Training Zones for Endurance

This article is a guest post from Hidde Weersma.  You can follow him on Instagram here.

What is Endurance?

The last article I wrote on this site focused on the ‘Interference Effect’ and how to optimally plan the combination of Endurance and Strength workouts. In this article we’ll dive deeper into training for endurance.

The definition for endurance I prefer the most comes from Wikipedia (yes, I use Wikipedia sometimes): ‘Endurance is the ability of an organism to exert itself and remain active for a long period of time, as well as its ability to resist, withstand, recover from and have immunity to trauma, wounds or fatigue.’ The reason I appreciate this definition is because it is very complete. To have great endurance an athlete does not only need to able to maintain a certain amount of physical effort over time, but also needs the mental capacity to deal with this effort and the ability to recover from one or multiple efforts.

For a HYROX athlete these qualities are of major importance. First, you need endurance to be able to maintain your physical effort over the course of the entire race. Secondly, you need endurance to recover from the different workstations. And finally, you need endurance to mentally cope with all the physiological challenges during the race.

It is clearly crucial to train your endurance in the weeks, months or even years leading up to a HYROX race. The different physical adaptations caused by endurance focused training are almost all focused on improving the cascade of subtracting oxygen from the air, transporting it to the working muscles and ultimately being used to produce Adenosine Triposphate (ATP) – an important organic compound which supports many processes in living cells, including the contraction of muscles.

During the remainder of this article when the word ATP is mentioned, think about “energy”.

How ATP is Produced

Endurance training comes in many forms.  For example, you can vary the exercise mode (running, rowing, SkiErg etc), total training volume (duration and frequency), and intensity. The remaining of this article will focus primarily on the training intensity during endurance efforts.

Before we move on, it is important to quickly explain how ATP for muscle contraction during endurance activities is produced (during very short activities there are also other mechanisms, but we won’t be covering that today)…

During an endurance effort, almost all ATP is produced by the oxidation of both fat and carbohydrates. The human body has a big reservoir of fat that can be used to produce ATP. Furthermore, fat is a very efficient source of fuel. On the downside, the speed with which ATP can be produced with fat is not that high. On the other hand, the reservoir of carbohydrates that can be used to produce ATP is relatively small (enough for around 2 hours of high-intensity activity) but the speed at which ATP can be produced with carbohydrates is quite fast.

This means that the body will use relatively more fat and less carbohydrates during low-intensity efforts. The amount of ATP required is not that high during these efforts, meaning it is most logical to primarily use fat for fuel. However, at the moment the exercise intensity increases the amount of ATP required might be more than fat alone is able to deliver. Then it will be required to produce some from the faster fuel carbohydrates. This means that a gradual increase in exercise intensity will results in a gradual increase in the use of carbohydrates and a gradual decline in the use of fat for fuel.

This has more consequences than just a faster production of ATP. To keep up with this production the muscles will need more oxygen, which will cause the rate of ventilation and heart rate to rise. Furthermore, the faster production of ATP will produce more metabolic biproducts like lactate or CO2. Because the body wants to maintain homeostasis (a state of balance) it wants to process or get rid of those biproducts. This will cause the rate of ventilation and heart rate to rise even further.

The Importance of Training Intensity

Because of the processes described above, the intensity of an endurance training can have an impact on the following two factors:

• The physical adaptations realized after the effort. For example, low-intensity exercise will stimulate the adaptation to use fat for fuel and increase the capillary density. High-intensity exercise will stimulate the adaptation to use carbohydrate for fuel and improve pulmonary function.
• The physical load or stress that the physical effort entails. This will have an effect on how long it takes to recover both physically and mentally from the effort (which then has an impact on how long it takes to be able to train again).

Therefore, work intensity is a crucial factor to consider during your training, and so coaches and athletes often use different ‘training zones‘ in their exercise description.  These training zones can be used for multiple reasons:

– to standarise the intensity domains of an athlete in a certain exercise mode

– describe a preferred intesity during endurance training

– in an exercise test to measure the endurance capabilities of an athlete

How are Training Zones Defined?

There are multiple ways to define training zones. You can use physiological variables like heart rate or ventilation rate, but you can also measure the concentration of metabolic biproducts.

One of the most famous metabolic biproducts is lactate, which is constantly produced during the oxidation of carbohydrates. Hence, when exercise intensity rises, and more carbohydrates are used to produce ATP, the concentration of lactate will rise. After a while all this lactate cannot be processed by the working muscles and it will be released in the blood. The concentration of blood lactate can be measured and therefore be used to analyse the metabolic state of the body.

Purely metabolically speaking there are two physiological thresholds:

• The lactate threshold (LT; the point during exercise where the lactate concentrations rise significantly above resting levels.)
• The maximal lactate steady state (MLSS; the highest workload that can be sustained over time (+/- 30-60min) without continual lactate accumulation)

An example of this can be seen in figure 1 (below). Over the course of the low-intensity effort the lactate concentration does not significantly differs from the start. During the medium-intensity effort the lactate concentration rises somewhat compared to resting values, but during the effort the concentration stabilizes. The last effort showed in figure 1 is during a high-intensity effort. Here it can be seen that the blood lactate concentration keeps on rising during the entire effort, which means that the body is no longer able to create a ‘steady state’.

