Understand the Science to Create Great HIIT Workouts

High intensity training is great, until it is isn't! Learn the science of how and why to train max effort intensities.

Let me start off by saying, I love high-intensity exercise.  Let me jump, sprint, and power lift my way to exhaustion and I’m happy.  While this is a far cry from the endurance runner and marathoner I’ve been over the past 10 years, I discovered that short anaerobic bursts of work are where I thrive.

The challenge though with this type of training is that it requires a lot from the body.  It pushes the body to reach anaerobic threshold, maxes out fast twitch muscle fibers, and pushes VO2max levels to new heights.  While doing all this the body is producing massive amounts of lactic acid and EPOC (Excess Post-Exercise Oxygen Consumption).  Sounds fun, right?!

For us crazies out there who love huffing and puffing during out workout sessions, yes it is tons of fun.  BUT, that fun comes at a risk.  All of those lovely things that are going on during the anaerobic intervals push the body and can actually over time be too much for it.

But don’t they say high-intensity exercise is the best?  Don’t we want to maximize gains in the shortest amount of time?

The short answer to that is, yes.  HOWEVER, there is a MAJOR caveat to that yes.  While it is the best way to achieve results in the shortest amount of time, it is very important to take into account what the body is experiencing and provide it with the adequate rest, fuel, and rejuvenation it needs to perform at these levels.

The problem is that most fitness professionals don’t fully grasp (or care) about what those proper work:rest ratios are, use the term “Tabata” to simply mean training the body with a certain time protocol, and aim to push their team to throw-up zones EVERY workout session.  This lack of focus on the science behind training causes participants to not get the training benefits they desire, causes burnout in the body, and ultimately could result in injury.  The exact opposite of what we are aiming for when teaching group exercise classes.

Ask Yourself…

Have you taken the time to really learn and understand the physiology behind high-intensity interval training before teaching it in classes?

Can you reach throw-up zone?!

People who have taken high-intensity classes with me know that I have a MAX intensity zone that I refer to as the “throw-up zone”.  This is my simple way of describing that max effort push I want them to achieve.  I joke about it, but in reality that is the true goal and purpose of anaerobic conditioning.  It is to push this “throw-up zone” a little further away, so the next time you try to workout there you can go a little bit harder.

My classes know that we are only going to get there 1-3 times in a class, and only hold it for 10-30 seconds (because I tell them at the beginning, and then again throughout class).  I don’t want them to feel like they are going to throw up for a long period of time (because how miserable would that be), but instead for a short burst of effort that helps push them to their anaerobic threshold, burn out those fast-twitch muscle fibers, and improve their overall metabolic conditioning.

It’s tough, but it is a valuable part of anaerobic conditioning.

But have we taken throw-up zone too far?

As much as I like pushing people, I have to ask – have we taken this throw-up zone too far?  Have we started to ask participants who don’t have the aerobic conditioning level to begin with to get anaerobic for too long of periods of time?  Are we actually doing a disservice to our team by asking them to reach this?

I would argue and say it depends; but a lot of the times YES — today’s fitness society has almost given “throw-up zone” challenges a “badge of honor” and idolized them in a way that makes people believe that more is ALWAYS better…that they MUST achieve anaerobic threshold with every workout or it was a waste of time.  Or they believe if they don’t feel sore the next day then the workout wasn’t hard enough.

This is what people aim for as this is what our fitness industry has glorified over the past few years.  Look at the high performing programs on the market – CrossFit, Insanity, P90X, OrangeTheory, etc. – each of these thrive on the idea of pushing people to the point of complete exhaustion day in and day out.

What they fail to mention in these programs is that people shouldn’t be doing these workouts every day.  They minimally explain that the body needs time to rest and recover between these sessions with activities such as mobility and stability work.  And most importantly, they fail to discuss that this “balls-to-the-walls” mentality day in and day out is actually breaking down your body, and not allowing it to get stronger (since muscle growth actually occurs during periods of rest).

I don’t disagree that these programs are awesome, and the endorphin rush at the end is amazing.  However, over the past few years as I have pushed through one medical issue after another, I’ve learned how important rest, fuel, and rejuvenation truly are to maximizing efforts in High-Intensity Interval Training.  It isn’t easy to take advantage of rest periods, but from a scientific standpoint I understand that those down periods are what the body really need in order to feel good, perform well, and stay injury free.

Going too far in the Group Exercise Studio

The goal of this article is not to talk about how we as a population need well-balanced workout programs (that will come at a later date).  Instead, I want to focus on how this affinity for high intensity classes, Tabata, and little rest is transpiring in the Group Exercise studios.

