Train like a beast. Train harder. No pain = no gain. That’s what you can see nowadays on every second social media page. Pure focus on high intensity training seems to be the answer to getting fit, strong, fast, gorgeous and overall great athlete.

But is it really?

To be fair, intense sessions do feel good in the moment. While training for shorter races I used to go all out on every session. Isn’t that what athletes are supposed to do? I felt I was killing it and getting stronger every day. Yet, when it was time to race I didn’t seem to have that spark, energy, speed or positivity I had just a couple of weeks earlier.

As I started researching it and talking to exercise specialists and physiologists I found out it’s not only about training harder. For competitive athletes it’s rather about holding yourself off to avoid over training. Consistently. Not so simple anymore, isn’t it?

There is one key little detail I’ll share in this post that helped me to finally optimize my high intensity training. And now that I train for long distance races it’s more important than ever.

Low vs. high intensity training – what’s the difference?

Human body can produce energy in 2 different ways.

When exercise intensity is low or moderate, energy is generated by oxidizing fat in mitochondria of the muscle. This process is slow and requires oxygen, therefore is referred to as aerobic.

Training at low and moderate intensity improves mitochondrial density of muscles and builds cardiovascular pathways. This trains the body to be more effective at generating energy aerobically.

Marathon runners (even elite ones) spend more than 90% of the race in aerobic mode. For them it’s critical to focus on low intensity training to build muscle efficiency and economy.

Long low intensity training sessions (like easy cycling) train the muscles to be more efficient.

As the intensity of the exercise increases, mitochondria can no longer produce energy fast enough. Instead, the body starts to split glycogen stored in muscles and liver into glucose and use it to produce energy. This process does not require oxygen and is referred to as anaerobic.

Glycogen stores contain only around 350 grams of carbohydrates (1,400 kcal of energy), which is enough for roughly 2 hours of intense exercise. When an athlete starts to run out of it his body begins to ‘shut down’.

Often referred to as ‘hitting a wall’, this tend to happen after 30 to 35 kilometers in the marathon when athletes already spend 2 hours racing.

High intensity training builds power in the muscles, making them stronger and more resilient to the build up of lactic acid. In short races it helps to maintain really fast speed for longer. And, of course, it means more speed in both long and short races.

How to measure training intensity

Training intensity as a concept may sound quite ambiguous. Who knows what exactly 70% of maximum is, right?

As a matter of fact, there are several ways to quantify it.

The best and most precise way to see how your body responds to training intensity is to take blood samples before, throughout and after the session to determine the level of lactate.

Generally, anything above lactate level of 2-3 mmol/L is considered high intensity. That correlates to heart rate Zones 4 and 5.

A much easier and more convenient way to measure training intensity is to use heart rate as a guide. The higher the training intensity the faster the heart beats, moving the blood to muscles.

Exercise heart rate zones categorize every intensity level and provide a very easy framework to follow during the training session.

Related: Train Smart – A Practical Guide to Exercise Heart Rate Zones

More experience athletes would also benefit from using speed or power to measure their exercise intensity. Unfortunately, it does not account for decreased performance caused by accumulated fatigue or illness. So, in this case knowing your body, how it responds to stress and when to back off is critical.

Bottom line is, whichever way athletes prefer to measure training intensity, the most important thing is that they actually do. Knowing exactly how hard you train serves as a reference point and allows to take the most of every session.

Physiological impact of High Intensity Training (HIT) on the body

Intense exercise taxes athlete’s body reserves quite a lot. While low intensity training can require as little as simple refueling, high intensity training (HIT) may require several days before body is ready to train again.

In addition to burning glycogen stores, HIT creates bigger micro traumas in the muscle tissue that take a while to heal.

At high intensity blood glucose is converted into pyruvic acid that is then converted into energy and lactic acid as a side product.

HIT (or HIIT) is a misunderstood concept. Many athletes strive to exhaust their muscles, which only leads to loss of endurance.

