Zone 2 Training & Lactate: Dissecting Iñigo San Millán’s Advice
Recently, there has been a surge in interest in the benefits of ‘Zone 2’ training, spurred in part by the success of Tadej Pogačar and recent interviews with his coach Dr Iñigo San Millán, who has espoused the benefits of Zone 2 training.
As head of performance at UAE Team Emirates, and professor at the University of Colorado School of Medicine, San Millán certainly has some insightful points to make. So in this article, we’ll sum up the key information from some of his most prominent interviews, and also discuss our own thoughts on some of the nuances of San Millán’s advice, and address the questions we’re commonly asked off the back of these interviews.
The most detailed discussion into Zone 2 training San-Millán has given is with Peter Attia, spanning 3 whole hours of discussion. The points below are taken mainly from this interview, together with some others, including a couple of GCN interviews, which can be found here and here. We’ve tried to accurately summarise San Millán’s points and any clarifying details added by us are shown in square brackets.
A note on lactate
We’ll be talking throughout this article about lactate, so we first wanted to include a very brief summary of lactate and its role before delving into San Millán’s advice…
When carbohydrates are broken down to produce energy, they are first converted into a substance called pyruvate. Some of this pyruvate is burnt directly to produce more energy, and some is converted into lactate and other metabolites.
Contrary to traditional view, lactate is a very useful molecule, as it provides a fast source of energy, that can be transported between muscle fibres and around the body in the blood. However, the other metabolites that are produced alongside lactate have been linked with muscular fatigue, and thus lactate levels in the blood have long been used as a marker of fatigue and metabolic ‘instability’.
If you want a more comprehensive discussion of lactate, check out this article.
Defining Zone 2
When it comes to defining Zone 2, San Millán comes at this from a bioenergetics standpoint - which is to say, that he considers the energy systems that are being worked at different intensities.
San Millán uses a 6-7 zone model when describing training intensities, and suggests that Zone 2 is an intensity that stimulates mitochondrial function, fat oxidation and lactate clearance the most, and mainly recruits ‘Type I’ muscle fibres [HNP: these fibres are also known as ‘slow twitch’ fibres. They are the most aerobically adapted muscle fibres in the body, and are the most responsive to aerobic training. They are also good at producing energy through fat oxidation].
San Millán explains that at a Zone 2 intensity, you’re using both fat and carbohydrates to produce energy. However, in contrast to Zones 1 and 3 [which also use fat and carbohydrates], in Zone 2 fat oxidation rates are near maximal. This is an intensity that’s sometimes referred to as ‘FatMax’, and is a key characteristic of Zone 2 training.
At the very top end of Zone 2, San-Millán notes that you also see a marked increase in glucose metabolism (i.e. the break-down of carbohydrates to produce energy). This corresponds to a marked increase in blood lactate levels, since lactate is a downstream product of carbohydrate metabolism.
Our Comments
This is also the way that we would define Zone 2 in an ideal world. The top end of Zone 2 is the wattage or heart rate where you first see a notable step-up in blood lactate levels, as illustrated in the figure below at the point marked ‘LT1’.
The midpoint of Zone 2 is broadly where FatMax is achieved (although as shown in this scientific paper, the actual rate of fat oxidation is very similar at wattages slightly above and below FatMax, so knowing the precise location of FatMax is not all that important).
San Millán doesn’t strictly define the cut-point between Zones 1 and 2, but we would define Zone 1 as an intensity where fat oxidation rates are meaningfully lower than in Zone 2 and/or where the % of muscle fibres being recruited is meaningfully lower. Generally this will be between 40-60W below the top end of your Zone 2.
In practice, unless you can buy a lactate meter or go into a lab for gas analysis to determine your FatMax, this definition of Zone 2 is hard to apply in the real-world.
A common approach to defining Zone 2 is therefore to use a fixed percentage of FTP (Functional Threshold Power), max heart rate or threshold heart rate. While this approach can give a ball-park estimate of where Zone 2 probably sits, it’s not always accurate on an individual level.