During my time in the studio and as a master trainer, I have discovered that people are just bastardizing the terms HIIT, HIT, and Tabata.  They are using them to mean things different than how science intended them to be used and ultimately giving a disservice to participants because they are doing crazy wrong stuff.  I have been to classes where it actually scared me to see how the instructor had put together circuits, intervals, and blocks of work because it was so counter-intuitive to what science tells us the body physiologically goes through in a burst of anaerobic and aerobic work that I didn’t know what we were trying to achieve.

It is these instructors who are always just trying to get their team into throw-up zone again and again that are ultimately impacting the experience and negatively training their team members.

So what are these five most common mistakes you ask, well they include:
  1. Misuse of the terms High-Intensity INTERVAL Training (HIIT) versus High-Intensity Training (HIT)
  2. Not fully understanding EPOC and the power of the recovery period
  3. Not fully understanding the purpose of anaerobic training zones
  4. Crazy use of work:rest ratios
  5. Complete misinterpretation or misuses of a Tabata

Have you been guilty of any of those?  Have you gone into a high-intensity class without the basic fundamental knowledge of what proper work:rest ratios are?  Have you unknowingly taught a “Tabata” class with 4-8 “Tabatas”, not allowed participants to have breaks after hard work segments, or stopped caring about technique in order to maximize effort levels?

I challenge you right now to take a good hard look at the upcoming science lesson and not become just another instructor who just butchers the poor term HIIT.

Science Lesson!

Common Fault #1: Misunderstanding HIIT vs. HIT

Let’s first examine the difference between HIIT and HIT as they are both buzz words and understanding is key to developing purposeful and successful workout plans.

Understand the Science to Create Great HIIT Workouts 1High-Intensity Interval Training consists of alternating periods of short, intense anaerobic exercises followed by less intense recovery periods.  The heart rate should alternate between periods of 90-100% and drop back down to 50-70%.

Think of it like an old-school rollercoaster in which you have a gradual build-up of intensity to the top of the hill, then suddenly drop super-fast to the bottom.  Your heart pounds and energy surges for those few moments, but then you get to quickly recover as you go through another building phase and repeat.

Could you imagine if a rollercoaster was just one drop, after flip after fast turn over and over again?!  You would probably feel really sick and uncomfortable the whole time.  You would not enjoy the ride as much, because it is those periods of buildup that allow for the greater intensity in the fall.

Understand the Science to Create Great HIIT Workouts 3Whereas High-Intensity Training consists of completing exercises at maximum intensity for a specific duration of time.  Limited recovery is giving and exercises are all designed to maintain a heart rate of 85% or greater.

This is more like completing the NASCAR Indianapolis 500 race.  Drivers are pushing to maintain high rates of speed over a prolonged period of time in an effort to push max limits and beat the competition.  In essence, the body is maintaining a high level of steady state training.

While both have their place in training, when you hear about the benefits of HIIT training, including that of “increased caloric burn for the 24-72 hours post exercise”, it is the INTERVAL component of the training that is actually making this possible, with EPOC.

Common Fault #2: Not fully understanding EPOC and the power of the recovery period

The term EPOC is thrown around quite a bit in gyms everywhere…but do people REALLY understand what it means?  It is a complex topic that essentially has to do with the amount of oxygen required by the body during activity to produce the needed metabolic functions required during the training period.  This involves times when there is:

  1. Not enough oxygen for the demands of activity
  2. A leveling off period during the training cycle
  3. An excess amount of O2 being consumed after the completion of exercise

Take a quick look at this picture to get a better understanding of how this process works in both Steady State and Interval based workouts.

Understand the Science to Create Great HIIT Workouts 5

This image of a steady-state workout session includes an initial oxygen debt, which is the delay (or lag) in oxygen uptake at the beginning of a workout session.  You know the feeling when you first start working out and it just seems like you are breathing harder than you normally would with the same level of intensity?  This is the oxygen debt in play.  Essentially this is caused by a rapid increase in oxygen needed for the demands of the workout.  It also suggests that the anaerobic energy system comes out to play super early in exercise, producing a few ATP quickly.

Depending on an individual’s conditioning level, it can take anywhere from 1-4 minutes to reach the steady-state level of conditioning or the work phase of a training session.

After a hard bout of work is completed (either an entire steady-state training session or an anaerobic interval), the body produces what is known as excess post-exercise oxygen consumption (EPOC).  The easy way to think of this is after exercise your body still thinks it’s going to work hard, thus it continues to take in more oxygen to help with that demand.

