Interval training. Speed Work. High-Intensity Interval Training (HIIT). Whatever you call it, performing fast interval work is often associated with anaerobic training. Many coaches, athletes, and trainers believe that the harder the effort, the more anaerobic the workout is. Take your classic Tabata sprints (10 sec on/20 sec off) or the football favorite “gassers,” highly fatiguing but are you utilizing more aerobic or anaerobic energy as the workout progresses?

If you answered anaerobic, it’s a common misunderstanding of physiology. We conflate the feeling of fatigue–the burning muscles screaming at us to stop– with the energy system being used. Most athletes and coaches assume, the more fatigue I feel, the more lactate is accumulating, thus the more “anaerobic” energy I’m utilizing. The reality is the opposite.

When we look at intense exercise, we actually utilize more of the anaerobic glycolysis system at the beginning of the race than at the end. Let’s say we are running a mile and the gun goes off. The intensity of the event determines our energetic demands. The higher the demand, the more we have to tap into all of our systems to meet the demand. If we are running a marathon, after we settle into our rhythm, the aerobic system can supply the vast majority of the energy because it is well within its capacity. Run a mile on the other hand and our aerobic system can’t meet the demands.

As we dart off the line, our immediate energy sources (ATP, Creatine Phosphate, etc.) dart into action supplying the majority of the energy for the first few steps. As we dart off the line our anaerobic or glycolysis system (whichever nomenclature you prefer is fine), as well as our Aerobic system jump into action. The problem is they take different amounts of time to rev up to full capacity. So our anaerobic system jump starts pretty quickly and takes it’s fair share of the load quickly. So 200 meters into our mile race, we’re relying primarily on anaerobic processes while our aerobic system works in overdrive to meet the demands. Our “slow” aerobic system might take ~90 seconds before it is fully revved up and working at its full capacity. Once it’s fully revved up, then we have to cover the gap between the energy needed (based on intensity we are running) and what our aerobic system can supply with our leftover supply of anaerobic energy. As that begins to fade towards the end, we might slow down as our aerobic system alone can’t match the full demands.

What in the world does this all mean? That in the beginning, when we are the least fatigued, we are actually relying on more anaerobic energy, while when we are at our most fatigued, our anaerobic system is likely faltering a bit, and we can’t cover that energetic gap. As we progress in the workout or race, fatiguing by-products may continue to increase as the energetic supply gap persists.

Now think about this same dynamic in terms of interval training. We tend to assume that if we do a HIIT session, we are training “anaerobically,” the reality is that we often are training the opposite. If we take our set of 10-20 second reps “all out” (Which is another pet peeve and dumb thing to do), number one might be almost entirely anaerobic, but by rep 3 or 4, we’re cranking along on aerobic energy, especially if they went “all out” the first rep and now are slowly dying. Don’t take my word for it, researchers have known this for decades, thanks to some innovative studies. So the next time you see a bunch of football players running gassers, what are they actually training? It’s not “speed!” It could be tolerance of fatigue or by-products, but we also get a challenge of the aerobic system, as well.

What is often lost is that our HIIT gurus think “Oh! So this trains the aerobic system, so all we need to do is do intervals and never run slow or steady! No more jogging!” The problem with this rationale is that the aerobic system isn’t an all-encompassing box that we check off. There are a myriad of ways to attack the development of it, and each different way will work on a different weak link. In some ways, you might get development of mitochondria in the slow twitch fibers, in others the fast twitch fibers. Another aerobic workout might help with lactate transport into and out of the cell, while another will directly attack the cardiovascular system. We could go on and on, but the point is that solely from a physiological standpoint, there isn’t just one magic aerobic workout. And we haven’t even considered the psychological, neural, or even specificity related components of training.

While this post is mostly to educate, there are profound implications. Don’t fall into the trap of thinking that interval work is all anaerobic work. It depends entirely on how you set up that interval set. Take long rest in between, and you’ve just shifted the demands of the workout. The magic is in the manipulation of the workout, not in the acronym or label that we assign it. Secondly, from a team sports conditioning standpoint, PLEASE consider what the effect of the workout you are designing is. Don’t simply assign HIIT intervals because they are the latest fad. Don’t simply try to get your athletes tired, for the sake of toughness. Understand what the workout you are assigning actually does.


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    1. Michael I. on November 20, 2017 at 9:46 am

      I really like your approach to things, but will the predominant bias ever change? There is so much “out of context” information out there, and if you’re the type that only scratches the surface, then you often look for the truth that suits you best. I am sorry to say, but I think that we have to go back to teach in the first chapters, so to speak. I don’t think that the vast majority have the right prerequisites to evaluate what information they are reading. That’s also one of the reasons why I admire your approach to sharing knowledge like the last post. Sharing your favorite literature with all of us signals high levels of confidentiality.

      Anyway, this blog and the past books have inspired me a lot over the past several years with a coach approach, which does not necessarily focus on running alone. Of course it is often centered in the first place about running, but the overall approach is more universal and works throughout the field of performance – thanks!

      Michael I.

    2. Gary on November 20, 2017 at 1:01 pm

      Love it, what I do is train often and vary my training, both aerobic and anaerobic will gradually improve. Flexibility of training is much more enjoyable too 😉


    3. Paul on November 21, 2017 at 5:10 pm

      Interesting and all makes sense when you break it down like this. However, do you consider the metabolic cost of the workout. i.e. the training effect on the mitochondria relating to the type of fuel they primarily metabolise due to intensity of exercise. As I understand it, the base phase works due to training predominantly in an aerobic zone which improves the mitochondria ability to metabolise fuel aerobically which benefits work at all intensities. . . As you outline, even higher intensity work can be largely aerobic, but due to the intensity, the and the higher energy demand, it doesn’t improve the mitochondrial density or ability to metabolise fuel as much, and will tend to degrade such capacity which has been created by high volume aerobic exercise. Is this roughly correct?? . . Therefore, should such high intensity (even largely aerobic, intervals etc) training really be used sparingly to sharpen for racing, and not often during a base building phase?

    4. […] Kilde: Science of Running – A Misunderstanding of High Intensity Interval Training […]

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