In the absence of lab or lactate data, we therefore like to use the ‘talk test’ to define Zone 2. When riding in Zone 2, you should find that your breathing rate is ‘conversational’. This means that you are able to speak full sentences, although you should still be able to tell from your breathing that you’re working a bit. The key point is this…
If your breathing is so easy that it’s effectively the same as when you’re at rest (e.g. similar to your breathing when walking easily), then you’re likely in Zone 1. If you’re having to take breaths at unnatural points mid-sentence, then you’re in Zone 3.
The reason the talk test works so well, is that there’s a marked ‘ventilatory’ threshold (i.e. change in breathing rate) which corresponds with the increase in blood lactate levels. We write more about the talk test, as well as other methods for determining the top end of Zone 2 here.
Using the talk test is also something San Millán recommends in his interviews, and something we strongly endorse too.
There is also new technology from companies like Tyme Wear, which can measure your ventilation rate in real-time, and determine the top end of your Zone 2 based on your breathing rate.
Technologies like these will likely become more prevalent in the future, as they allow a more individualised definition of training zones, without the need to visit a lab or perform regular lactate testing. Another benefit of these technologies is that they allow day-to-day iteration of your training zones to account for things like fatigue and prior nutrition.
Benefits of Zone 2
San Millán lists three key benefits of Zone 2 training:
Improving your capacity to oxidise fats (meaning you can burn more grams of fat per minute, and your FatMax occurs at a higher wattage)
Improving ‘mitochondrial function’
Improving lactate transport between muscle fibres
In contrast, San Millán states that higher-intensity training largely helps train the ‘glycolytic’ system - that’s the system whereby energy is produced from the break-down of carbohydrates, to produce lactate and other metabolites.
Our Comments
It’s widely accepted that Zone 2 is a good intensity for improving fat oxidation. Building fat oxidation is highly important for almost all endurance cyclists, because it contributes both to endurance and to threshold power.
It’s worth noting though that Zone 2 riding is not the only intensity domain that helps build fat oxidation capacity, and is possibly not even the ‘best’ intensity for achieving this. For example, a study from the infamous Ronnestadt research group (responsible for designing the ever-popular 30/15 microburst intervals) shows some early indications that adding blocks of short 30-second sprints to an endurance ride, might enhance the stimulus for improved fat oxidation (Almquist et al., 2020). However, more research is needed before we can be confident in this training approach.
The suggestion that Zone 2 optimally develops mitochondrial function and lactate transport is also debatable. In our view there are other complementary ways to develop these abilities, and we probably need a combination of different intensities to develop these abilities optimally (in much the same way that building VO2max needs different intensities in order to train different contributors to VO2max).
As an example, there’s notable research from David Bishop’s research group (e.g. Bishop et al., 2014) that suggests that while lower-intensity riding helps greatly with developing the number and/or size of mitochondria in the muscles, higher-intensity riding is needed in order to improve the functioning of these mitochondria (e.g. by stimulating enzymes that play a role in aerobic metabolism).
Similarly, if we look at lactate transporters, which are responsible for moving lactate between muscle fibres, we’ve seen evidence that certain transporters (specifically the transporter MCT4) specifically require high intensity riding to be developed optimally (McGinley et al., 2016). We’ll discuss these transporters more in the next section…
Ultimately, in our experience, Zone 2 is very valuable for stimulating a wide range of aerobic adaptations. It also has the benefit that it’s not overly stressful, so it’s possible to spend a lot of time in Zone 2, while also still having energy left to perform 1-2 higher-intensity sessions each week. As stated by San Millán, Zone 2 does contribute specifically to improving mitochondrial capabilities within the muscle and lactate clearance abilities. However, riding exclusively at a Zone 2 intensity probably wouldn’t develop these attributes maximally, and other intensities are needed in order to optimise training.
Lactate transport
San Millán speaks a lot about lactate transport in his interviews, as we’ve already alluded to above.
He explains that the ability to transport lactate between muscle fibres is a key determinant of performance among endurance cyclists, allowing fuel to be redistributed to fibres that need and have the capacity to combust lactate.
There are two key lactate transporters: MCT1 and MCT4.
MCT1 is mainly responsible for transferring lactate INTO muscle fibres, and also moving pyruvate (the precursor for lactate) into the mitochondria, where it can be turned into energy. This transporter is mainly found in Type I fibres.
