The Science of HIIT Cardio
What in the world do these crazy acronyms HIIT and LISS mean? HIIT stands for high intensity interval training, which consists of short sprint intervals coupled with low-moderate intensity work. An example of this would be a 10-30 second sprint followed by a 3-5 minute steady pace walk to cool down and bring your heart rate back to normal and then repeating it. LISS stands for low intensity steady state cardio, which consists of purely low-moderate intensity work. An example of this would be walking on the treadmill or riding the bike and being able to hold a conversation. Now that you have a basic understanding of the two forms, let’s dive into some more detailed stuff.
Lactate and Anaerobic Thresholds
Why testing the lactate threshold (LT) and anaerobic threshold (AT) is a good idea. The AT and LT are extremely powerful predictors of performance in aerobic exercise (cardio). (1) There are 2 ways that muscle can burn glucose (blood sugars) and that is through aerobic work (with air) and anaerobic work (without air). For example, long bouts of LISS cardio is considered aerobic work and weight training or HIIT cardio can be classified as anaerobic work.
The AT and LT are a great test for HIIT and LISS cardio because it gives a great predictor of which type of work produces ATP (Adenosine Triphosphate). ATP is a quick burst of energy that we get in our muscles when we contract them (Example: every time you do a bicep curl, you are getting a quick burst of ATP).
HIIT produces better changes in exercise capacity as opposed to LISS cardio. High intensity training will hit the AT and LT, that’s what causes the body to make metabolic changes. When you are doing LISS, you are considered below the AT and LT. A simple test is being able to hold a conversation while doing cardio. When doing HIIT you are above the AT and LT and when you are above the AT and LT you push for greater improvement in metabolism which thus leads to better fat loss over time.
So why not do a more intense, more time efficient, and more enjoyable form of cardio? Why sit there and dread doing cardio when you can look forward to it and have a variety of different modalities to choose from? When we say variety, we are talking about 15 different forms of HIIT cardio modalities that we will demonstrate for you further in the guide. Doesn’t some good olé car pushes sound more appealing than running on a treadmill? Let’s delve into some research and science on HIIT cardio.
Time Efficiency with HIIT Cardio
With lack of time being one of the biggest elements people always identify as “what’s wrong” with their health, HIIT cardio seems to be the most time efficient exercise strategy.
(2) A study by Metcalfe et al. showed if you perform what Metcalfe and colleagues call the “minimal amount of exercise for improving metabolic health” a 3 time per week, 10 minute exercise regimen, with no more than two all-out sprints, everything you’ve got, you will make changes to your metabolic rate. This 6 week exercise program was compared to the results of a 10 month intervention program in subjects who exercised 3 times a week for 40 minutes (steady state). Metcalfe’s study goes to show that it’s a more efficient way to burn fat by doing 3 times per week for 10 minutes with only 2 all out sprint intervals because the steady state endurance group was not only four times more time-consuming, but it also failed to improve the glucose tolerance test and produced no improvements in insulin sensitivity.
(3) Moreover, Trapp et al. Showed 20 minutes of HIIT, performed three times per week for 15 weeks compared to the same frequency of 40 min of steady state intensity training was associated with significant reductions in fasting insulin, total body fat, subcutaneous leg fat, and abdominal fat.
(4) Another study by Burgomaster et al. demonstrated that short interval training is a time efficient strategy to induce changes in whole body and skeletal muscle carbohydrate and fat metabolism during exercise that are comparable to changes induced by traditional endurance training.
The above evidence shows that high intensity intervals induces fat loss in the most time efficient manner with half the time commitment compared to regular steady state cardio. If time is a huge barrier for you, then HIIT would be a viable option.
HIIT for Fat Loss
Another big element people always identify as “what’s wrong” with their health is producing results, which in this context will be achieving fat loss. HIIT cardio seems to be the most optimal exercise strategy to achieve fat loss when performing cardiovascular aerobic work.
(3) In a 2008 study, the researchers showed that high-intensity intermittent exercise led to a significant decrease in central abdominal fat, whereas the steady state intensity and control group had non-significant decreases in central abdominal fat.
(5) To support this, research from the University of Lethbridge showed that six weeks of HIIT three times per week reduced both body fat and waist circumference.
(6) Furthermore a study conducted by Wilson et al. From the University of Tampa, shows when you add in LISS you get a temporary boost in weight loss. Subjects lost a couple of pounds the first week and after that they lost nothing. This happened because their metabolism completely adjusted to that and it became their new set point to what they had to do just to maintain.
A huge adaptation that HIIT cardio makes is small changes to your metabolism as well as activates fat burning enzymes. If you want to change your metabolism, you have to increase muscle mass and increase your muscle’s oxidative capacity. Your muscles have these energy producing units called ‘mitochondria’ and this is where ATP are made and fats are burned. The more mitochondria you have and the more active they are the greater oxidative capacity you will have for fat loss. HIIT increases mitochondrial capacity and you actually increase the amount of mitochondria you produce.
(7) In a new study by Perry and colleagues showed that interval training, repeated bouts of “all out” exercise (HIIT intervals), result in comparable adaptations in both performance and skeletal muscle metabolism. Specifically increases in markers of mitochondrial density in muscle which could improve fat loss capacity. The mitochondria is known as the power house of cells, where fats are mainly oxidized. It seems these short “all out” sprints increase the skeletal muscle mitochondria and allows fats and carbohydrates to be used more efficiently to burn fat and thus improve fat loss capacity. These findings are also supported by Scribbans et al.
(8) Dr. Layne Norton stated “The body is very adaptive. We tend to see a lot of people doing hours and hours a week of LISS and according to calculations they should be losing pounds, but they can’t lose anything because your metabolism adjusts to low intensity exercise. It just doesn’t cut it because it’s just a calorie burn at that time, not 24 hour energy expenditure like HIIT cardio induces.”
