There is perhaps no training modality amongst endurance athletes that has as much hype surrounding it as Zone 2. Slow, easy running has been described as the bedrock of endurance training with its benefits being touted not just by influencers, podcast hosts and YouTube channels but also by elite runners and coaches. Zone 2 - and its close cousin, polarized training - is portrayed as the royal road to build endurance and aerobic capacity, improve cardio-metabolic function and fitness while cementing the necessary base for further race-specific sharpening.
Nonetheless, despite the enormous attention and interest Zone 2 has generated (it would be hard as a recreational runner not to hear about this in some forum or group), the actual evidence supporting these claims happens to be very limited.
Before we get there let me briefly describe my flirtation with this type of training.
My experience with low heart rate training
Well before Zone 2 reached its crescendo of popularity (about a year and half or so ago), I had been doing something of a polarized training where the majority of the runs were very easy. And by easy I mean a pace that was typically slower than 6min/km (even though my HM pace then was around 4:30min/km or faster) and at a heart rate that I tried to contain within 138bpm (although I didn't go out of my way to achieve it). For reference, my max HR is over 195bpm. To be clear, I did see some benefits from this type of training but again any running in any zone would provide some benefits. However, some of the things that frustrated me back then were the comparatively high relative exertion required to maintain a pace that is even 20-25s/km slower than a marathon-effort. The subjective perception of effort to push to anything faster than 5:40min/km seemed more than what I think it should have been considering the volume of training I was doing. Heart rate would also shoot up unpredictably high from time to time.
Fast forward to about 7 months ago where I eliminated these super easy runs completely and aimed for what I considered as general aerobic runs. They had no specific heart rate or pace range attached to it per se but involved some level of exertion and more significantly there was no attempt to reduce the pace when the HR unexpectedly went up or to box in the pace to fit into some HR territory. I tried to focus on the form, the breathing and natural sense of rhythm. As a result the typical pace of these runs climbed up to 5min/km or faster. This has led to reduction in average heart rate by about 10bpm not just on these runs but for most categories of workouts including threshold and long runs. There is less random spiking of HR and my running form seems to have improved as has my cadence. Although the accuracy of the estimate is questionable, the trend in Garmin’s VO2max prediction serves as an indicator of gain/loss in maximum aerobic capacity - in my case, it went up from 55-59 to 59-63 (ml/kg/min). Overall I felt I became a better runner after I quit “Zone 2” running
Well, I am an intermediate amateur runner and everything above is 100% anecdote and 0% science. One can always find anecdotal evidence for practically any viewpoint regardless of its scientific merit and there is no reason to treat the n=1 case described above any more seriously than just that.
Therefore let’s actually try to understand Zone 2 training more systematically. What exactly does it represent and what are its purported benefits? What is the theoretical basis (in terms of endurance physiology or other mechanisms) for such benefits? What is the empirical evidence to support the claim that Zone 2 training confers these benefits to athletes?
We will first examine some of these questions in the context of the more popular interpretation of Zone 2 that one typically comes across online and in discussion in amateur running circles. We will then consider the scientific underpinnings of it and the claims associated with it.
What is Zone 2 training?
As is evident in the terminology, Zone 2 training is based on the concept of distinct heart rate zones. Specifically, this is the second interval in the the five-zone convention of splicing the permissible range of heart rate values in ascending order of intensity with the boundaries between them specified in terms of percentages of maxHR (the upper end of Zone 5 is 100% maxHR).
Zone ambiguity
Odd as it may sound, there is no universally agreed demarcation of Zone 2 in terms of the heart rate range. A quick search online would lead to different divisions of the allowed range. To get a sense of how large these variations are, for someone with a maxHR of 200, 60-70% translates to 120-140 bpm whereas 65-80% translates to 130-160 bpm. Now anyone who has any experience with running would know that the difference in the perception of effort at 140 and 160bpm is considerable!
Given this variability how does one pick the correct heart rate range to get the benefits of Zone 2 ? And what is basis for picking one over the other? I have never seen any article or video where these zones have been subjected to empirical validation.
And it actually gets worse.
Heart Rate Reserve Calculation
There is an alternate. approach to determine the training zones based not on the maxHR (as is done above ) but on what is known as heart rate reserve, defined as MaxHR - RestingHR (HRR). The calculation of the heart rate boundary corresponding to x% is given by RestHR + (x/100)* HRR. For the same range of say 60-70% and max HR of 200 and resting HR of 55, the first method will give 120-140 bpm and the second approach will get us 142-156 bpm. In other words, there is not even an overlap in these windows.
