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A Deep Dive into Training Zones: Shoddy Math and a Partitioned Understanding of Physiology?

Training zones are simply used to prescribe exercise intensity. Zoning became a thing when people wanted a little more detail in their training prescription than 'train easy & train hard.' However, this has led us down a road of poorly applied mathematical models and a reductionist partitioned view of physiology.

How do we find your training zones?

Many cyclists are familiar with FTP (Functional Threshold Power). FTP is found, in the traditional way, by sustaining your highest average power for 1 hour. This is typically done in a more tolerable and time saving 20 minutes, in which your end wattage is then x95% to attain your FTP. However, FTP can only tell you a number, not exactly how you achieved it. Percentages based off your FTP are used to ascertain training zones. FTP zones are achieved from math, not physiology. Theoretically, everyone who achieved the same FTP will have the same training zones. We know however that physiology can differ greatly. One individual may achieve the same as another via a completely different set of biological conditions. Same can be said for heart rate. Therefore, a prescription for one person might be totally inappropriate for the other.

So, in order to understand the how, you can undergo physiological testing. Like our VO2 max test. VO2 max is considered the gold standard measurement for identifying accurate training zones. Simply because VO2 max testing allows us to look at our physiology with a breath by breath analysis. We are able to see your individual physiological responses to incremental ramps in exercise intensity. We can look at substrate utilisation, breathing frequencies, O2 inhalation vs. CO2 expiration, and work out things like economy and gross efficiency. All of these metrics give us an understanding of what is happening in real time. We then use these metrics to set zones based on our understanding of bioenergetics. We can identify for the individual things like the crossover between more carbohydrate and fat, and where your "anaerobic threshold" lies.

However, whilst this is helpful to know, and certainly more accurate than percentages from things like FTP and HR, it has sent many into a frenzy. The rise in polarised training and the mass prescription of zone 2, for example, has got many people believing that zones are like strict boundaries, in which they have almost become more valuable and important than the very thing they are trying to describe; it kinda misses the point.

When zones are used to prescribe training, the idea that only certain adaptations occur at specific zones is not entirely true. Your training isn't worthless if you go a few watts outside of zone 2. Your adaptations aren't going to be ruined when you climb a hill on your 'zone 2' ride. This is a partitioned view of physiology, and not one that accurately depicts the physiological landscape. Additionally, when you advocate for these blanket prescriptions like 80/20, you miss individual context. 80/20 for someone on 4hrs a week total training time is just under-cooking it...

Let's look in more detail at the physiology, specifically what happens in zone 1, 2 and 3. As a recap, above is part of the descriptions, and distinctions, between the zones and how we identify them through physiological testing.

When we start exercising we see an incremental rise in substrate utilisation (fat and carbohydrate). Essentially, we start to use more energy. The start point is defined when we see an initial and obvious rise in substrate utilisation, this is considered the start of "zone 1". Zone 1 typically ends at something we call the 'fat max'. This is the exercise intensity at which we are using the most fat for fuel. We then go into zone 2, in which we continue to use predominantly fat for fuel until a crossover point, where we start to use more carbohydrate. This happens for everyone, you will see this crossover point occur because as intensity of exercise increases, so does our reliance on carbohydrate to help fuel higher intensity activity. This transition marks the end of zone 2 and beginning of zone 3 where we utilise even more carbohydrate.

VO2 max test - metabolic analysis looking at substrate utilisation across 3 zones.

Now, you can see if we look at the fat utilisation, fat max is achieved and then stays pretty consistent up until the dip before crossover, and so highlighting a specific 'fat max' is not really that much of a focal point per se. Actually, we are seeing a great amount of fat utilisation all the way to the end of zone 2. In real world terms, we're looking at about a 3% fluctuation in fat utilisation across 100 watts here for this individual. All that is happening simultaneously is increasing levels of carbohydrate in a linear fashion (naturally as the exercise intensity becomes greater). So, is there any need for a distinction between zone 1 & 2? Probably not. As long as training is hard enough to signal for a response, zone 1 & 2 predominately utilise fat for fuel, and limited fatigue is accumulating in these zones. And to be honest, you measure this on another day, potentially with a different amount of stored glycogen in tissues, you will see a little bit of a different response.