What happens after makes all the difference. Lactic acid in itself is quickly split into lactate and positively charged hydrogen ions (H+).

These hydrogen ions create a ‘burning sensation’ in the muscles and are the cause of all problems. Accumulation of these molecules lead to decrease in performance and speed.

Which is why it’s, sadly, not possible to run very fast for too long…

What happens after too much high intensity training?

Accumulation of H+ ions is basically accumulation of fatigue. It’s not the end of the world (or training session, for that matter).

At low intensity mitochondria in the muscles are very good at utilizing H+ ions quite well. It’s at high intensity when most of the athletes take it too far and exhaust themselves thinking it provides a better training benefit.

High intensity training is done in intervals to avoid accumulating too much fatigue at once. This is why it’s often called High Intensity Interval Training (or HIIT).

Related: How To Measure Post Workout Recovery And Avoid Accumulated Fatigue

Theoretically, as long as the athlete doesn’t accumulate fatigue, he can perform indefinitely even at high speed. If he consumes enough nutrition, of course.

However, there is one thing I ignored for too long and learned the hard way:

The worst possible thing an athlete can do for endurance is to allow the muscles to reach exhaustion.

What too much high intensity training or training to exhaustion does is produces more H+ ions than mitochondria can handle.

Overloaded with these molecules, mitochondria are forced to work extra hard to utilize them. This even leads to the burnout of some of them, causing loss of endurance. So, all that hard work to build endurance goes to waste.

I got this wrong for myself for too long. I trained until exhaustion thinking I’m getting stronger, when actually I was just taxing all my reserves and compromising all the endurance it took so long to build.

How to include high intensity training into a training plan

Focusing on high or low intensity alone is not efficient.

If all training sessions are done at high intensity, muscles don’t have enough time to recover and endurance will suffer. On the other hand, focus only on low intensity work does improve endurance a lot, but will do little to improve top end speed.

While hard training sessions is what we see most often on TV or on elite athletes’ Instagram channels, in reality it’s only a small fraction of what they actually do.

It’s the most eye-catching, for sure, which is why it works so well for advertising.

Best way to balance a training plan and maintain motivation is to include low intensity activities and focus on having a good time with friends.

If we take into account all what athletes do, they spend vast majority of time at low intensity, focusing on general endurance and strength, technique, flexibility, tactics and other skills.

Apart from recovery and endurance benefits, lots of consistent low intensity training also helps athletes build mind-body connection. Repeating a movement over and over for thousands of times improves and optimizes the form and helps to build supportive tissues, which then translates into muscle economy at higher intensities.

Not to mention that low intensity helps to improve muscle efficiency and speed up recovery process, allowing athletes to tolerate bigger training load and improve even more.

The 80-20 rule

In reality, athletes do a lot of polarized training when they focus on very short but very hard efforts (several seconds to several minutes) or long and easy efforts (half an hour and longer).

Balancing the overall training load is a form of art. It is really tricky to get that training intensity just right, to have a significant improvement, but avoid over training. In many cases it’s trial and error.

Every coach tends to keep their specific ‘recipe’ a secret, which is why it can be challenging to create your own training plan without past experience.

A good place to start is to dedicate 80% of the time to low intensity work and spend 20% doing high intensity training.

Of course this ratio will vary from session to session or even from week to week. As long as over prolonged period of time (several weeks to a month) it totals to 80-20, the training plan will be balanced and athlete will have enough energy to recover and go hard where needed.

Specific split between each of the training zones will depend on the training phase and the goal race distance.

For instance, in the off-season or early season athletes may spend a lot of time in Zones 1 to 3 to build up the aerobic base and Zones 4 and 5 may only account for 5% or so. Closer to race day, however, athletes may spend more time doing high intensity training to reach the most optimal condition for the race.

What is your experience with training intensity? Have you got yourself over trained or in peak condition at exactly the right time? Do take a moment to share it in the comments below.