What is striking to me of course is how little this discrepancy is talked about during discussions of heart rate training zones and their calculations. Amazingly, the two distinct ranges are published on the same page without so much as a passing remark on why there is a difference in the first place or what should be the guiding principle to identify the correct range to work with!
When I searched online and scoured discussion threads to learn about how to resolve or interpret this discrepancy, what I found was very unconvincing. There were suggestions that these zones were approximate divisions and it would be a mistake to over-interpret them. This is a strange argument to make: if two distinct approximations here involve error margins so large that there is literally no overlap, perhaps then one might as well randomly select zone boundaries and train according to that?
If you are going to claim that there are significant physiological benefits to Zone 2 training (as opposed to other zones), is it not important that one is able to demarcate the boundaries with some reasonable level of accuracy? Conversely, if the claim is that these physiological adaptations are not overly sensitive to heart rate values, then does that not undermine the notion that Zone 2 offers superior benefits over and above training in other zones?
The fact that this arbitrariness does not bother most people who passionately advocate for “Zone 2 training” but are unable to resolve the ambiguity is somewhat troubling.
Yet, merely because online training websites and videos are unable to define Zone 2 training well doesn’t mean there is no support in literature for something similar. Indeed, there is and we will come back to this later.
What exactly are the purported benefits?
From an endurance physiology perspective, advocates for Zone 2 training mention numerous benefits. However these can sound a little vague invoking phrases such as “aerobic adaptations”, “base building”, “fat-burning” etc. It would be more helpful to get specific about potential benefits in order to understand which aspects of the physiology of distance running Zone 2 has an impact on.
Here are some (certainly non-exhaustive) endurance adaptations:
Mitochondria related:
function
size
density
oxidative capacity
enzymes
Capillaries around the muscles involved in the movement
Biasing the fuel substrate (fat vs glycogen)
Metabolic flexibility
Stroke volume
Oxygen carrying capacity of the blood
Oxygen uptake by muscles
Running economy
Lactate thresholds (LT1, LT2)
Recovery
Now let’s be clear about one thing. Most training, including Zone 2, is likely to improve one or more of these parameters. The question therefore is not whether Zone 2 impacts some of these attributes (which it most certainly does) but rather the specificity of it, ie, does training in Zone 2 elicit a relatively greater improvement as compared to other zones keeping some aspect of the training fixed (distance/duration)?
For example, does Zone 2 increase capillary density more than say training in Zone 3 or 4? I don’t see much evidence for it. It may bias the fuel substrate but again by how much and compared to what?
Furthermore in relation to what we discussed earlier, given the uncertainty in the boundaries of these zones, is there a well-established sweet spot for these cardio-metabolic adaptations?
We will revisit these questions.
Interpreting the successes of athletes
Ultimately though, the real reason why so many people seem to be ardent believers in this mode of training is probably because it works for them. They have seen tangible results in terms of race performances, the quality of the hard workouts, and overall perception of gains in fitness. One can of course dismiss all this as pure anecdotal but that wouldn’t be very helpful.
At the same time, we need to be somewhat skeptical about such claims because it is very easy to mis-attribute an element of training as being critical to success when there are multiple other factors. Here are some things to consider when one is claiming that Zone 2 training provided significant benefits:
Any additional training is most likely going to have a positive effect on fitness and performance. If Zone 2 training was accompanied by an increase in volume (weekly mileage), then it is unclear whether it is the additional mileage or the Zone 2 training that made the difference.
For any comparison to even be close to being reliable, one needs to keep the weekly mileage constant and then perhaps switch out some Zone 3 (or higher zones) running for Zone 2 and find out whether there has been any distinct improvement from that. If there are examples of athletes who have done this and observed improved performance, that would be pretty good evidence.
It is entirely possible that doing more easy runs enabled someone to recover better and quicker and hence execute the faster workouts on subsequent days better. The real training adaptations came from being able to perform those workouts well. Now if the claim were to be that the majority of the benefits arise from the fact that running in Zone 2 makes you less fatigued and enables quicker recovery then I have no disagreement. That claim is very different from talking about its impact on physiology such as oxidative enzymes and capillary growth.
The fitness and performance gains happened independently and for reasons such as accumulated training over the years. And Zone 2 training was just incidental to it.