Figure 1. Example of blood lactate concentration during different exercise intensities.

Another example of these two physiological thresholds can be seen in Figure 2 (below). This is an example of a graded exercise test where an athlete needs to run at a certain pace for 4 minutes. After 4 minutes the pace increases (in figure 2 by 2 km/h), and this is repeated until exhaustion. When you measure the lactate concentration during this test, it is possible to estimate the pace at the two physiological thresholds (see the two vertical dotted lines in figure 2).

Figure 2. Example of blood lactate concentration and heart rate during a graded exercise test.

Additionally, when you also measure the heart rate during this test it is possible to associate a physiological threshold to your heart rate. In figure 2, this would mean that the LT is around heart rate 150 and the MLSS around 168. This can be useful during a training session when there is no information about your running speed or when it is unreliable (like running in the forest, uphill, downhill, headwind etc.).

What Kind of Training Zone Models are there?

To complicate things, there are a lot of different training zone models. For example, there is a 3-Zone model; 5-Zone model; 5-Zone model where zone 5 is further differentiated in zone 5A, 5B and 5C (does that not make it a 7-Zone model?!); 7-Zone model (ahh, there it is) and I even read somewhere about a 9-Zone model.

Furthermore, there are a lot of different methods and terminologies to differentiate between the different training zones. Examples are the aerobic threshold, anaerobic threshold, ventilatory threshold, lactate threshold, 1mm delta lactate threshold, maximal lactate steady state, onset of blood lactate accumulation (OBLA), heart rate reflection point, functional threshold power (FTP) and critical power to name a few examples.

It is important to not fear all these different models and thresholds, they all are used to achieve the same things as described before, namely 1) standardise intensity domains; 2) describe training intensity and 3) use as a measurement outcome after an exercise test.

In figure 3 (below) you can see how all different training zone models are related to one another. There are a large number of similarities between them, with the real differences simply being is the number of different zones and the terminologies over the entire training intensity spectrum.

Figure 3. Examples of different training zone models.

How to Use Training Zones?

I hope it sounds logical that you first need to estimate your training zones before you can start using your training zones during training sessions. There are multiple ways to estimate your training zones:

• The most reliable way is by doing an exercise test with a sport physiologist or a sport medicine clinic. Here they can measure both your heart rate (often including analysing possible heart-related symptoms), lung ventilation and blood lactate concentration.
• Via an exercise test measuring only heart rate and blood lactate concentration. You could even do this yourself if you buy a blood lactate testing kit.
• Using a sport watch which can estimate your training zones. Keep in mind that normally heart rate values alone are not sufficient to accurately estimate your training zones. The large amount of data these watches use, in combination with advanced algorithms/formulas, make it possible to estimate your training zones, but they can still come with inaccuracies.
• By working to a percentage of your max heart rate (like in figure 3). Personally, I’m not a big fan of using percentages of HRmax since values like HRmax, and heart rate values at certain intensity thresholds, are highly individual. The values shown in figure 3 are averages, and since there is no average human, it is unwise to copy these values and use them in your own training sessions.
• Using a rate of perceived exertion (RPE). This is something I am a big fan of. I believe all athletes should be able to tune into their own body and evaluate how they are feeling before, during and after training sessions. An important notion regarding RPE is that its value is not only depended on the training intensity, but also in the training duration. When exercising at VO2max (Zone 4 in a 5-Zone model and Zone 5 in a 7-Zone model) the RPE will probably start at 8, but when the effort lasts longer and longer this will surely go to a 9 or even a 10.

During the training process an athlete ideally needs to combine their own subjective perception (RPE) with objective measures (heart rate, running speed etc.). In my opinion, brings the best of two worlds together. On one hand you are using systematic information to guide your training intensity, but on the other hand you are keeping in touch with your own body to evaluate the training process from your own subjective point of view.

Points to Note

To finish this article, I want to address the following three important notions:

• The values appointed to your training zones are not fixed but change over time. The border of a training zone today can be different than it is tomorrow because of factors such as training stress, life stress or fatigue. Therefore, it is wise to avoid training too close to your thresholds and always keep in check with your body during your training sessions. Additionally, over a longer period of time, your training zones can change due to physical adaptations. Hence, it is also wise to perform a standardised exercise test every once in a while (i.e. every 3 months).
• When you use heart rate values to differentiate between different training zones, it is crucial to realize that those heart rate values only apply for the specific sport that you tested it for. If you did an exercise test while running, for example, the heart rate values for those zones only apply for running and not for other sports like rowing or cycling.
• When you use heart rate values during a training session it is important to realize it can be influenced by factors other than just your training intensity. Your heart rate is, for example, somewhat higher when training in the heat, when training while (slightly) dehydrated or training when stressed. On the other hand, your heart rate can be somewhat lower than expected when training in the cold or when you are (slightly) overtrained (one of the symptoms of overtraining is that you cannot reach high heart rates).

Summary

All things considered, training zones are a very helpful as a guide to prescribe and control training intensity, but it is important to realize that the human body is a complex whole. It is simply impossible to comprehend all this complexity into a training zone model and it is, therefore, unwise to stubbornly stick to the values appointed by your personal training zone profile. Use training zone models as a systematic approach for your training description, but always listen to your own body!