EPOC is what many times is referred to when people suggest that interval training burns more calories in the 24-72 hours post exercise.  While there is conflicting research out there as to how long EPOC may stick around, it is true that high-intensity exercise does produce higher levels of EPOC due to:

  1. high levels of body heat generated during activity
  2. the level of phosphocreatine (PC) broken down and depleted in the muscle during activity
  3. levels of epinephrine and norepinephrine in the blood

There are many factors that might contribute to EPOC, including:

  1. Resynthesis of PC in the muscles
  2. Lactate removal
  3. Restoration of muscle and blood oxygen stores
  4. Elevated blood temperature
  5. Post-exercise elevation of HR and breathing
  6. Elevated hormone levels
In Practice

As you can see, when you are doing steady-state training you only have one point when you are in a period of oxygen surplus – at the end of the workout.  Therefore, there is only one point in time when the body is being called to work harder than it needs to for the demands of that activity (ie. rest).  Thus, this period of oxygen surplus will not burn extra calories for a long period of time.

Alternatively, when you look at an interval training workout, you have multiple points of work and recovery.  Thus the body is constantly going from having not enough oxygen, to having too much.  And it is this point when it has too much that true EPOC is able to kick-in.

Understand the Science to Create Great HIIT Workouts 7

In practice…

It is when you are actually lying on the floor AFTER throw-up zone that you are going to receive those benefits of EPOC.  It is for the brief moment when you are RECOVERING that you’re actually helping increase your VO2max.  Because without these points of recovery, you would not be able to push into that lactate threshold point again.

Thus, without points of intensity and recovery, the body doesn’t need to fluctuate it’s oxygen needs and therefore doesn’t need to produce greater oxygen demands for the hours after a workout is completed.

THIS is what must people get WRONG in their HIIT workouts.  They expect to achieve these gains by staying at 100% for 60-minutes, and science just doesn’t allow that to be the case.

Simply put, too many people are doing High Intensity STEADY STATE Training and not producing the necessary highs and lows to allow for greater physiological adaptations and that much desired caloric afterburn.

Common Fault #3: Not fully understanding the purpose of anaerobic training zones

I myself have been guilty of maintaining high levels of steady state training by creating drills and workouts that caused the heart rate to stay as high as possible for as long as possible.  At that point, I figured that was the only way to TRULY get to the next level.  It wasn’t until I studied for the NSCA Certified Strength & Conditioning Specialist exam and really dove into understanding training methods and work:rest ratios that I realized just how wrong I had been!

I realized that maintaining a max heart rate for a prolonged period of time was actually not allowing the body to reach maximum performance gains.  Instead of making my participants better, I was actually making them worse.  Instead of giving them better anaerobic conditioning levels, I was just forcing them to work at higher aerobic conditioning points.

Additionally, when I did ask them to push into an anaerobic zone, they couldn’t do it because their bodies simply didn’t have the energy stores needed to take it there.

Anaerobic exercise includes short bursts of energy that increase the heart rate to a very uncomfortable and breathless zone.  Bouts of work last from 10-seconds up to 3-minutes.  The longer the bout of work, the less anaerobic it becomes (lowering the heart rate).

There are two energy systems that produce fuel during anaerobic activity, these include:

  • Phosphagen system: super high to max intensity levels (90-100%), 1-10 seconds of speed and power moves
  • Anaerobic glycolysis: high intensity (80-90%), 2-3 minutes of speed and strength moves

Both of these systems produce ATP stores without the presence of oxygen, which is why they can only be sustained for a short period of time.

What are the anaerobic training zones

First, take a moment to watch this great video of Usain Bolt’s 100m World Record setting race.

Amazing, right?!

Now let’s think about Usain Bolt and that run as we discuss anaerobic training.  While he has a knack for making the runs look effortless and super easy, he is really pushing his body into the anaerobic zone (90%+ Heart Rate), and utilizing what is called the Phosphagen System of energy production.

That world record for the 100m is an amazing 9.58-seconds.  To put it in perspective, that is the equivalent of running 1-mile in 2:34 (a feat that has not happened)!  This is pure speed and power that he is pushing out for that short burst of effort.

Due to the super high demands on the body, the phosphagen system burns out at 10-seconds.  At that point it has produced only one poor ATP, and caused the body to gasp for air.  As much as Bolt may want to stay in that zone during his runs, after the 100m it is just not possible.

After those 10 seconds are over, the body simply can’t continue to produce ATP at that fast of a rate.  It yells and says “I need more ATP…Give me more energy”!  Thus, the Anaerobic Glycolysis system comes out to play.  This system allows the body to work at 85%-90% heart rate levels for up to 3-minutes while completing speed or strength activities.

