Training intensity – how HIIT may compromise progress

Training intensity

Putting in consistent hard work and not getting back expected results sucks. I’ve been there.

I listened to my coach and worked every interval as hard as I could. I felt great during training (mostly because I was working my butt off), but when races came I didn’t have enough speed and endurance to show even those times I had during training. 

The problem – incorrect training intensity.

After I started to read more about it and talk to specialists I started to understand the whole concept much better. Ultimately, it all comes down to this:

You need to give your body just enough intensity to stimulate adaptation, but not too much so that it doesn’t overload


Well that’s as helpful as your girlfriend saying “I don’t know what I want, but bring me something”. How should I know what’s “just enough”?

I get your confusion and my first reaction was exactly the same. As usual, the devil is in the details, so let’s look at it more closely.

Why training intensity is such a big deal?

Training or working out is about developing many aspects of our physical form – strength, endurance, speed, technique and other important skills. All of these areas cannot be trained with one universal exercise or a speed interval.

Every workout should be targeting an improvement in a specific area.

So it’s bad news. We can’t sit on both chairs – for every workout we need to choose one and after choosing we need to ensure that we are working out with the correct intensity throughout the workout to get those improvements.

When we train long endurance, for instance, we can’t at the same time train speed, because different muscle fibers are involved. To train maximum speed you need to have a short and very high intensity workout (intervals of 10 to 20 seconds), whereas for endurance you need to exercise at much lower intensity and much longer.

Nike, Adidas and other brand commercials don’t really help much here as well. From their angle it’s all about flying, doing the impossible, breaking limits and so on. This is confusing, as we don’t necessarily need to give all we got every single time.

That’s the worst mistake one can make in training.

How high intensity training influences our body

A little anatomy lesson before we get to the practical stuff.

At lower intensities our body produces energy in the presence of oxygen (aerobically) and mostly uses fat storage for that. This sounds good and handy-dandy for getting lean and stuff, but that process is rather slow. The good thing is that the body has the time to not only burn fat, but also recycle accumulated lactic acid or in normal language – recover.

When the intensity of our workout gets high (like when we are running from a bear or something) our body can’t transport oxygen that fast, so glycogen stored in our muscles and liver does the work instead (and our body moves into anaerobic work).

The problem is that glycogen stored in our muscles and liver is not infinite. According to various estimates these reserves are enough for around 2 hours of intense exercise.

Don’t get the wrong idea, though. When you run out of glycogen you can hardly move and it’s exactly why many marathoners are bonking after 30 kilometers (somewhere exactly past 2 hour mark).

So during an intense effort it depends on how good the body is trained to deliver energy from glycogen sources to muscles, which ultimately translates to speed.

There is just one thing in between.

During intense effort lactic acid starts to accumulate in our muscles, preventing them from using full capacity. In normal words, performance, speed and sometimes even technique start to deteriorate after a while. This is why you can’t run at maximum capacity for hours (bonkers).

I’m getting to the point, I promise.

Lactic acid accumulates exponentially in our muscles – at first it builds up slowly and then gets more and more noticeable. When it gets hard to breathe and you feel that your legs are getting stiff when you run, for instance, that’s when you know it’s there.

Finally, my point is:

  • when we train at low intensity we teach our body to better utilize oxygen, which allows to recycle lactic acid during the aerobic exercise. This results in faster recovery between intervals and also after a workout

This is why it’s always suggested to walk a bit after a hard run, or give preference to active leisure than passive one.

  • when we train at higher intensity we teach our body to prevent the lactic acid to build up super fast during the anaerobic exercise. This means that you will be able to go faster for longer.
Low training intensity workouts should be a major part of the training plan regardless of the sport you're in.
Low training intensity workouts should be a major part of the training plan regardless of the sport you’re in.

Intensity load

Balancing intensity load is a form of art, so every coach can be considered an artist. It’s really tricky to get that intensity just right and not over-train.

In that sense my coach is like Monet – as crazy as him. Somehow for me it was always much easier to over-train than under-train.

In most cases it’s trial and error until the proper balance between high and low intensities is found. And the fact that everyone is different doesn’t make life any easier.

When there’s too much intensity our energy supplies get depleted during the workout and it takes 1-3 days to fully recharge those. In particular, too much of HIIT training (High Intensity Interval Training) that pushes our bodies to anaerobic mode are very demanding from the energy and glycogen perspective and require a proper recovery.

Now honestly, who takes 2 days off after a workout, or makes one hard workout and 2 easy ones in consecutive days? Answer – elite and professional athletes do and so should we.

Elite and professional athletes sometimes have high intensity workouts several days in a row, but they also spend all remaining time of the day on proper recovery and sleep

Most of the people, however, have jobs, life and other commitments that put additional stress on the body, preventing it from recovering as fast as possible. Therefore, including too much of high intensity workouts will only make the situation worse and prevent us from progressing as fast as we could or expect to.

Interested how you are doing? Try measuring your blood pressure every now and then. If you have a crazy daily schedule I bet it will not be ideal (120/70) towards the end of the day when we tend to plan our workouts.