If you do LISS all the time, you’re basically trading calories in and calories out and you can cut these same calories through diet and still get the same effects. For example: You burn 200 calories over 30 minutes of LISS, you can cut out 200 calories through carbs or fat and basically get the same effect as opposed to getting a 24 hour energy expenditure through HIIT cardio.
(3) We will leave you with Trapp et al’s conclusion:
“The high intensity interval training produced a greater fat reduction effect compared to steady state intensity, this supports the results from Tremblay et al. Despite exercising half the time, HIIT subjects in the present study lost 11.2% of total fat mass with steady state subjects experiencing no fat loss. Collectively, these results demonstrate that intermittent sprinting compared to steady state is a more effective and efficient way of controlling body composition.”
HIIT for more Muscle Retention
A very key element when dieting is muscle retention and keeping metabolic rate elevated. The more muscle mass you have, the better the chance you have for keeping metabolic rate high. Thus you must be able to retain muscle mass when in a calorie deficit. HIIT cardio seems to be the best exercise strategy for muscle retention.
(3) A study by Trapp et al. showed subjects in the high intensity interval group gained lean mass in their legs compared to the steady state intensity and control group.
(4) Moreover, Burgomaster et al. stated that the very intense nature of sprint interval training might stimulate rapid skeletal muscle remodeling (possibly due to fiber recruitment), whereas adaptations to lower intensity accrue more slowly.
(6) Furthermore, Wilson et al. found that LISS caused more muscle loss than HIIT. HIIT caused more muscle retention because when you are doing LISS (say fast paced walking) you’re not activating muscles the same way as if you were lifting weights. So when you sprint you have hip flexion, knee extension, and these are all weightlifting movements. Think squats, leg presses, leg extensions, etc. HIIT is another way to overload the muscle and activate the type II muscle fibers.
The above study by Wilson et al. is hard to argue with because the point about HIIT activating hip and knee movements. Hip flexion and knee extension are the same movements when doing leg workouts. Also, by doing high intensity work you are activating muscle fibers and anytime you activate muscle fibers you are primed for growth. LISS unfortunately can’t stimulate muscle fibers the same way.
Now if you have never performed high intensity intervals, you will notice that the burning sensation is very similar to a high rep leg workout. This supports one of the three mechanisms of hypertrophy, cell swelling[i]. Think of cell swelling like a balloon, you keep blowing air into it, and it will eventually pop. When the cell is getting stretched and it is also swelling, there are these other cells that sense this stress and stretch and they try to avoid this bursting of the cell from stretching and it then increases protein synthesis, decreases protein breakdown, and thus leads to hypertrophy. There are some trends with HIIT data leading towards causing some cell swelling. (9)
Now we know a lot of you have gotten the hint as to why HIIT cardio is more advantageous than LISS cardio for muscle retention, fat loss, and time efficiency. We know it seems as if we totally bashed LISS cardio to the ground, but keep in mind that this doesn’t mean LISS is useless. We’re big believers in doing both HIIT and LISS combined. Here are the following reasons why:
- You can’t do HIIT 5-6 days a week because eventually it will have a negative impact on your weight training and interfere with growth.
- Many people have legitimate orthopedic, cardiac, and even psychological reasons to avoid HIIT, so LISS is their only option.
- HIIT could be dangerous if not used correctly and could lead to injury.
- Everybody has different tolerance levels for HIIT, some can do more than others.
- HIIT and LISS on either a combined, cyclical, or rotational basis seems to be the best formula in our opinion.
HIIT is quicker and more time efficient, proves to be more effective for fat loss, creates metabolic changes, and helps with muscle retention but not everybody can do HIIT. LISS is safer, but takes twice as long to accomplish similar things and it still has its place for fat loss in moderate amounts, from a pure calorie burning standpoint (meaning only to burn calories & not make changes to your metabolism). Then again, many people enjoy doing LISS cardio for cardiovascular and relaxation purposes and there’s nothing wrong with that.
So to sit there and say that HIIT is hands down more superior than LISS for improvement in body composition is as bad as saying that 6 reps per set is better than 20. We’re firm believers that both HIIT and LISS cardio have unique benefits unto themselves. We feel they both should be incorporated into your routines since each have specifically different effects. Bottom line is doing the type of cardio that you have a personal preference for. Whichever one fires you up the most because you will most likely work harder at it, but always keep in mind what the data shows is more superior.
(1) Baechle, Thomas R and Earle, Roger W. Essentials of Strength Training and Conditioning/ National Strength and Conditioning Association. USA: 2008 by the National Strength and Conditioning Association
(2) Metcalfe et al. Towards the minimal amount of exercise for improving metabolic health: beneficial effects of reduced-exertion high-intensity interval training. European J applied Physio. 2011.
(3) Trapp et al. The effects of high intensity intermittent exercise training on fat loss and fasting insulin levels of young women. Int J of Obesity. 2008
(4) Burgomaster et al. Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans. J Physiol. 2008
(5) Hazell et al. Run sprint interval training induces fat loss in women. 2014
(6) Wilson, et al. Concurrent Training: A Meta-Analysis Examining Interference of Aerobic and Resistance Exercise. University of Tampa, FL. J Strength Conditioning.
(7) Perry et al. High Intensity Aerobic Interval Training increase fat and carbohydrate metabolic capacities in human skeletal muscle. Appl. Physio. Nutr. Metab. 2008
(8) Layne Norton. PhD
(9) Shoenfeld, Brad. The mechanisms of muscle hypertrophy and their application to resistance training. NSCA. 2010
Want to learn how to incorporate HIIT cardio into your training routine? Grab a copy of our ebook the Ultimate HIIT Cardio Guide.