MCT4 is mainly responsible for transferring lactate OUT of muscle fibres, and is largely found in Type II fibres [sometimes known as ‘fast twitch’ fibres, these are mostly activated at higher powers above Zone 2 and are well adapted to produce energy quickly through glycolysis - i.e. carbohydrate break-down].
As mentioned above, San Millán suggests a key goal of Zone 2 training is to improve the capacity to transport lactate between muscle fibres by developing these transporters.
Our Comments
Lactate transport is a performance determinant that’s often overlooked, and not spoken about all that often. However, it plays a key role in a number of cycling abilities, including the ability to sustain power across a wide range of wattages, as well as the ability to recover quickly after high-power efforts. So we’re really pleased that San Millán has brought more attention to this area.
Although lactate is a useful product from glycolysis, it’s linked with the presence of other metabolites that can cause fatigue. Thus, the ability to transport lactate between muscle fibres not only helps with redistributing fuel around the body, but it also helps with reducing the levels of fatiguing metabolites and thus the physiological stress caused by a given exercise intensity.
We usually refer to this ability as ‘lactate clearance’ and it depends not only on the ability to move lactate around the body, but also on aerobic capacity (i.e. VO2max), since oxygen is required in order to combust (and therefore remove) lactate.
Using lactate to guide training
San Millán uses lactate testing heavily in his coaching, and noted several key things:
Lactate levels at a given race-relevant wattage are strongly predictive of performance. For example, he describes testing lactate levels of world-tour athletes while they ride at 5W/kg, which is a typical climbing wattage in the Tour de France, and using this as a benchmark for an athlete’s fitness. Indeed, San Millán mentioned that one of the first things he noticed about Pogačar and his strong racing potential was his very low lactate values relative to other riders.
The same lactate concentration in an elite athlete does not correspond to the same lactate concentration in a recreational athlete in terms of the level of metabolic stress induced. This is because the elite athlete will be producing lactate at a higher rate, but also clearing this a higher rate. San Millán suggests that recreational athletes can actually ride for longer than elite athletes at the same lactate level (e.g. 4mmol/L) due to this discrepancy in metabolic stress.
Our Comments
The most important point above is that lactate levels are not comparable across athletes. Riding at 2mmol/L of lactate can induce a very different stress level in one athlete vs another, depending on how well each athlete is able to use fats, and clear lactate. If you’re training according to lactate levels, it’s important to understand what your own lactate profile looks like, and base your training zones on this. Traditionally, coaches and athletes have often prescribed training at generic lactate levels (e.g. 2mmol/L), but in reality is not much better than riding at a fixed percentage of FTP.
Lactate meters can be bought for use at home, and the basic testing protocol described by San Millán to assess your lactate levels at a race-relevant workload is a good way to track progress. We write more about lactate testing at home here.
On the topic of lactate testing, it’s worth explaining that lactate levels in the blood represent a balance between the extent to which the body needs to rely on carbohydrates (rather than fats) to produce energy, and the ability of the body to transport and combust lactate. If you reduce your lactate levels at a given wattage, this might reflect an improvement in fat oxidation, lactate transport, and/or lactate combustion capacity (e.g. improved VO2max). Without further testing, it’s hard to disentangle these different components.
Targeting top vs bottom of Zone 2 range
Zone 2 can span quite a wide wattage range (e.g. as much as 50-60W). One question we’re often asked relates to whether to focus training at the top end of this zone for the biggest benefits.
On several occasions, San Millán suggests that the top end of Zone 2 is where the biggest training benefits are gained, and he mentions that he encourages his athletes to target this upper end.
Our Comments
We believe the question of whether to target the upper or lower end of Zone 2 is more nuanced than this.
Riding right at the upper end of Zone 2 can be quite stressful, especially for highly trained athletes, who will be producing very high wattages at this intensity, burning large amounts of both fats and carbohydrates, and achieving a higher level of muscle tension.
Moreover, at least when it comes to stimulating fat oxidation, research suggests that fat oxidation rates are very similar both slightly above and slightly below FatMax (Schwindling et al., 2014). Thus the stimulus for fat oxidation is likely quite similar at a lower Zone 2 level as compared to the upper end.
