Zone 2 training builds aerobic base and fat oxidation capacity, but its benefits depend entirely on training at the correct individual intensity, determined by FatMax, not a generic heart rate or power percentage.
Zone 2 training is promoted as your go-to training intensity, whether you’re a professional athlete or simply looking for exercise health benefits. Zone 2 training gained popularity when the coach of Tour de France winner Tadej Pogačar revealed that his athlete spends 80% of the training days in zone 2. In this article we talk about the benefits of zone 2 training. We also share tips on how to apply it in training. Let’s get started!
In this article
Iñigo San Millán “invented” zone 2 training. In a podcast with Peter Attia, Iñigo shares his definition of zone 2 training: zone 2 is the exercise intensity that stimulates mitochondrial function most.
Mitochondria are often referred to as the energy factories of the body. They play an important role in athletic performance and health, as we’ll soon find out.
Zone 2 training is the intensity at which athlete stress the mitochondria, so they improve their ability to burn fat and clear lactate. At this exercise intensity, you:
Says Iñigo San Milán, coach of Tour de France winner Tadej Pogačar.
It’s important to note that the zone 2 intensity Iñigo talks about is not part of any famous training zone model, like the 3-zone-model, 5-zone-model or the power zone model of Andrew Coggan.
You could however say that Iñigo’s zone 2 intensity is the intensity you should aim for 80% of the time, in a polarized training model. In fact, Iñigo reveals that Tadej Pogačar trains roughly 80% of the days in zone 2.
Now that we know the physiological theory behind zone 2, you probably wonder how you can determine an athlete’s zone 2 exercise intensity. Here are 3 ways to determine zone 2 intensity:
Let’s have a closer look.
According to Iñigo, the gold standard method of determining the zone 2 intensity is via a metabolic test. This test will tell you at which intensities athlete burn the most fat. This is athlete’s zone 2. Note that for (untrained) runners, this could mean they need to walk to stay in zone 2.
INSCYD gives coaches and athletes many options to perform a metabolic test. Both in a lab setting and in a (remote) field test, using power (cycling) or GPS (running) only.
As a result, you get athlete’s zone 2 intensity (= FatMax intensity) based on power, pace and/or heart rate.
Book your metabolic test with one of our certified INSCYD coaches.
In their podcast Peter Attia and Iñigo San Milán mention that zone 2 often occurs at a lactate concentration between 1.7 – 1.9 mmol/l. For trained athletes, this lactate concentration is slightly above resting values. This corresponds very closely to the intensity of lactate threshold 1 (LT1).
If you have a lactate analyser, you can measure blood lactate during exercise to see if the athlete is in zone 2. Here’s a free online education guide to taking valid lactate samples.
INSCYD coaches and athletes can see what the exercise intensity is, that corresponds to a lactate concentration between 1.7 – 1.9 mmol/l. They don’t even need a lactate analyzer for this (not during training, nor during the metabolic test).
Simply create a custom training zone in the Training Zone Builder, based on a lactate concentration lower- and upper limit of 1.7 and 1.9 mmol/l. It then shows the corresponding power, pace and/or heart rate.
Coaches and athletes, take the guesswork out of your training. With INSCYD’s Training Zone Builder, create custom zones based on accurate lactate levels, and get precise power, pace, and heart rate data. Elevate your training and results, see the Training Zone Builder in action!
If accuracy is less important to you, you can determine the zone 2 intensity without any equipment. The talk test is one way to do that.
Here’s how the talk test works: exercise at an intensity you think equals zone 2. If the athlete is indeed in zone 2, the athlete should be able to have a conversation on the phone, but the person on the other end of the phone would notice that athlete is exercising.
In other words: zone 2 training feels a bit uncomfortable when talking, even though it’s possible.
Experts agree that you cannot determine zone 2 based on power only. This means zone 2 can not be expressed as a percentage of FTP.
The same is true for heart rate. You cannot determine zone 2 using a heart rate monitor only. It’s not a percentage of maximal heart rate, for instance.
You can however determine zone 2 via a metabolic test or lactate sample, and connect it to a power, pace and/or heart rate number.
In short, the benefit of zone 2 training is that athlete increase mitochondrial function. These mitochondria generate energy aerobically, using fat and lactate as a fuel. As a result, an improvement in mitochondrial function increases the ability to burn fat and clear lactate.
All other things equal, an increase in mitochondrial functioning due to zone 2 training should:
Let’s dive a little deeper into the details.
When we fully want to understand the benefits of zone 2 training, we need to know more about muscle fibers. Here’s a quick summary:
Type I muscle fibers are dominant at low exercise intensities. These fibers have many mitochondria that produce energy aerobically and use fat as a fuel.