For the anaerobic glycolysis system look over to the current 1-mile record holder, Morocco’s Hicham El Guerrouj, with a time of 3:43:13.

Let’s take a look!

Holy shit, he is running SICK fast!

But notice it is over a minute slower than Bolt’s time in the 100m (if he were able to maintain that speed).  This is because the body simply can’t maintain that push of speed and power for periods greater than 10-seconds.  Instead, it transitions to a small combination of anaerobic and aerobic energy supply, producing ~3 ATP over a period of 1-3 minutes.

This very challenging zone is where many group exercise classes try to live. Instructors and participants aren’t respecting how demanding this zone of work is on the body, and continue to try and push into it over and over again. Ultimately, this takes the body to a point where there is no physiological way it can maintain the levels of intensity that are being demanded of it.  It becomes “hungry” and demands energy stores (ATP) greater than what are able to be produced at that level intense work.

Even Dennis Kimetto, the current marathon record holder is only maintaining a pace of 4:41 of the course of 26.2 miles and 2-hours!  If you’ve got nothing else to do and want to watch an amazing race, here is footage of the entire race.  But if time is limited, just check out his amazing finish.

During the 2012 Winter Olympics I watched a great interview with one of the middle distance Speed Skaters in which he was talking about what it feels like to be in this anaerobic glycolysis zone.  He discussed that the point when you feel that burn in the legs, that tightness in the chest, and the complete agony of wanting to stop, that is when you know you are crossing into the lactate threshold and calling upon this energy system.  (I can’t find the video clip to let you watch it yourself!)


Make sure you fully understand when you want your team to be 100% max effort (the phosphagen system), 85-90% (anaerobic glycolysis), or <85% (aerobic system).  And ensure you are using the correct work:rest ratio’s (discussed next) when in these zones.

Common Fault #4: Crazy use of work:rest ratios

This is the place where you see classes go insanely wrong.  From work periods of 3-minutes with only a 10-second break to completing too many negative work:rest ratios (when the work is longer than the recovery period).  Instructors often believe that by removing the break periods (ie. recovery) they are able to further push their teams further.

When in reality the opposite is happening.  If you notice without a break, your team just can’t stay at the levels you are trying to demand of them.  Remember, you aren’t doing the class at 100%, so it may not feel bad for you, but out there if they truly are responding to everything you ask of them, they WILL burn out before it is over.

Oh, and how many times do you hear instructors fearful over giving rest breaks because they think participants may get bored?

While you may have a few of those ADD people in your class who you tell they’re going to go get water and next thing you know are wandering the floor trying to find something to do, the majority of your team will enjoy a break.  Many need it in order to go into the next block of work with renewed energy and the true ability to push max levels.

Oh, and if you really educate your team on how hard they should be working during the work phase then they will understand and value the recovery period you are giving them.

Please remember too that a recovery is either a COMPLETE recovery, in which the person just sits and breaths, or as an active recovery when you do lighter activity to rest the body part just worked (mobility, balance, and stability work great here).

What research tells us

As seen in research, properly spaced work-to-rest intervals allow more work to be accomplished at higher exercise intensities with the same or less fatigue than during continuous training at the same relative intensity.  Showing that more training can be accomplished at higher intensities with interval training.

One of the most challenging parts of interval training is determining the appropriate ratio of work-to-rest as research has shown benefits in a variety of different time intervals.  Therefore, understanding the time intervals and recovery periods for each energy system is key for maximizing intervals.

While definitive research still needs to be done to determine the best intervals, the below chart from NSCA outlines general suggestions for each energy system.

% effort
Energy system
Work duration
Recovery duration
Work-to-rest period ratios
10-seconds 30-60 seconds 1:3 – 1:6 6-10
Anaerobic glycolysis
1-minute 2-minutes 1:2 4-8
3-minutes 3-minutes 1:1 6-12
Common Fault #5: Complete misinterpretation or misuses of Tabatas

I could write a full article just on this topic (and probably will before too long), but for now, I’m going to keep this short and to the point.  Tabata is NOT just another interval training protocol.  Tabatas are NOT to be done multiple times in an hour.  Tabatas are NOT to be completed with single-joint, low-intensity exercise.

Why you ask??  Because this was not the intent of a Tabata!  If you want to achieve the benefits of using this style of training, then you sure as hell better do it correctly.

What the study did…

Did you know that Tabatas were actually based on a research study in an exercise science testing lab at a prestigious university?  They worked long and hard to discover how to best use the system…take a look!

