercise duration will not last much longer than 30 seconds. If you chose RPE 7, your exercise could last minutes depending on your fitness.
Picking your modality and style of exercise will influence the other factors listed here. If you wanted to use a higher intensity such as an RPE 9 or 10, it is probably best to use the exercises that allow for true sprint style training. A tabata style workout opens the possibility for you to incorporate some different movements like kettlebell swings.
Rest intervals can either be active and submaximal (RPE of 3-6) or passive where you are completely at rest. In either case the goal of the rest interval is to allow for lactate and metabolites to clear from the muscle in order to perform another effort.
Number of Efforts
An important factor to consider when determining how many efforts you will perform is how much time you spent above the anaerobic threshold. The time interval you choose will depend on what your goals are. If you are a middle distance runner who is using HIIT to increase your VO2max, it might be a good choice to aim for a similar amount of time spent at or above the anaerobic threshold as it would for you to run your event.
The number of HIIT sessions you should be doing per week is dependent on what your goals are. It also depends on the modalities of HIIT training you are doing. If your goal is strictly to build muscle and increase lean body mass then your resistance training needs to take precedence over any kind of cardio. Most of the changes in fat loss come about as a result of proper nutrition, not exercise. For these people, I wouldn’t be doing more than 2-3 sessions of HIIT per week. Younger, more resilient individuals who recover better may be able to handle 3 sessions of HIIT a week. I think a case could be made that no cardio needs to be performed outside of cutting phases for bodybuilders and those who are looking for maximal muscle growth.
If your goal involves improving your overall health and muscular growth is just an added bonus to exercise, then you have more freedom to incorporate HIIT into your plan. Exercising without a specific goal in mind means that you can really do whatever you want so long as you are able to recover from that exercise before the next time you want to work out again. Doing higher intensity workouts for people in this group can realistically be done every day if done intelligently and in a way that allows for proper recovery and mitigation of any repetitive use injuries. Running sprints every day at RPE 10 should be kept for the track and field athletes as working up to that volume takes time to achieve. However, if you wanted to incorporate daily crossfit style WODs, which in many cases would be defined as HIIT, there is a case that could be made to support this as being beneficial.
Here is a table that is included in the paper titled “High-Intensity Interval Training, Solutions to the Programming Puzzle” by Buccheit et al. in 2013. It gives recommendations for how you could set up your HIIT workouts for maximizing the time spent at maximal oxygen uptake.
Looking at your overarching goals is crucial when it comes to adding HIIT to your training. Most of you reading this will probably be wanting to add HIIT as a way to augment weight loss. It is important to remember that any amount of weight loss comes from being in a caloric deficit over a defined period of time. The calories/fat you burn in a HIIT cardio session means nothing for weight loss if you don’t maintain proper dieting strategies in the time surrounding the workout. Research is a great tool to tell us important information about different factors in training. Understanding how something like HIIT actually affects your body on a physiological level is insanely beneficial. However, what research can’t do is tell us about the practicality of adding something like HIIT into our training and how it is actually applied in real life scenarios. In my own personal experience as a coach and exerciser myself, HIIT training seems to increase appetite more than other forms of exercise does. If you have specific dieting goals related to weight loss, it might be counterproductive to include exercise that is going to make nutritional adherence more difficult.
Certain types of HIIT training may lead to the accumulation of fatigue greater than other forms of cardio. Sprinting in particular is a very taxing form of exercise. It is, in my opinion, the most potent form of high intensity exercise but it comes at a cost. The average exerciser who is looking to add HIIT into their week probably hasn’t full out sprinted since high school. Some of you probably blew out a hammy just reading that sentence. Running repeat 100m sprints probably isn’t a great idea for a 40 year old, normal guy who just wants to lean up a bit and gain muscle. The injury risk of sprinting is too high to outweigh the benefits. If you pull a hamstring you can sideline yourself for weeks. I ran some sprints with friends this past August and even I tweaked my 26 year old hamstring doing it. I was messed up for almost a month. The repeated impact of exercises like sprinting, sled pushes, or burpees is something to take into consideration as well. Exercise selection matters. If you are going to perform HIIT training multiple times per week it would likely behoove you to pick exercises that lower impact so you don’t develop too much fatigue to inhibit your regular resistance training.
Now Summarize That For Me
HIIT Training is a specific form of cardio training. It is not interval or circuit training. HIIT training can be broadly defined as repeated bouts of exercise lasting between 10 seconds and 5 minutes completed at an intensity greater than the anaerobic threshold. This anaerobic threshold is likely equivalent to somewhere around a 7 on a scale out of 10. If your “HIIT” workout isn’t making you breathe and cross over this threshold, you are not going to receive the benefits of HIIT training that the literature has observed.