Type II muscle fibers are dominant at high intensities. They use carbohydrates as a fuel and produce lactate.
This lactate is transported out of the type II muscle fiber via a MCT-4 transporter. It then travels into a type I muscle fiber via a MCT-1 transporter. The type I muscle fiber uses this lactate as a fuel. This happens in the mitochondria.
So mitochondria can use both fat and lactate as a fuel. Simplified speaking: the higher the exercise intensity, the less fat and the more lactate is used as a fuel in the mitochondria.
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Zone 2 aims to increase mitochondrial function by using an intensity at which type I muscle fibers, with their mitochondria, are dominant. At this intensity, fat combustion is at its maximum. When mitochondrial function improves, both fat and lactate combustion is considered to improve.
There’s no doubt that an increase in mitochondrial mass and an increase in mitochondrial functioning is beneficial for endurance athletes.
It increases fat combustion, and therefore saves the most precious fuel: glycogen. This is often referred to as metabolic flexibility: the ability to switch in fuel source, depending on the exercise intensity.
It also increases lactate clearance, which enables athletes to exercise at high intensities without accumulating lactate. A high lactate clearance rate also improves the recovery after high intensity work.
In fact, the benefits of a good mitochondrial functioning might be one of the most important factors that separate the good from the best World Tour professional cyclists.
However, even though zone 2 training may be effective, there’s not a lot of literature that shows it is the most effective way to improve mitochondrial functioning.
You could for instance argue that at intensities above zone 2, type I muscle fibers and their mitochondria are still highly active. Additionally, at intensities above zone 2, lactate clearance also remains high. However, fat metabolism decreases above zone 2. But is training at FatMax also increasing ability to burn fat the most?
After 30 years of testing with (professional) athletes and patients, Iñigo San Milán concludes that zone 2 is indeed the most effective training zone for increasing mitochondrial functioning.
With INSCYD you can find out whether this is true for your athlete too. Simply perform an INSCYD test before and after your zone 2 training program. Then look at whether the athlete’s fat combustion (kcal/h) and lactate clearance rate (mmol/l/min) increased. According to Iñigo, these are the two markers for improved mitochondrial functioning.
Mitochondrial functioning and metabolic health are strongly related to longevity.
As we age, mitochondrial functioning decreases. The same process takes place when we have an inactive (sedentary) lifestyle.
Mitochondrial dysfunction is linked to type 2 diabetes, cardiovascular disease, cancer, and Alzheimer’s disease.
So far, there’s no medicine that increases mitochondrial functioning, except for exercise.
Contrary to high intensity training (HIT), zone 2 training is a sustainable exercise intensity for most people, from sedentary patients to professional athletes. That, combined with the observation of Iñigo that zone 2 training is also the most effective way to increase mitochondrial functioning, makes zone 2 training an obvious choice.
When zone 2 training is considered a medicine, it is important to optimize and individualize the dose (“prescription”). INSCYD can help to determine the right zone 2 intensity. The software can also help to better understand lactate dynamics and fat combustion, which are considered markers for mitochondrial functioning. Lastly, INSCYD can help track mitochondrial functioning progress, looking at the lactate clearance rate and FatMax.
Now it’s time to put zone 2 training into practice. Let’s get started with these 5 steps:
Determine your zone 2 intensity via a simple metabolic performance test. Cyclists and runners can even do the test remotely, using a power meter or GPS watch only. Book your test with a certified INSCYD coach or if you a coach or lab book a call to see INSCYD in action. This test gives the right training intensity and serves as a baseline for the mitochondrial functioning.
You need to spend a certain amount of time in zone 2, to enjoy the benefits. But how long should a zone 2 training be? Tour de France winner Tadej Pogačar probably needs 4-5 hours of Z2 intensity per training. For starters, the minimum zone 2 training time is 30 minutes per workout. According to Iñigo San Milan, 1-1.5 hours of Z2 is already effective for trained athletes.
If you think of Z2 training as a medicine, it’s probably better to dose daily than to dose all of it once a week. But how much zone 2 training per week is ideal? Iñigo recommends 3-4 days of zone 2 training per week. He also says 1 day is probably not enough, while 2 days might enable athletes to maintain current mitochondrial functioning, depending on thier fitness level.
You now have a baseline for athlete’s mitochondrial functioning. You also know the intensity, duration and frequency of your athlete’s zone 2 training program. If you want to see whether your athlete is making progress, simply repeat the metabolic test (step 1). Look for improvements in maximal fat combustion (kcal/h) and lactate clearance rates (mmol/l/min), shown in the INSCYD report. These are two markers for mitochondrial functioning.
