For those that don't know HIIT training stands for High-Intensity Interval Training and has become an extremely popular training method for exercise enthusiasts and athletes alike; and for good reason.
If we were to define HIIT we would say something like
"Maximum effort intervals to achieve muscle fatigue and maximum oxygen use in a quick burst of exercise"
- sound familiar? It should. If you've been to any AOD PT sessions or received programming off us it's likely you've been exposed to said "death" intervals. We even have a HIIT Programme that incorporates this type of training entirely. But the negative connotations are the only bad thing about this type of training. Because, as you'll see below HIIT training has some HUGE benefits, backed up by the science of course:
Optimal muscle building and muscle retention during fat loss
Maximum increased metabolic rate
Increased calorie burn during and after workouts
And it's that last point I want to elaborate on...
What Factors Effect Our Endurance?
Before we get into HIIT any further, it's probably worth understanding what factors actually help make us fit and what factors can actually help us "burn fat"(i hate that term). So, know your cardiovascular performance is based on three primary factors:
Heart Rate (beats per minute)
Stroke volume (amount of blood per beat)
Heart contractility (force per beat)
As each of these variables increases, we increase the amount of blood and oxygen being pumped towards the working muscles. The above factors have been shown to improve significantly using aerobic training protocols  but are these the only things that affect our endurance? You may have guessed, not quite.
Recently, a lot more studies have been published referring to other factors that could highlight the way for endurance performance such as your mitochondria size, density & the number of oxidative enzymes.
Ok, well that sounds well and good. But didn't you promise this article would help me burn fat too? Firstly, let's start by rephrasing that. We don't burn, but rather, use fat as a fuel. Now since our mitochondria are responsible for creating energy by using carbohydrates and fats as fuel - I'd say we're barking up the right tree (but more on that below). Let's get straight into some studies to see how HIIT training faired against more typical aerobic training.
Can HIIT Trump Cardio For Endurance Performance?
Your mitochondria are the powerhouses of your cells. They use oxygen to manufacture high levels of ATP (energy) by breaking down carbohydrates and fats. Within the mitochondria are certain oxidative enzymes (citrate synthase, malate dehydrogenase and succinate dehydrogenase). These enzymes are responsible for more effectively breaking down fat and carbohydrates for fuel; thus accelerating ATP and energy production.
Let's look at one study  which compared 1 vs 2 for oxidative enzyme activity (promoting more effective metabolic function of the skeletal muscles - particularly by causing more effective fat and carbohydrate breakdown for fuel).
4 x 30s maximal-effort cycling sprints each followed by 4.5 minutes recovery, three times a week (HIIT)
40-60 minutes of steady-state cycling (65% VO2 max), five times a week
Well, it may come as a surprise to you that the levels of oxidative enzyme activity were significantly higher in the HIIT programme - even though they were only training for a fraction of the volume!
This increase in oxidative enzymes can actually change your metabolism causing you to oxidise more fat during exercise; which may seem controversial as high-intensity exercise is normally associated with a carbohydrate fuel source. But multiple studies [4,5] have shown that after a few weeks of HIIT training, fat burning increases significantly and carbohydrate burning decreases significantly.
So, there we have it. Multiple studies showing an increase in fat utilization as a fuel, increased oxidative enzyme capacity and increased mitrochodria size and density when performing HIIT training.
Excess Post Exercise Oxygen Consumption (EPOC)
But it doesn't stop there - check out how HIIT training can help you continue to burn calories even after you've finished exercising.
These high-intensity intervals are so intense in fact they can cause you to continue to burn extra calories for up to 48 hours after your workout. Here's why:
Anaerobic exercise is so intense that our body can no longer supply the muscles with enough oxygen. Once we have performed multiple reps at this intensity our muscles are in oxygen debt. We continue to pay off this debt post-training while are muscles are still recovering.
A review study  found HIIT training to have higher EPOC values when compared to continuous aerobic training - which translates to into more calories burnt after your workout is over.
It takes approximately 5 calories to consume 1 litre of oxygen and so the more debt you can create, the longer you will continue to consume extra oxygen post-exercise.