To prevent this from happening mix up your high intensity training with low intensity training, like easy and recovery runs. And make an emphasis on those lower intensity workouts. That’s where you build your aerobic base, economy and perfect technique that all will help in making you faster.

This principle works the same way for strength training as well. If you do a killer workout, don’t expect to be fresh the next day for another workout, unless you spent the previous afternoon having massages and lying on sofa like a Pharaoh with women feeding you grapes.

The key is to proportionally increase the intensity/workload and include recovery days and weeks where the intensity is dropped. This will allow the body to rebuild torn tissues and become accustomed to the increased workload and eager to take on the world.

When after recovery you feel that you got faster – that’s when you know you’re improving.

How to measure training intensity

The best and most precise way to understand how your body responds to intensity is to take blood samples before, throughout and after the workout to determine the level of lactic acid.

This is not something super convenient, right? In fact, this kind of tests are mostly done in laboratories.

Lab tests help to determine lactate threshold and training zones quite precisely. That's me at around 95% effort with a bleeding ear (they take blood samples from there every minute)
Lab tests help to determine lactate threshold and training zones quite precisely. That’s me at around 95% effort with a bleeding ear (they take blood samples from there every minute)

There are also tools like these that allow you to take your blood sample and check the level of lactic acid in the blood yourself. These are quite handy to test how well rested you are after previous workouts or how intense was the workout that has just ended.

Another way (and much easier) of checking the intensity of the exercise is by listening to your heart. The higher the intensity of the workout the faster the heart beats and transports the blood to all the organs and muscles.

To help you navigate the intensity ship by heart rate, there are 5 training zones that define various types of intensities. I’ll describe each and every one of those in this post.

Training intensity zones

Finally, we’re getting to the real stuff (yay!).

Training zones are very useful to categorize intensity and create a better and more structured training plan. Each intensity level, or training zone, triggers different processes in our body and trains different area.

I’ve added efforts range for every intensity zone, so that if you have a heart rate monitor you are able to control the intensity you are working out in.

Heart rate is different for every person and depends on many factors. So to calculate your personal heart rate zones check the next chapter.

Zone 1 – recovery and warm-up (50% – 60% effort)

At this intensity all energy comes from fat sources in our bodies. Training at this pace trains our hearts to become bigger and transport more blood, which then brings more oxygen to our muscles faster.

After this point it’s only the heart rate that increase, not the amount of blood transported.

Zone 1 is great for fat burning, warm ups and recovery.

Note, though, that you won’t burn all fat at once with one training. You need to spend quite a lot of time in this zone to see real results. 1 gram of fat contains 9 calories, so you need to spend 900 calories (or around 1:30 hours of easy running) to lose 100 grams of fat. 

Zone 2 – easy pace (60% – 70% effort)

This intensity trains our body to develop capillary network which will help to transport oxygen more efficiently.

It will lead to improved muscle economy and will allow the body to faster recycle the lactic acid that accumulated during intense effort (or from previous exercises).

This is why it’s often suggested to do an easy run or bike spin after intense training or even strength training – it helps to recycle lactic acid faster and, therefore, assist recovery.

One of the best ways to speed up recovery is a low intensity workout with couple of hard bursts to stimulate adaptation. Volleyball and other games are perfect for that
One of the best ways to speed up recovery is a low intensity workout with couple of hard bursts to stimulate adaptation. Volleyball and other games are perfect for that

Zone 2 is ideal for building general endurance, as well as for fat loss (the body still partly uses fat stores).

For reference, this is often referred to as “conversational” pace, or a pace that you can sustain for 4 hours (with food and drink, of course)

Zone 3 – aerobic threshold (70% – 80% effort)

Here we work on speed endurance and training at this pace lets us build that nice cruising speed to maintain during longer distances, like 10K, half marathon and marathon.

Aerobic threshold is the point at which lactic acid starts to slowly accumulate in our body (meaning our body cannot recycle it fully anymore), but not as fast that you lose speed significantly. You just start to feel that muscles get a little “pumped” after spending some time in this zone.

Note: All elite athletes spend 80% of their time in these 3 zones to have a solid aerobic base to build their speed on. This allows them to recover faster and be much more efficient in oxygen transport to muscles. What’s important is that it doesn’t matter if they run a marathon or 5K.

Zone 4 – anaerobic threshold (80% – 90% effort)

Somewhere between 80% and 90% of effort there is an anaerobic threshold where the lactic acid starts to build up even faster and our body needs energy much faster that it can produce in the presence of oxygen. As a result, the body starts to operate in anaerobic mode (no oxygen).

It is highly individual, so there is no specific number where exactly that threshold is that would suit everyone.

Training in this zone and around the anaerobic threshold in particular, improves speed and speed endurance. Adaptation to this training would mean that lactic acid would accumulate slower in our muscles which will allow us to maintain higher speed for longer.

Anaerobic threshold pace/heart rate can can be estimated as an average heart rate or pace during a maximum 5K run, or during a maximum 20-minute effort.

Zone 5 – maximum effort (90% – 100% effort)

Training in this zone focuses purely on maximum speed. Similarly as explosive work with 1-2 repetitions in strength training target the maximum weight you can lift, training at maximum intensity lets you develop that peak speed which you can reach. Read more about strength training in this post.