There are many benefits of HIIT training. It is often purported that HIIT training is better for fat loss. When workloads and energy expenditure are matched, HIIT training and more traditional lower intensity exercise are equal when it comes to weight loss but HIIT training does seem to lead to a bit more absolute reduction in fat mass. HIIT training is also quicker than traditional cardio. The amount of work it takes to burn a certain amount of calories takes up to 40% less time using HIIT than with lower intensity cardio. HIIT training may also be the more muscle sparing form of cardio exercise. High intensity exercise can also increase your metabolic rate in the time period following exercise, this is known as excess post exercise oxygen consumption. Although it is increased, it is not by a large amount.
When it comes to programming HIIT exercise, you should factor it into your normal training plan so that it fits your goals. Considerations should be made for intensity/duration, modality, rest intervals, number of efforts, and weekly frequency. The time you spend above the anaerobic threshold in conjunction with the intensity of the exercise is a good metric to use in order to determine the number of efforts and duration of your workouts.
HIIT training is not the silver bullet to fat loss. It is a tool that can be used for improving your fitness and as a mode for increasing energy expenditure. Any amount of weight loss comes about from being in a caloric deficit over a period of time. HIIT for weight loss is merely a method that you can use to help create a caloric deficit. HIIT for increasing aerobic capacity can be a powerful tool especially for people who are not already of high fitness. In order for one to receive the benefits from HIIT training the exercise has to actually be intense enough. It is far too common for the average everyday gym goer to think they are doing HIIT training when they are actually just doing circuit training. Social media is part of the problem, promoting false information. If you are going to include HIIT into your training do it intelligently and do it with intensity.
- Laursen, P. B., & Jenkins, D. G. (2002). The Scientific Basis for High-Intensity Interval Training. Sports Medicine,32(1), 53-73. doi:10.2165/00007256-200232010-00003
- Shephard, R. J. (2000). Anaerobic Metabolism and Endurance Performance. Endurance in Sport,311-327. doi:10.1002/9780470694930.ch22
- Scott, C. (2005). Misconceptions about Aerobic and Anaerobic Energy Expenditure. Journal of the International Society of Sports Nutrition,2(2). doi:10.1186/1550-2783-2-2-32
- Viana, R. B., Naves, J. P., Coswig, V. S., Lira, C. A., Steele, J., Fisher, J. P., & Gentil, P. (2019). Is interval training the magic bullet for fat loss? A systematic review and meta-analysis comparing moderate-intensity continuous training with high-intensity interval training (HIIT). British Journal of Sports Medicine,53(10), 655-664. doi:10.1136/bjsports-2018-099928
- Kiens, B., & Richter, E. A. (1998). Utilization of skeletal muscle triacylglycerol during postexercise recovery in humans. American Journal of Physiology-Endocrinology and Metabolism,275(2). doi:10.1152/ajpendo.1998.275.2.e332
- Wewege, M., Berg, R. V., Ward, R. E., & Keech, A. (2017). The effects of high-intensity interval training vs. moderate-intensity continuous training on body composition in overweight and obese adults: A systematic review and meta-analysis. Obesity Reviews,18(6), 635-646. doi:10.1111/obr.12532
- Matsuo, T., Saotome, K., Seino, S., Shimojo, N., Matsushita, A., Iemitsu, M., . . . Mukai, C. (2014). Effects of a Low-Volume Aerobic-Type Interval Exercise on V˙O2max and Cardiac Mass. Medicine & Science in Sports & Exercise,46(1), 42-50. doi:10.1249/mss.0b013e3182a38da8
- Milanović, Z., Sporiš, G., & Weston, M. (2015). Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials. Sports Medicine,45(10), 1469-1481. doi:10.1007/s40279-015-0365-0
- Wilson, J. M., Marin, P. J., Rhea, M. R., Wilson, S. M., Loenneke, J. P., & Anderson, J. C. (2012). Concurrent Training. Journal of Strength and Conditioning Research,26(8), 2293-2307. doi:10.1519/jsc.0b013e31823a3e2d
- Balabinis, C. P., Psarakis, C. H., Moukas, M., Vassiliou, M. P., & Behrakis, P. K. (2003). Early Phase Changes by Concurrent Endurance and Strength Training. The Journal of Strength and Conditioning Research, 17(2), 393. doi:10.1519/1533-4287(2003)0172.0.co;2
- Borsheim, E., & Bahr, R. (2003). Effect of Exercise Intensity, Duration and Mode on Post-Exercise Oxygen Consumption. Sports Medicine, 33(14), 1037-1060. doi:10.2165/00007256-200333140-00002