Is there a well-established scientific view on this?
Unlike what several (by no means all) runners assume Zone 2 to mean, the Zone 2 definition used by researchers would suggest that the intensity involved is a notch or two above easy running. One of the foremost experts in this field is San Inigo Millan, a professor at Colorado School of Medicine’s Exercise Physiology & Human Performance Laboratory. San Milan defines Zone 2 as the intensity at which fat oxidation is maximized.
Quick background: When we run or do any activity, the substrate that fuels our energy demands is a combination of fats and glycogen (for the purposes of this discussion, read glycogen as glucose or carbs). At low intensities the proportion of fat utilization is higher (though not 100% and in fact not even beyond 70% in most cases) but as exercise intensities increases, glycogen usage goes up, both in absolute and relative terms. Beyond a certain point, the glycogen utilization is almost 100%.
Identifying the correct Zone 2
Now Zone 2, according to San Milian and other researchers in the area, is the intensity at which the fat oxidation (in absolute terms, not as percentage) is maximum. That intensity occurs close to LT1 or the first lactate threshold, which is defined (or rather one definition of which ) as the intensity at which the blood lactate rises above the baseline level (~1mmol/L) and reaches a steady value of about 1.5-2mmol/L .
![](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F17825bfa-4026-40ee-adec-0917b63339c8_4403x2510.png)
This suggests that Zone 2 conceptually is not a range but a specific intensity, and the notion of a range arises from the fact that training at intensities close to it will also incur the same benefits. San Millan defines LT1 as the upper end of Zone 2 and in one of the podcasts mentioned that the ideal heart rate range is around 5 bpm in the subjects (presumably elite cyclists) that were tested in the lab.
LT1 however does not occur around 60-70% of HRMax but rather close to 75-80% of HRMax (see Table 1 in this paper)1. This would come as a surprise to a lot of people who typically advocate training at a lower intensity for Zone 2 benefits. This is evident for example in practically every case where runners and physios do the actual test and are surprised to find out that their LT1 ( and hence the true Zone 2 range) occurs at a higher heart rate range than what they had assumed.
Even Stephen Sailer who popularized the idea of polarized 80/20 training (80% of the time in low intensity and 20% at high intensity) defined low intensity as anything below LT1.
So, whatever one believes the benefits of Zone 2 to be, the range prescribed by researchers in this field happens to be significantly higher than what is commonly assumed. There are runners practicing Zone 2 who will studiously ensure that the HR does not rise over 135bpm (~70%HRMax). Conversely, running around LT1 is not something that I would characterize as “easy” and my assumption is that this would be shared by most other runners too.
Rationale for fat-oxidation
Why do researchers like San Millan suggest training at intensity close to where fat-oxidation peaks? The reasoning has never been made clear as far as I know but having listened to several podcast episodes featuring San Millan, the claim seems to be that training at the intensity where maximum fat oxidation occurs improves mitochondrial function and the ability to clear lactate2 the most.
Nonetheless, this is not a view that has been shared by every researcher. Despite the fact that San Milan, who has been a champion of Zone 2 training and has successfully coached elite cyclists in premier events like Tour de France, argues that the best intensity to spur mitochondrial improvement lies where maximum fat oxidation occurs, I have not been able to come across a single research paper that actually demonstrates this. In fact, even in podcasts, he indicates that this is what he believes and I am not sure how to interpret that: is this something for which there is hard evidence or it is that he has personally witnessed the gains in so many world-class athletes that it is an article of faith?
In fact, we can consider various claims in this context separately:
Does training at maximum fat-oxidation improve all-round mitochondrial function more than any other zones? That does not appear to be the case and in fact interval training may induce even greater benefits (increasing the oxidative capacity).
Is it necessary to train at maximum fat oxidation intensity to improve fat-oxidation function of mitochondria? It may intuitively seem that the answer to this question is an obvious yes but that need not be the case. Partly because there is significant overlap in the metabolic processes of fat being converted to energy and glycogen converted to energy. And research looking into this question seems to suggest that the answer in fact may be no.
It is also not very clear why fat-oxidation so important for endurance performance. If we focus on running and more specifically marathons and distances shorter than that, glycogen happens to be predominant fuel source for these events.3 I understand that improving fat oxidation implies that we could perhaps reduce the reliance on glycogen but in the current elite world of running, there is no shortage of gels and fueling strategies to increase carb intake so that one would never run the risk of glycogen depletion. Maybe it is more relevant for grueling cycling events or ultramarathons where fat may be the primary source of fuel? Or is fat-oxidation even at higher intensities critical for performance?