Group 1: these guys performed moderate intensity (70% VO2 Max) steady state cardiovascular exercise for one hour on 5 days/week.  This would be along the lines of what most people would be accustomed to doing in the gym.

Group 2: they used the Tabata protocol which consisted of a 10-minute steady state warm up followed by 7-8 sets of 20 seconds at 170% VO2 Max on a mechanically braked cycle ergometer.  Subjects were given 10 seconds of rest between each set.

On 4 days of the week the Tabata group performed this exact protocol.  On the fifth day, they actually did 30-minutes of steady state exercise at 70% VO2 Max followed by 4 Tabata style intervals (20:10 x4).

The results found that Group 2 was able to produce higher levels of aerobic and anaerobic conditioning using these short burst protocols versus Group 1 who performed traditional steady state training.

Are you like me and love reading the actual research study, then check out the original Tabata Study.

Bringing the Tabata study to the Group Exercise Studio

While Tabatas ARE a type of high-intensity interval training, they are a very specific protocol and training method.  The athletes only did 1 bout of the Tabata protocol during their training sessions, for a total of 4-minutes of really hard work.  They completed the exact same exercise for all 8 rounds, aiming to complete as many reps or the same wattage efforts the last time as they did the first.

They were NOT asked to repeat that 2, 3, or 4 more times.  If they had tried, their body would not have been able to respond the same ways.  As discussed earlier in regards to anaerobic energy systems, it can not work at the same levels of intensity without adequate recovery periods.  This protocol is so challenging that the recovery period needs to be at least 24-48 hours.

It is also important to note that Tabatas are meant to push people to 170% of VO2max levels.

  • They are hard…
  • They are challenging…
  • You are supposed to be gasping for air, running for your life, and in throw-up zone…
  • This is a place where you can’t (and don’t want to) return to shortly thereafter
But Summer…

While you can argue with me and say that you teach your classes using the Tabata timing style of 20:10 x8, but can do multiple ones in your classes because you aren’t really pushing your team to their max levels of 170%, I would argue back and say then you are simply doing an interval.  Don’t call something a Tabata if you aren’t fully acknowledging and appreciating the system as it was designed.  That is the same as calling a Toyota Camry a Corolla because they have the same general make, look, and feel.  Nope, they are two different cars and need to be acknowledged as such.

You may also argue and say “well participants don’t know the difference”.  To that I say, of course they don’t…because YOU aren’t taking the time to teach them the difference.  Just like you probably don’t spend your free time reading up on different criminal cases and law practices (like my lawyer friends do), remember your participants aren’t spending time outside of the gym really learning different training protocols.  They are trusting YOU to give them the information and teach them about proper science and research.

Do your team a favor and don’t just follow the norm of bastardizing poor Dr. Tabata’s name and protocol by learning and teaching a well designed Tabata class.  Check out this great article from the Indoor Cycling Association about creating a true Tabata style cycle class.

Another really great resource for Tabata information can be found at

So what does all this mean?

Whew, that was a lot of information and a lot of science!  I hope you loved it all as much as I did!  But in all seriousness, this science lesson hopefully got you really thinking about your own classes and how you are including intervals, anaerobic conditioning, Tabatas and work:rest ratios in your classes.

A quick example of a class could include:
  • 20-minute active-dynamic warm-up (read more about the importance of those here)
  • 10-minute stability work
  • 10-minute interval bout
  • 10-minute strength work
  • 4-minute Tabata
  • 6-minute strength work
  • 10-minute balance & mobility cooldown

I would love to chat with you further and answer any questions you’ve got on the science behind high-intensity interval training.

Happy Teaching!


Oh, and if you felt lost during the read, don’t forget to check out the exercise science 101 review guide.


  • American College of Sports Medicine. (2010). ACSM’s Guidelines for Exercise Testing and Prescription. Baltimore, MD: Lippincott Williams & Wilkins.
  • Baechle, T. R. (2008). Essentials of Strength Training and Conditioning. Champaign, IL: Human Kinetics.
  • Iserman, M., & Walker, K. (2014). NETA: The Fitness Professionals Manual. Minneapolis, MN: National Exercise Trainers Association.
  • Laursen, P. B., & Jenkins, D. G. (2002). The Scientific Basis for High-Intensity Interval Training. Sports Medicine, 53-73.
  • Powers, S. K., & Howley, E. T. (2004). Exercise Physiology: Theory and Application to FItness and Performance. New York, NY: McGraw-Hill Companies, Inc.
  • Swain, D. P., & Leutholtz, B. C. (2007). Exercise Prescription: A casestudy approach to the ACSM Guidelines, 2nd Edition. Champaign, IL: Human Kinetics.
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