Breaking Down Some Common Issues With Zoning. A Cycling Example

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 time saving 20 minutes, in which your end avg watts is 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, anyone who achieved the same FTP will have the same training zones. We know however that physiology can differ. One individual may achieve the same score as another via a different set of physiological parameters.

Same can be said for heart rate. Therefore, how do you know your prescription of zone 2 based off HR or % is accurate? This is where VO2 tests can help. With a VO2 you 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. We then use these metrics to set zones. Great.

However, whilst it can be helpful to know, and more accurate than percentages from things like FTP and HR, zoning in general has become a bit hyperbolic. The rise in polarised training and the mass prescription of zone 2 for example, has got people missing the wood from the trees (That is, if people actually appreciate the above. I’ve seen posts where people are criticising Tadej Pogacar for saying his zone 2 is at a HR that ‘is not zone 2 bro’. Hopefully some of that has cleared that up).

The issue though, when zones are used to prescribe training, the idea that only certain adaptations occur at specific zones is not entirely true (away from the extreme ends of the spectrum). People are usually not concerned with that though, instead they are concerned if their training sesh is ruined when they go few watts outside of zone 2... No. If you go on a climb on your zone 2 ride, chill out. This is a partitioned view of physiology, and not one that accurately depicts the physiological landscape.

Secondly, like with all things, the application of zone 2 and polarised splits are about time / opportunity cost when scaled back to the individual. Zone 2 and its benefits are not magical, instead, lots of zone 2 is just a logical way to manage training load for people with high volumes (where most of this research comes from aka the pros).

Lets explore both. The VO2 analysis breakdown: When we start exercising we see an incremental rise in substrate utilisation (fat and carbohydrate). Essentially, we start to use more energy to sustain work. 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.

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. Like, really anything under crossover can be considered a range of fat max.

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. That’s a lot of watts.

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 anyway? 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 (to a point). I’m sure, if you measure this on another day, potentially with a different amount of stored glycogen in tissues, you will likely 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 exercise 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 shutdown so I don’t shit myself'.

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 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. Small amounts of lactate is also still being produced in zone 2, it’s just being utilised in the cell and not ‘spilling over’ into the blood. As mentioned, it's not like we have this completely different physiological environment across 3 zones. Therefore, improvements in things like the capillary network, mitochondrial content, and cardiac adaptations still happen with consistent training across these zones. Things don't just switch off when you cross the boundary of a zone.

This is also just looking at things like substrate utilisation... a lot more is going on.

So what does this all mean and why am I rambling about it?

We know that volume is the main driver behind elite endurance performance. So you will see a lot of elites adopt an 80/20 approach with a lot of zone 2. This isn’t about zone 2 being special though, instead it’s just a logical way to manage training volume whilst still moving the needle... Where training is hard enough, but not so much that it leaves you wrecked.

More time at zone 3 and 4 (still below threshold) for example is more costly than zone 1 & 2 because of the mechanisms above. Therefore, our total time training is less and reduces our ability to recover for higher intensity sessions. However, to go further, whilst an 80/20 split is just a logical way to manage training load for high volumes, you need to be careful when synthesising this information. The only thing that matters is what is appropriate for where you’re currently at, and that is different for everyone based on their time available and training history.

You’ll find research showing higher volumes of training is the norm amongst faster runners (and across all endurance sports). What you also will see is what I said above, most of which done at easier paces (zone 1 & 2). However, is this applicable to you? If you’re limited on time, an 80/20 split is probably undercooking it on something like 3hrs a week. Actually, by pursuing more intensity it can go a little way to making up the gap. You’ll have to accept your genetic ceiling won’t be reached and you won’t be winning Le Tour but... you probably wouldn’t anyway!

Take homes:

-HR and FTP % is not a good way to zone. The best way if no VO2 is RPE, rating your sessions ‘easy or hard’.

-Zone 2 isn’t uniquely special.

-In general, you don’t have to be super specific and bound to a certain zone. Think ‘ball park adaptations’.

-High training volumes however mean you should be more cautious in making your easy sessions truly easy and hard sessions truly hard.

-An 80/20 split is a logical management of training load when training volumes are high. That enables people to stay consistent. Being consistent over years = what matters most. Does this matter for you currently though?