What is Zone 2 Training?
What's all the fuss about?
If you’re lucky enough to see or hear from health and fitness experts on your social media or in the news, then over the past couple of years, you’ll probably have heard the phrase ‘Zone 2 Training’ a fair amount. This article aims to remove the fog surrounding the phrase: Outline what exactly ‘Zone 2’ is, why is it proposed to be beneficial, and whether it is as effective as it is made out to be…
What does ‘Zone 2’ mean?
The name ‘Zone 2’ or Z2, depicts a certain intensity range for endurance based exercises, such as running, rowing, cycling, walking etc. Typically, Z2 training appears on the lower end of this range: Not hard, but not effortless. However, Z2 may be specific to the method used to measure exercise intensity.
Exercise intensity can be measured using many objective and subjective methods. I have listed a few below. The list starts with accessible methods but gets progressively more professional/physiologically determined…
RPE (rating of perceived exertion) - ‘How hard on a scale of 1-10 are you exercising?’ ‘Are you able to maintain a conversation whilst exercising?’
Heart Rate - ‘What percentage of your maximum heart rate are you currently working at?’ Can be measured using a wearable device.
%VO2 - ‘What percentage of your maximum aerobic capacity are you currently working at?’ Requires either a lab-based assessment of your aerobic capacity, or estimates provided by well known equations or through wearable technology.
Lactate concentration - ‘How much lactate is expected to be circulating in the blood at this intensity?’ Requires previous, accurate, exercise testing.
Most commonly, especially for general populations, RPE and heart rate are used to measure exercise intensity. This is because these methods don’t necessarily require any technology or prior exercise testing. More experienced and capable athletes may well undertake VO2max testing or lactate profiling, but this tends not to be the case for the everyday athlete. When using RPE and/or heart rate to assess exercise intensity, a total of 5 zones are commonly created. I have outlined the features of these zones below:
Zone 1) RPE 1-2; can speak in full sentences. Heart rate 50-60% of maximum.
Zone 2) RPE 3-4; can converse, but not comfortably. Heart rate 60-70% of maximum.
Zone 3) RPE 5-6; hard to hold conversation. Heart rate 70-80% of maximum.
Zone 4) RPE 7-8; forceful breathing and no conversation. Heart rate 80-90% of maximum.
Zone 5) RPE 9-10; maximum effort. Heart rate >90% of maximum.
Therefore, in its simplest form, Z2 training represents an exercise intensity which elevates the heart rate to slightly over a half of its maximum potential, but still enables the exerciser to be able to talk. In theory, one would be able to maintain this level of exercise for at least an hour.
The Theoretical Benefits
Z2 training is a low enough intensity to allow the exerciser to go for an extended period of time. If we take the government recommended guideline of 150 minutes of moderate-intensity exercise per week - Z2 training would be a good intensity to aim for. It is reachable and much easier to accumulate Z2 minutes than Z5 minutes, for example. In theory, Z2 is key to improving fitness. Not only does it allow for sustainable, low-injury-risk addition of exercise volume, but it also aids recovery from more intense exercise sessions. You would also be surprised at just how ‘easy’ Z2 pace is. Most recreational athletes tend to naturally pace themselves at Z3 on an easy day, and the popularity of Strava will have some people trying to convince others that their 30-minute ‘easy’ run, with an average heart rate of 195bpm, was a walk in the park…but that’s a discussion for another day…
It has also been suggested that Z2 training improves mitochondrial function. Within our cells, mitochondria are responsible for energy production, so it makes sense, then, that training at an intensity which requires more energy, improves the efficiency of the mitochondria which produce energy. Regular training can actually increase the density and size of the mitochondria within the cells, leading to improved aerobic fitness and metabolic function.
Zone 2 training potentially also has a much easier ‘buy-in’ compared to other forms of cardiorespiratory exercise. For example, research has shown that enjoyment of steady state exercise, such as Z2, to be higher than high intensity interval training (HIIT), at least when first starting an exercise routine. This may well lead to increased adherence to an exercise plan, which in itself has many benefits, including improved mental health and reductions in rates of chronic inflammation and disease.
So, is zone 2 training as special as its made out to be? Well…not exactly…
The Small Print
Here’s the thing: Many of the benefits of zone 2 training aren’t exclusive to zone 2 training. In fact, Z2 itself is a huge range, especially at the physiological level.
For example, let’s take a 20 year old exercise beginner. In theory, their maximum heart rate would be roughly 200bpm. Their Z2 would then encompass anything from 120bpm-140bpm. I can guarantee that this person would be able to tell you pretty quickly whether they were exercising on the low, or high end of their Z2 range. So, let’s consider that they aim for a Z2 run, but the route is quite hilly, so they accidentally dip into Z3 by averaging a heart rate of 145…disaster! Not really. They’ll still benefit from this exercise in almost exactly the same way than if they had average 140bpm. These borders/cut-offs are great for us as practitioners, but in reality, the lines are blurred, not exact. Theory is clean, but reality is messy. This is particularly true with regards to heart rate zones, where warm weather, dehydration, caffeine intake and other external factors can play a large role in heart rate differences. An easy pace on a temperate day will be much faster than an easy pace in the height of a summer heatwave with 90% humidity.
Let’s consider that this person ditches heart-rate monitoring and instead uses the RPE method for measuring intensity. Would they genuinely trust themselves to know, for sure, the difference between a 5 RPE and a 6 RPE? No, but again, that’s not a problem, because even though a 6 would be considered Z3 training, there will be minimal difference between this and Z2.
So what does this all mean?
My personal opinion is that Z2 has attracted a lot of attention because it sounds ‘scientific’ yet fairly easy to understand for the general public. It also convinces us that exercise doesn’t have to be ridiculously hard all the time, which is a pleasant contrast to the HIIT popularity wave which preceded it. If you tell someone you’re going for an ‘easy run’, it doesn’t sound too impressive, but a ‘Z2 shakeout’, well, don’t you sound like a pro? Don’t get me wrong, the theory behind Z2 training is absolutely fine. We should all be looking to improve our aerobic base fitness, and lower-intensity sessions like Z2 are key for this. In fact, the majority of our cardiorespiratory exercise sessions should probably be at a lower intensity. However, it is important to remember the other end of the spectrum.
Some of you may have heard of the often cited ‘polarised training’ method. This refers to the theory behind spending 80% of exercise sessions at low intensities such as Z2, and 20% at higher intensities like Z4/5. I personally find that this method works well for many exercise beginners or recreational athletes: the majority of cardiorespiratory sessions can be low intensity, social, and require much less recovery afterwards. For those coming from a lower baseline, these could simply be long walks or hikes with the family. However, at least once a week or so, you can still go for it with an interval-style session, class, or whatever is best to get that heart rate close to maximal, just to remind yourself what that feels like. In reality, the hype around Z2 pushes us to misunderstand, and make us more likely to undertrain. We mustn’t forget the benefits of higher-intensity exercise for health and also exercise performance.