Is Zone 2 training really worth it?

Zone 2 training is all the rage at the moment being promoted on many of the top podcasts… and for good reason.

When we assess the performance of top endurance athletes, they tend to use what is referred to as polarised training. This is when around 80% of their training is done at an ‘easy’ effort and 20% of their work is done at moderate to high effort levels.

If this is the away elite athletes train, then we should do too, right?

Before we get into some of the assumptions and misconceptions around Zone 2 training, it is important to lay some foundations of the point of different types of endurance training and the adaptations these ‘types’ create.

For each of the effort levels we use during training there is a list of adaptations associated with them.

These take place along our respiratory system to increase the delivery, uptake, and usage of oxygen. The more oxygen we can get into the body the better we can perform as we have more ‘fuelling potential’. Think of this as the size of your engine.

When we combine this with the correct gearing, so that we are efficient at each effort level from easy to hard, we have two major components of endurance performance that we can manipulate throughout training.

Now, we don’t want to turn this into a physiology lecture so we will simplify this down to the following.

For each gear (zone) we identify from physiology testing, the time we spend in these zones will target the adaptations most associated with them.

We have intentionally highlighted the word most here because ultimately, all types of endurance training will cause the adaptations we desire for most of our endurance performance.

This isn’t strictly true especially when looking at higher intensity work but looking through it from the perspective of Zone 2 training, the adaptations we get from higher effort eundurance work are fundamentally the same. The difference is to the extent at which each zone turns up each adaptation dial.

How do we define Zone 2?

If you use a heart rate monitor or sports watch you may be aware of the different heart rate zones. For example, in two of the leading wearable tech brands, Polar and Garmin, they break their heart rate range zones into 5 zones.

Active Recovery (Zone 1).

Easy Effort (Zone 2).

Moderate/Tempo Effort (Zone 3).

Threshold/Sweet Spot Effort (Zone 4):

Anaerobic/High Intensity Effort (Zone 5).

When we are talking about Zone 2 training this is based on the idea of a 5-Zone system. Simple, so far?

This simple allocation of the above zones is determined on wearable tech and heart rate monitors by using percentages of maximum heart rate. Although there is a clear link between heart rate and the zone we are in this isn’t actually how we would define training zones if we were to take a science-based, pure physiological, approach.

When we define these zones in physiology testing, we define these from either accurate measurement of oxygen uptake and/or blood lactate. Lactate testing is the most accurate method of determining these zones as it offers a more direct measure of what is going on with our internal physiology and therefore, we can ‘know’ how this directly relates to our heart rate to create accurate much more accurate training and in turn allocate appropriate heart rate zones.

When we run people through our lactate testing the zones we define using this method are often very different to the zones determined by wearable devices and simple percentage of max heart rate estimations.

When we are defining Zone 2 accurately, using lactate testing, these are the lactate values that are used to determine zones.

Zone 1: Active Recovery - Lactate levels below 2.0 mmol/L.

Zone 2: Easy Effort - Lactate levels between 2.0-2.5 mmol/L.

Zone 3: Tempo - Lactate levels between 2.5-3.0 mmol/L.

Zone 4: Threshold - Lactate levels between 3.0-4.0 mmol/L

Zone 5: Anaerobic Zone - Lactate levels above 4.0 mmol/L

An important observation here; there is also a 3 zone system that is often reported. The three zones, as you can see in the image below, are defined by 2 key lactate ‘turn points’ LT1 and LT2.

The 3 Zone sytem determined by LT 1 & LT 2.

LT 1 occurs when lactate increases above baseline measures, practically this would be detectable around 2-2.5mmol/L. LT 2 is determined where there is the second big lactate turn point, around 4mmol/L, occurs.

As it so happens LT1 also coincides with what is known as ‘ventilatory threshold 1’ (VT1) and LT 2 coincides with the second ventilatory threshold (VT2). These ventilatory thresholds are, to oversimplify it, changes in breathing that correspond to changes in measures of oxygen consumption (there are several methods to calculate these thresholds, but we won’t go into that here and focus on the lactate-based method).

Zone 1 using this approach (Zone 1 & 2 in our 5 Zone model) would be any pace/power/heart rate that is below this first Turn Point.

Zone 2 (which combines Zones 3 & 4 from our 5-zone model) is between LT 1 & LT2, and Zone 3 corresponds to Zone 5 and is the work we are doing after LT2.

Got that?

There is some argument to be made that in terms of adaptation, this is all you really need to focus on in polarised or pyramidal training. As Zone 2 in this model (3 & 4 in the 5 Zone system) create similar adaptations, require similar adaptive durations and generate similar types of fatigue.

Now we have defined this properly, this highlights the first two problems most people have when trying to utilise Zone 2 training.

Problem 1: The problem of individualisation.

Let’s say you’re an athlete who is already maxed out on their training capacity for the week, there is no more room for increases in training volume, you are at your ‘maximum recoverable volume’.