Lactate clearance and its impact on LT1 and LT2
It would seem that a natural consequence of improvement in lactate clearance rate (which we are told happens over time training in Zone 2 ) while keeping lactate production rate fixed (at some specific intensity) leads to shift the lactate threshold4 . Presumably that adaptation happens – namely enhanced lactate clearance – at all training intensities and specifically applies in the vicinity of LT1 and LT2. Would that not imply then, that LT2 (and likely LT1) would both improve (ie, the heart rate or the pace where the threshold occurs is higher)? If so, does Zone 2 training alone improve LT2 as well? That would be interesting but I have never seen anyone make that claim before. I wonder whether the effect is small or there is something wrong in this interpretation.
Final thoughts
Let’s be clear about one thing: there is no debate that the majority of the training for most athletes happens at relatively lower intensity. This has been true for long distance runners, be they amateur or elite, well before anyone had ever heard of “Zone 2” running. No one disputes the fact that endurance adaptations increase with increase in weekly mileage and to achieve that increased volume while avoiding injury requires athletes to do most of that volume at lower intensities. The more well-trained an athlete it, the more pronounced will be the distribution of training at lower intensities. There is nothing unique , surprising or novel in this.
However proponents of Zone 2 training don’t merely recommend it on the basis that it allows greater volume while staving off any potential injury. Had that been the case, it would not be such a revolutionary idea and wouldn’t have garnered as much attention. Zone 2’s appeal stems from the claim there are unqiuely advantageous cardiometabolic adaptations that occur in a well-specified band of heart range values.
It is in the context of this claim that I have identified two issues broadly. First, the heart rate range corresponding to the correct Zone 2 as interpreted by researchers occurs at intensity that is higher than what most (at least amateur) athletes assume it is. Zone 2 should not considered as “low heart rate” training given that LT1 occurs at close to at least 75% of MaxHR.
Second, there are significant doubts, and insufficient evidence, to back up the claim of unique cardiometabolic adaptations happening even when the training is done at the correct intensity. There is some research to suggest that similar adaptations can occur even at higher intensities and there is no reason to treat Zone 2 as something very distinctive.
Does this all mean that I think Zone 2 is bunk and we should never talk about it again? Not quite.
First, it is possible that there is some high quality study that I happen to have missed demonstrating the benefits of Zone 2 quite comprehensively. As someone who remains open and curious, I would be more than happy to revise my opinion in that scenario.
Second, let us not forget that absence of evidence is not evidence of absence. Longitudinal studies seeking to identify optimal training are going to difficult to design and carry out owing to the need for closely matched subjects following training programs that are sufficiently different in just one aspect (amount of time spent in Zone 2). It may very well be that in the years to come we will be able to gather more and better data and provide a definitive answer to this question.
Going back to my personal experience, I want to also note that the general aerobic runs that I mentioned as replacing thelow heart rate training would have my heart rate lie around 146-154bpm. Although I have never taken a test, I wouldn’t be surprised to find that my LT1 is close to the top end of that range (or a little beyond). If that is so, could it be the case that my improvements have come precisely because I am running in the correct Zone 2?
Finally, at an individual level, all this may or may not be very relevant. If following some training protocol (even if that means lots of time in the incorrect Zone 2 training) has led to significant improvements in fitness and performance, there is good reason to continue doing the same. It would be sensible to experiment and tweak things a little bit but if one has discovered a program that seems to really work well then how does it matter whether there is theoretical and/or systematic empirical evidence for it? Just keep at it!
This of course not only depends on the particular individual but the threshold rises for the same individual with training HRMax.
Lactate clearance refers to the rate at which lactate that is generated by Type IIx muscle fibers (in anaerobic conditions) is transported into the mitochondria of Type I muscle fibers and further broken down in aerobic conditions to generate energy. Mitochondrial function in this context would again refer to fat and glycogen (Krebs cycle?) being oxidized to release energy. San Milan stresses on fat oxidation abilities more than glycogen.
Marathon pace lies between LT1 and LT2 where glucose contribution would be about 70%; that percentage would be higher for shorter distances naturally
This is a simple difference equation: rate of increase in blood lactate = rate of production. - rate of clearance
Awesomely written