If we look at zone 3, we see that crossover occur, but it is important to note we are still way below threshold here. Threshold being defined as the point where homeostasis cannot be achieved through ventilation, and we see a drop in pH, thus leading to failure. This is commonly termed anaerobic threshold, or lactate threshold, or VT2, or the other 20 scientific terms one might use for 'I'm about to fall off a cliff'.

Zone 3 gets a lot of heat for being empty junk miles. And whilst this may be more true for seasoned endurance athletes who have extremely high training volumes, there isn't anything hugely different about this zone, other than we are using a bit more carbohydrate... so what? The pace is just is a little more costly over time, but it isn't a hugely different biological environment from zone 1 & 2. The only real difference here is a rise in lactate. Whilst not a fatiguing by product, lactate starts to rise just as a consequence of increased carbohydrate consumption. However, it still remains stable i.e we actually are able to utilise lactate for fuel in the mitochondria and at other sites in the body. We call this zone 'tempo' training, and the benefit being we see more improvements in development of the 'lactate shuttle' and the ability for the mitochondria to utilise lactate for fuel. It also typically simulates race pace and gets you comfortable at those paces. But as mentioned, it's not like we have this completely divergent physiological environment. Aerobic adaptions, things like development of the capillary network, mitochondrial content, and cardiac adaptations all still happen. They don't just switch off when we cross the boundary of a zone. That is an outdated perspective of bioenergetics.

So what does this all mean?

Well, as previously mentioned in the previous blog post, it's not necessarily about an 80/20 split, it's not necessarily about zone 2 either, it's about time accumulated at these sub maximal intensities overall. That is what matters most, and that is different for everyone based on their time available. The problems then with zone 3, and where the 'empty miles' discussion comes in, is that it is difficult and tiring spending loads of time at that 'tempo' intensity as training time increases. Therefore, it reduces our total time training and harms our ability to recover for higher intensity sessions. It is purely logical for someone on say 20hrs a week to polarise their training so they are able to recover "keep the high, high and the low, low". If Kipchoge was to run all the time at tempo, then he wouldn't be able to achieve as much time on feet. He would accumulate more fatigue. He would have a harder time recovering and hitting higher paces. Less time on feet = less time to signal for aerobic adaptations. On the flip side, it is not necessarily that important for someone on 4hrs a week total training time. And arguably this is the majority of people who recreationally train. If anything zone 3 or 'surfing the zones' probably gives you more bang for your buck when time is short. Zone 3 is definitely not empty mileage or sub-optimal when your mileage is pretty sparse! Plus, you will still be getting aerobic adaptations, it's not like someone turns off the tap once you cross into zone 3! Even at VO2 max we see improvements in mitochondrial capacity and aerobic adaptations, it's just far more costly!

Take homes: I always just frame the type of training to athletes as being either more vs. less appropriate for a given outcome. Yes, I think physiological testing is helpful in understanding your own physiology, seeing where things like fat oxidation and where threshold occur is useful. But more importantly I'm concerned with making the best training choices in the time that you have to maximise your adaptations. Yes, if you are doing a load of volume then understanding and training at zone 1 & 2 is more appropriate when signalling for aerobic adaptations. But this is primarily due to time spent at those intensities, rather than anything specifically magnificent about zone 2, specifically when compared vs. zone 1 and zone 3. If you go up a hill on a zone 2 session, using a bit more carbohydrate won't matter that much in the great scheme of things.

Zones are just things which we use to describe what happens across a spectrum of exercise intensities. We should not become married to them in prescription, and skew our understanding of responses and adaptations to training.


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