You read some sexy article on Zone 2 and decide that you are going to drop some of your higher effort work for more Zone 2 work. The question we should ask here is how do you know that you need more Zone 2 work?

Or more appropriately, what does your performance profile look like to decide how you should spend your training ‘budget’ to get the most out of your adaptive energy and improve performance.

Here in lies the problem of individualisation.

What if we were to run you through some physiology testing and we found that your distribution of training zones showed that your Zone 2 performance was solid. We then combined this with a review of your current training and this showed that you were still spending a significant amount of time in and around Zone 2 with your current program and in fact, your clear weakness was at higher intensity/higher power output efforts.

Note: Before it seems like we are bashing Zone 2 completely, this idea of bolting on more Zone 2 work is completely reasonable IF you have the time and energy to do this to increase overall training volume, alongside ticking your other training and recovery boxes.  

Unfortunately, for most athletes who don’t have the time, or are from sports where they prioritise other training methods, we must understand their individual needs to justify implementing a specific training strategy… Zone 2 or otherwise (Sweetspot, HIIT and SIT etc. do not get a free pass here).

Problem 2: Training accuracy.

This point is more of an observation that may highlight the futility of Zone 2 training for some people. It is not uncommon that when we test people, they simply do not have a Zone 2 to target.

Ok, that’s not strictly true, they do have a Zone 2 but this only occurs at such a low level of effort, here we are talking about walking at a brisk pace at best, and any form of running will push them quickly out of Zone 2 and into Zone 3.

Note here that cycling is less prone to this as we are typically dealing with a broader range of powers/speeds, so that we can still be performing a sports specific activity and developing economy and efficiency of movement and be in the correct zone, this is often not the case for running.

Back to running. Now, we can still perform Zone 2, easy effort training using other related metrics. For example, perceived exertion scales, the talk test and even the heart rate zones we ‘bashed’ earlier.  However, this still would not strictly be Zone 2 in the physiologically determined sense of the term.

You might ask, “what is wrong with just walking until you build fitness and build a Zone 2 at a higher pace?”.

The answer to this is, absolutely nothing at all. However, we would contest that if a person were at a point where they lack the fitness to have a Zone 2 during what we would consider running rather than walking, then their time will be better off spent in higher Zones for the ‘experience’ of running for several reasons we will now move on to.

Problem 3: Training time.

Let’s say your Zone 2 is walking, or very low power cycling. How long do think you would need to perform that for to cause enough stress to trigger an adaptation? If you are very unfit then this might be a short walk to get some serious benefits, however what if you are already a pretty fit sportsperson who just so happens to have a profile with a very small Zone 2.

This is not uncommon in the athletes we test. We have athletes with relatively high VO2 max, that are fit in the context of their sports, but do not have a classical endurance profile and Zone 2 seems to have gone on holiday and left Zone 3 & 4 to hold the endurance fort. Their Zone 2 according to the data would be at effort levels so low that it would be absolutely pointless for them to get any benefit of doing direct Zone 2 work, either because they wouldn’t have the time to spend in this zone (say walking for a few hours a day) or most importantly is because there are other ways to improve Zone 2 without doing Zone 2 work directly.

This is a really important point. If an athlete doesn’t do any ‘pure’ endurance work, say they only operate in Zone 4 and above in their training with any easy effort work being during bouts of recovery.

Then even though we might want to improve their endurance base then Zone 2 training isn’t likely to be the most efficient way of doing it. Indeed, even an increased focus on some Zone 3/Tempo/Moderate effort work will have carry over into improving the adaptations that will give this athlete a more defined Zone 2 and be much more time efficient.

When you look at elite, or good level amateur athletes, their Zone 2 is often in the region of only 3-5km slower than their race pace. Take a marathoner who races at 20km/hr, their Zone 2 work is likely conducted at 15-17km/hr!! This is still insanely fast and operating at this pace will mean that significant training stress and the accompanying endurance adaptations will take place within a few hours of this ‘easy’ pace and most importantly they are CLEARLY running.

This is related to how economical these athletes are (we’ll come back to this on the next point). If we consider breaking from a walk to a jog takes places around 7km/hr that means that if we considered a run being 8km/hr and above, their range of running speeds keeping them at Zone 2 (and even being able to run in Zone 1!!) would be from 8-17km/hr.

This means that even if they started at the higher end of Zone 2 and wanted to stay in that Zone after a few hours running, they have plenty of pace to drop down and stay in Zone 2 even as they become fatigued and heart rate and lactate potentially start to creep up they will still be doing Zone 2 work.

Compare this to someone who has a Zone 2 of 8-10km per hour and much poorer running economy. Within 30 minutes of running at 10km/hr they may have to adjust pace downwards by some margin to stay in Zone 2, and as time extends and fatigue increases even 8km/hr will have them working in a higher Zone as their economy breaks down with fatigue.

At some point they will no longer be doing what we would consider to be running and this comes with a problem we will come back to in point 4.