This effort should be reserved only for short intervals of up to 30-40 seconds. In fact, it was scientifically proven that you can sustain peak speed for only around 8 seconds.

I’ll write a whole post on specific workouts you can do to train in every zone, but for now this should be enough of theory.

How to calculate your own training intensity zones

I’ve mentioned effort in percentage previously, but that’s not only how you feel. That would be too confusing and vastly imprecise to train in “around 70% effort”.

The best way to determine your heart rate for each training zone would be, of course, to take a lab test, but even with estimation and trial and error you’ll be able to determine zones quite precisely.

Zones are based on the percentage of the target heart rate, that is calculated using the resting HR value. This is done to account for the current fitness level. The formula to calculate the effort is:

[ (220 – age) – Resting HR ] * XX% + resting HR

The (220 – age) part of the equation is the estimation of a person’s maximum heart rate. It can be imprecise as for serious athletes the maximum heart rate can be higher than estimated. If you know yours from lab tests or from your heart rate monitor feel free to use that. For me, at least, this estimation is almost precise and I don’t shy away from using it.

Resting HR is your heart rate measurement taken first thing in the morning before you even get out of bed to go to the toilet. This shows your overall fitness and how well rested you are, so make sure you check it several times on different days to make sure you have the figure that is not impacted by the workout the day before.

For example, a person named Ben is 30 years old and his resting HR is 50 (a well-trained guy, probably). His estimated max HR is, therefore, around 190 and 50% effort for him would be (190 – 50) x 60% + 50 = 134 beats per minute. 70% effort would then be 148 beats per minute. 

So for him to train in Zone 2 the heart rate should be between 134 and 148 beats per minute

Putting it together – how to get training intensity right every time

This is the part where I’ll start making sense (sorry it took so long).

To take the most from your training, intensity should be planned for a specific race distance. So once again, without a goal you can’t score. Only when we know where we want to go we can take steps to get there. Otherwise we’re just marching in the wrong direction.

For example, if you’re training for a marathon which should be run somewhere around your aerobic threshold for most people (70% – 80% effort), majority of your training time should be spent in that zone.

If you’re training for 5K, however, you need both endurance and speed, so your training should be focused on aerobic threshold for general endurance and then anaerobic threshold for speed endurance and speed intervals. 

I suggest to include high intensity and low intensity work in 20:80 percent proportion and make that high intensity work very focused on the distance-specific needs.

Also, to get consistent progress the training plan should be like an upward spiral where you give your body a significant training load and then recover for a while by reducing the workload to give it time to adapt. In the next cycle you give a slightly bigger load until you recover again.

Remember to include a recovery week where the volume is dropped, but leave some degree of intensity.

A so-called super-compensation is exactly this adaptation of muscles to the load that is slightly bigger than one those can tolerate (Read more about it in this post). To achieve this the plan has to be created with progression and recovery in mind

Remember that for beginners weekly volume increase shouldn’t be over 10%, so that muscles can adapt.

Periodization guide for race preparation

Proper race preparation requires training prioritization.

This means that the preparation plan has to be split in several phases each devoted to a certain area, for example:

  • Aerobic base building – before starting any high intensity work we need to ensure that our cardiovascular system is able to efficiently transport oxygen to our muscles, so that we can recover fast. For that we need a lot of volume in Zone 2. There is no clear number how long to spend in this phase, but I recommend around 20 hours of pure Zone 2 training.
  • Transition – once you’ve spent significant amount of time on aerobic base building, you’ll feel that you have gotten much faster at that effort level – good sign. It means you have also trained muscle economy. Now you can start adding longer intervals in Zone 3 (10 to 30 minute intervals). This phase should not last long, around 5-10 hours of work is enough
  • Focused work – now that you have prepared a base you can go into a more focused work for your distance. Here you can start adding shorter duration intervals to improve speed and speed endurance. Remember to balance high and low intensity work for optimal recovery and performance
  • Peaking and tapering – around a month before the main race you need to have the hardest week in terms of volume/intensity followed by several weeks of progressive recovery (called taper). During taper you slowly decrease the volume, but leave some degree of intensity. Last week should be roughly half of your average weekly volume (in hours or kilometers) and should consist of low intensity work with several short strides (up to 10-20 seconds).
  • Recovery – after the race/competition take it easy and enjoy a couple of days off to allow your glycogen stores to recharge and then get back to aerobic base building for a couple of days. After that continue in aerobic base building phase if you have nothing more scheduled or come back to focused work if you have races soon ahead.
Make sure you have an audio book or a podcast ready to listen to or a training partner to talk to while you are doing the aerobic base building work. It can get pretty boring alone with all the lo
Make sure you have an audio book or a podcast ready to listen to or a training partner to talk to while you are doing the aerobic base building work. It can get pretty boring alone with all the lo

So that’s about it.

I’ll write a more comprehensive post on training periodization and how to plan your workout schedule, but for now this should be enough for you to avoid over-training and making the most of your workouts.

Let me know how your training is going on and if you have been using this theory in your preparation in comments below. Or share it with others so that we all can get fast and strong. Thanks for reading!

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