We therefore think it’s important to think carefully about your level of fitness, how much time you’re hoping to spend in Zone 2, and what your training priorities are, before determining where to pitch your Zone 2 rides.
More specifically, during the base-building phase (usually 3-6 months out from competition), the focus of training is usually on building your aerobic base, and there’s usually relatively little high-intensity training. During this phase, it might make sense to target the upper end of Zone 2, since these rides are your main priority.
In contrast, as the race season gets closer, then higher-intensity training will usually become more frequent and of higher priority. In order to be fresh for these sessions, then it might be beneficial to target the mid/lower end of Zone 2 during this phase, so that the high intensity sessions can be executed to a high standard.
This guidance needs to be considered in the context of fitness level and time availability too. For example, professional athletes will be doing very high volumes of riding during the base phase. In this scenario, even though Zone 2 riding is the priority and high-intensity riding is less prominent, it may still be too stressful for these athletes to target the top end of Zone 2 in all of their rides. Indeed, when we look at the training practices of elite cyclists, we can see that the majority of their long endurance rides through the base phase are often performed at the mid-lower end of Zone 2.
In contrast, if we consider a time-crunched athlete who is already at a relatively high fitness level and who is recovering well from their existing training, then a nice way to increase the adaptive stimulus without adding extra training hours might be to focus the majority of Zone 2 riding around the upper end, provided this doesn’t compromise recovery or performance in higher intensity sessions.
Avoiding power surges
One point that San Millán repeatedly makes is around the importance of keeping your intensity under control during Zone 2 rides, and avoiding surges in wattage. This is because when you ride hard, even for a relatively short amount of time, glycolysis (carbohydrate break-down) is stimulated and lactate levels rise.
San Millán notes that this is far from ideal, because lactate inhibits the break down of fats (a process known as ‘lipolysis’). It also inhibits the mitochondrial transporter responsible for moving fatty acids into the mitochondria, where they can be combusted for energy. Ultimately, therefore, lactate suppresses fat oxidation.
Moreover, this suppression of fat oxidation can last for as much as 30-minutes after each power surge. So even if you aren’t including all that many power surges in your endurance rides, if these are included sporadically throughout the ride, they will undermine one of the key goals of these sessions, which is to develop fat oxidation.
If you want to include some intensity within your endurance rides, San Millán suggests including this towards the end of the ride, so that the first part of the ride isn’t impacted by elevated lactate levels.
Our Comments
Controlling power within endurance rides is a key skill we try to teach all our coached athletes, and something we feel is very important for the reasons stated above.
It is worth noting though that - as with most aspects of training - there are some subtleties that need factoring in. For example, as mentioned above, including short 30-second sprints, within small portions of a longer endurance ride might actually enhance rather than suppress the stimulus for improved fat oxidation (Almquist et al., 2020).
We really like San Millán’s suggestion of including higher intensity work towards the end of an endurance ride in order to keep the first part of the ride to a ‘high quality’ (a.k.a low lactate levels). This is a strategy that could work well for time-crunched cyclists, doing rides in the region of 1.5-2 hours.
Another approach we like to take is to include intervals towards the beginning of a longer (2.5H+) ride. The benefit of front-loading the intensity is that you should be feeling more fresh for these efforts, and they will also deplete muscle glycogen levels, which is a key signalling pathway for a range of aerobic adaptations, including improved fat oxidation.
Of course, with this approach, fat oxidation might be suppressed for the 20-30 minutes after finishing the intervals. However, the adaptive stimulus from the remaining Zone 2 ride should be enhanced by the lowered muscle glycogen levels.
Importance of subjective feelings
San Millán speaks repeatedly about the importance of subjective feelings in training. This is often in the context of paying attention to breathing rates, in order to make sure you’re riding at a Zone 2 intensity.
However, he also speaks about using subjective sensations to determine the level of recovery on a given day, and adjusting training or racing tactics accordingly. Indeed, this is the primary method he uses to guide riders on tactics during the Tour de France and other major stage races, and notes that he prioritises subjective feelings above other metrics, such as resting heart rate, and heart rate variability, which can be unreliable.