At this point should they end the workout? Allow the zone to drift and do more work? Or simply accept the physiology and reduce to walking pace? All of these are appropriate to do in the right circumstances, but this comes down to understanding where you are as an athlete, the amount of work being done and where you are best placing your training efforts.

Remember, Zones are defined and ‘fixed’ in relation to our internal physiologically (using heart rate as a reasonable proxy after testing) not the output we are measuring such as speed, power or pace. Therefore, to maintain a physiological zone may mean adjusting the output as we fatigue and as we ‘drift’ across zones. We can use this drift as a good indicator that we have caused enough training stress.

The problem with this idea, is that for some people if they used performance or heart rate drift alone to determine a workout termination point, they just wouldn’t get the volume of work done with such narrow zones as the drift between them can occur quite quickly especially in less fit or heavier athletes.

Problem 4: Economy & efficiency.

This article I’m sure you’ll agree, has already gone on long enough! At this point I feel like we have covered most of the major considerations when looking at introducing more Zone 2 training especially if sacrificing other training Zones.

So, building slightly on to something I have alluded to earlier is this…

There is a reason why sprint and hill training are prescribed for many athletes and that is as well as targeting those Zone-specific adaptations they also promote better running economy.

We aren’t going to go too deep into the weeds on this here, but if someone hasn’t got the time to be doing lots of speed or interval work, then sacrificing this for more Zone 2 work may have some knock-on implications for performance.

Basically, for some athletes spending a significant amount of time running slower will reduce their running economy and efficiency and potentially make them perform worse.

Problem 5: Identifying the true effectiveness of Zone 2 in recreational athletes.

Trying to pick through the research that demonstrates how effective pure Zone 2 training versus pure higher effort training is challenging to put it mildly. In any case, if we go back to our original ‘hypothesis’ that elite athletes spend a lot of time in Zone 2 but still in fact do many hours training at higher efforts as well.

We believe the key is to do your higher effort work with high quality, then fill in the gaps with easy effort work. Even in elite athletes the focus is producing high quality, high effort work as the priority.

This poses an interesting question. What would happen if we pulled the easy effort work entirely to these athletes? Or even more interestingly, what if we replaced easy effort work with higher effort work but with an equated level of training volume and stress?

There actually some research including athletes across several sports that have manipulated training intensity distribution across our zones and looked at performance outcomes. What they have collectively found can be confusing to interpret and often contradictory, but to broadly summarise the trends these are:

Polarised training (high percentage Zone 2 with an amount of Zone 5 training of 10-20%) seems to create the best overall training benefits.

Doing a vast amount of work in Zones 3 & 4 and neglecting Zone 2 might be less effective than a polarised program, especially if you don’t do any higher effort work in Zone 5.

If you do a very high percentage of Training in Zone 2 without any higher effort work, then this probably isn’t a good idea.

Doing a high proportion of Zones 4 & 5 work seems to reach a threshold where it offers no more benefit, and the focus should still be mostly on Zone 2 work to build overall volume (however, this is one are where the research is mixed).

Ultimately, we must take into consideration that here we are talking about apparently well trained athletes doing a solid amount of training volume, so to interpret these effects to program for someone training for only a few hours a week is difficult and therefore would lower training hour availability benefit from more higher effort work.

For example, would a recreational athlete with only 3 hours available endurance training per week be better spent filling most of that time with Zone 2? We suspect not, and this is more reflective of most runners who will be training for less than 5 hours per week. In this case the focus might even shift to 50% of training or even higher being Zone 4 and above.

Whatever the thoughts on the current evidence, where you are as an athlete and the limiters you have on maximising your performance.

The one common theme here is that if you aren’t doing any high effort, threshold and above work, then you are probably missing out on performance, so exclusively Zone 2 training is probably not a good idea and neither, would we suspect, taking away higher effort work to accommodate easier work if you are doing less than 2-3 hours a week of Zone 3 and above work as it is! Zone 2 should be seen as something you add, rather than something to replace in most (but not all) cases.

Conclusions

Assuming someone is already moderately fit and conditioned to endurance work with a goal that is to get faster. Then Zone 2 work can, and arguably should, be added to a program to add overall volume but should not be the sole foundation it is built on.

If you are at a point where you are fit enough and have the training time to polarise their program properly, then they should.

But honestly, do we genuinely think that any athlete or coach would replace higher effort, performance specific work with Zone 2 work if they were limited in time in some way?

The answer may be yes under very specific circumstances, say a return from injury, illness, in a beginner or with certain physical constraint.

But if you could only run/cycle/swim for 2-5 hours a week and wanted to get the most out of your training would you choose Zone 2 training, or even a high proportion of your training in Zone 2? We would suggest with this ‘little’ amount of training, the goal would be maximising your volume across those sessions (again assuming you had built up to handle those volumes) and that as our volume is metric that contains both the duration of the sessions and the intensity, this would point to operating with larger proportion of your training distribution at higher intensities than a polarised model.