HIIT Your VO2 Max
So far, so good, but what about the cardiovascular factors we spoke about previously? Does HIIT training help our stroke volume or heart contractility? These studies looked at the effect HIIT has on your VO2 max.
Your VO2 max is essentially your bodies upper limit of consuming, distributing and using oxygen for energy production and rely on a host of cardiovascular and respiratory factors.
One of these factors is stroke volume. Which refers to the amount of blood pumped out of the left ventricle per beat. Well, one study  compared 4-minute high intensity running intervals(95-97% max HR), 3 times a week for 8 weeks against longer, slower distance training performed 3 times a week for 8 weeks. The results showed a 10% greater increase in stroke volume for the HIIT training group.
That's not all. Another study  showed a 13% increase in heart contractility & a 12% increase in left ventricle heart mass when performing HIIT training. Quite literally making your heart bigger and stronger.
So to summarise so far. HIIT training CAN be used as an effective way of utilising fat as fuel, upregulating our metabolism to burn calories post-exercise and increase endurance performance by improving:
Mitochondrial size and density
Oxidative enzyme capacity
Heart size (mass)
So what're the cons? Otherwise surely wouldn't every athlete, park runner and exercise enthusiast be using HIIT training to become a lean fitness machine? Well, do read on...
Should I Do HIIT Training?
HIIT training is extremely popular due to it being an incredibly effective method for improving fitness in a short time. However, it's also extremely taxing on the body. As you've read above a lot of these HIIT sessions involve training near or at some sort of max. Be it your max HR, a % of your max VO2 or somewhere in-between; performing high-intensity workouts all the time can be extremely taxing on your central nervous system (CNS), muscles and joints. So adequate rest between sessions is paramount to ensure your body recovers fully.
However, HIIT doesn't win in all aspects of fitness, one study  had 20 untrained individuals run three ties a week for 6 weeks. Half ran at a steady (65% VO2 max) intensity, starting at 30 and building to 60 mins; the other half did 30-second sprints with 4 minutes rest, starting at 4 reps and building to 6.
Here's the takeaway from the results:
All participants improved their VO2 max by 12% & their 2k time by 5%
The HIIT group lost twice as much fat
The endurance group improved their cardiac output by 9.5% while the HIIT didn't increase it at all
HIIT training will maximise the amount of blood your muscles can ustilize but aerobic training will help maximise they amount of blood your heart can send to your muscles.
So to get the most out of endurance performance you'll want to incorporate both styles of training. If it's purely fat loss you're after though; HIIT has shown to be superior.
If you have any questions involving HIIT training reach out to us and ask away by leaving a comment on our blog. If you're ready to get started on your HIIT training programme you can sign up by clicking here.
Thanks for reading,
 - Bartels, M.N., Bourne, G.W., & Dwyer, J.H. 2010. High-intensity exercise for patients in cardiac rehabilitation after myocardial infarction. Physical Medicine and Rehabilitation, 2 (2), 151-55.
 - LaForgia, J., Withers, R.T., & Gore, C.J. 2006. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption. Journal of Sports Science, 24 (12), 1247-64
 - MacPherson RE, Hazell TJ, Olver TD, Paterson DH, Lemon PW., 2011. Run sprint interval training improves aerobic performance but not maximal cardiac output. Medicine and Science in Sports & Exercise. 43(1):115-22.
 - Perry, C.G., et al. 2008. High-intensity aerobic interval training increases fat and carbohydrate metabolic capacities in human skeletal muscle. Applied Physiology, Nutrition, and Metabolism, 33 (6), 1112-23.
 - Talanian, J.L., et al. 2007. Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. Journal of Applied Physiology, 102 (4), 1439-47.
 Helgerud, J., et al. 2007. Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine and Science in Sports and Exercise, 39 (4), 665-71.
 Slørdahl, S.A., et al. 2004. Atrioventricular plane displacement in untrained and trained females. Medicine & Science in Sports & Exercise, 36 (11), 1871-75.
 MacPherson RE, Hazell TJ, Olver TD, Paterson DH, Lemon PW., 2011. Run sprint interval training improves aerobic performance but not maximal cardiac output. Medicine and Science in Sports & Exercise. 43(1):115-22.