Although zone 2 training seems very straightforward, there are some common mistakes that make the benefits disappear. Let’s address 4 common mistakes.
You might look at your Apple watch, Garmin or Wahoo, and see that athlete spent 60 minutes in zone 2. However, this does not equal 60 minutes of zone 2 training.
It does not equal zone 2 training, because in theory the athlete could have done zone 3 intervals with recovery in between. During the first minutes of your Z3 intervals, heart rate was climbing from zone 1 to zone 3. During the recovery phase, heart rate was decreasing from zone 3 to zone 1. As a result, the athlete spent quite some time in zone 2, without actually exercising at a Z2 intensity.
Continue reading the next common mistakes, to better understand why time in zone 2 does not equal zone 2 training.
Although many professional athletes spend a large portion of their training in aerobic training zones like zone 2, it’s not the only thing they do.
As Iñigo San Milan says, Tour de France winner Pogačar spends roughly 80% of the days in zone 2. That still leaves 20% of the days with other intensities, like high intensity interval training to boost VO2max and increase anaerobic power.
Zone 5 is a training intensity San Milan explicitly talks about, which brings us to the next common mistake.
Yes athletes should train at several training intensities, and yes you can combine those intensities in one single workout. But when it comes to combining high intensities with zone 2 training, you need to be careful.
If you add a zone 5 interval to your zone 2 workout, you risk losing the benefits of zone 2 training. That is because a zone 5 interval increases lactate concentrations. As a result, the mitochondria will first burn lactate – the preferred fuel – before they start burning fat again. This can take longer than you expect.
The lactate recovery & accumulation graph of INSCYD shows exactly how long it will take before you recover from the lactate accumulation, and start burning fat again.
Say our example athlete (image above) rides 5 minutes at 270 watts. The athlete accumulates 1.5 mmol/l of lactate per minute at this intensity. This equals an increase of 5 * 1.5 = 7.5 mmol/l lactate.
If the athlete then shifts to a zone 2 intensity of 125 watts, 0.35 mmol/l is cleared per minute.
To clear 7.5 mmol/l of lactate, takes 7.5 ÷ 0.35 = 21 minutes and 25 seconds. In other words, it will take more than 20 minutes of zone 2 intensity, before the athlete is actually burning the most fat again, and benefitting from “zone 2 training”.
This again shows that time in zone 2 does not equal zone 2 training.
You can simply prevent this phenomena by doing zone 2 work first. For instance in the first 1.5 hours of training. Then add some zone 5 intervals at the end of the training, and acknowledge that this part of the training is not zone 2 training.
The goal of a zone 2 workout is to increase mitochondrial function and therefore fat combustion. The zone 2 intensity is defined as the intensity at which athletes burn the most fat. This means that when your zone 2 workout is effective, you should increase their power or pace over time, to stay in zone 2.
You do this by regularly updating your training intensity, based on a metabolic test.
If you don’t do this, you quickly train at an intensity that is too low, while Iñigo says you’re probably better off training at an intensity that is a bit too high, than too low.
Zone 2 training is gaining popularity, both in the field of athletic performance and as a medicine for disease. The goal is to improve the performance of athlete’s mitochondria, which are the energy factories of their body.
When mitochondria work better, they will increase the ability to burn fat instead of glucose. They will also become better at keeping your blood lactate concentration low.
Whether zone 2 training is the most effective way of improving mitochondrial functioning remains a bit unclear. This leaves space for other training methods like polarized training and the Norwegian method.
If you want to learn which training method suits you best, you need to start with understanding your current metabolic profile. INSCYD allows athletes and coaches to do exactly that. A 360 metabolic profile allows you to make educated decisions on what to work on. It also allows you to retest and see whether you made the right decision.
Book a free demo to experience how INSCYD can elevate your coaching or lab services.
Markers for mitochondrial functioning can separate the good from the best World Tour professional cyclists:
San-Millán, I., Stefanoni, D., Martinez, J. L., Hansen, K. C., D’Alessandro, A., & Nemkov, T. (2020). Metabolomics of Endurance Capacity in World Tour Professional Cyclists. Frontiers in physiology, 11, 578.
Mitochondrial dysfunction is linked to type 2 diabetes, cardiovascular disease, cancer, and Alzheimer’s disease.
San-Millán I. (2023). The Key Role of Mitochondrial Function in Health and Disease. Antioxidants (Basel, Switzerland), 12(4), 782.
VO₂max, VLamax, FatMax, thresholds, fuel use and training zones, from a single test in the lab, in the field or fully remote.