Our Comments
We strongly agree with Iñigo that perceptions should be used above all else in training.
Metrics such as power, heart rate, heart rate variability, body temperature and so on can give useful data on things like recovery status, and standard training zones based off percentages of FTP or heart rate can be a good initial benchmark to set your training intensities. However, there’s nothing better than paying attention to how you feel. Asking yourself basic questions like ‘how sore do my legs feel?’, ‘how high is my motivation to train?’, ‘how does this intensity feel?’ and ‘does this power feel sustainable?’, and then adjusting your planned session accordingly should be a mainstay of your training.
Of course, this subjective data can also be bolstered with objective data, such as looking for abnormalities in the relationship between your power and heart rate to spot signs of fatigue. However, the key point is that subjective data on how you feel is usually the most reliable and all-encompassing marker of how well-recovered you are, and whether you’re riding at the right intensity on any given day.
We write extensively about how to use perceptions, in conjunction with objective data in our Cycling Physiology and Training Science Guide.
Using power or heart rate in endurance rides?
Another question we are repeatedly asked is whether to train with power or heart rate in longer rides, given that you may experience heart rate drift in the latter hours of the ride. This question was addressed by Iñigo San Millán in one of his GCN interviews.
San Millán explains that heart rate drift (where heart rate rises despite power being held constant) can generally reflect either dehydration or an increased metabolic stress. He notes that, if you stay well hydrated, you shouldn’t have a heart rate drift of more than 4-6bpm. Heart rate drift in excess of this level probably reflects dehydration (potentially coupled with increased metabolic strain).
In the absence of dehydration, then heart rate tends to correlate with lactate. For this reason, San Millán suggests that you may want to use heart rate as your primary metric to keep within Zone 2, rather than power.
Our Comments
In general we agree with San Millán’s suggestion of using heart rate to keep an eye on whether you’re staying within Zone 2 during your endurance rides.
However, we think there is a subtlety to this, and it depends what you want to achieve from the session.
Heart rate drift arises, in part, due to depletion of muscle glycogen within Type I fibres, and/or cumulative damage/dysfunction in these fibres. This causes some of the workload to be passed to less-efficient Type IIa fibres, resulting in an elevated heart rate.
If your goal from a training session is to develop these Type IIa fibres, so that they become more aerobically efficient, then it might be worth maintaining your wattage, despite seeing an upward drift in heart rate. Indeed, this shift towards utilisation of Type IIa fibres under conditions of lowered muscle glycogen is one of the main benefits on longer endurance rides, and is an adaptation that’s hard to stimulate in shorter rides. Therefore, you might be forgoing some very beneficial adaptations is your drop your power in order to keep heart rate in Zone 2.
The exception to this rule is if you’d be forced to cut your ride short by trying to maintain a wattage that feels subjectively too hard. In this case, we’d usually encourage you to decrease the wattage a bit, so that you can ride longer, since many training adaptations occur as a function of riding volume, rather than intensity.
We’d also still encourage use of power, even if you’re using heart rate as your primary metric. This will help you to avoid power spikes that, as discussed above, can reduce the quality and associated adaptations from a Zone 2 ride.
We hope you’ve found that round-up useful. If you have any queries, please do post below, and we’ll do our best to help!
References
Almquist, N. W., Ellefsen, S., Sandbakk, Ø., & Rønnestad, B. R. (2020). Effects of including sprints during prolonged cycling on hormonal and muscular responses and recovery in elite cyclists. Scandinavian Journal of Medicine & Science in Sports.
Bishop, D. J., Granata, C., & Eynon, N. (2014). Can we optimise the exercise training prescription to maximise improvements in mitochondria function and content?. Biochimica et Biophysica Acta (BBA)-General Subjects, 1840(4), 1266-1275.
McGinley, C., & Bishop, D. J. (2016). Influence of training intensity on adaptations in acid/base transport proteins, muscle buffer capacity, and repeated-sprint ability in active men. Journal of Applied Physiology, 121(6), 1290-1305.
Schwindling, S., Scharhag-Rosenberger, F., Kindermann, W., & Meyer, T. (2014). Limited benefit of Fatmax-test to derive training prescriptions. International journal of sports medicine, 35(04), 280-285.
