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The Triathlete Blueprint Newsletter #35-Z2 Training Decoded: What's Beyond the Hype?

Updated: Apr 8

Read time: 5min.

By Coach Yan Busset

If you're even a bit curious about training methods (which you certainly are if you're reading my blog), you would need to live under a rock to not have heard about all the hype surrounding Z2 training benefits for endurance athletes and overall health benefits. In a way, it's nothing too new under the sun; aerobic training, aka Z2 training, has always been around, but it has become more popular lately for a few reasons. Firstly, we moved away from the trend of the 'no pain, no gain' era, and a more polarized training method has emerged where 80% of the training is at or below the high Z2 limit, with less time spent in the 'grey' Z3 zone. The word is spreading that 'slow is the new fast.' People used to push hard all the time for a good reason: they wanted to progress. But when you understand all the positive adaptations that happen when training at Z2, you start to discipline yourself to resist the temptation to always go hard or go home.

Z2 training has multiple benefits, such as:

  • Enhanced Fat Oxidation

  • Increased Mitochondrial Density

  • Improved Cardiovascular Health

  • Enhanced Lactate Clearance

  • Improved Aerobic Capacity

The beauty is that all these adaptations made at this lower pace will be beneficial not just for long-distance athletes, but also for shorter races. By training for hours in Z2, you can also increase your VO2max, and you will be able to perform back-to-back, more intense efforts as your lactate clearance rate will be higher.

Also, Z2 training has gained popularity as the side health benefits of training at this intensity match not only performance gains but studies also show a correlation with longevity. Especially the fact that it increases mitochondria density, which has crucial functions in disease prevention (type 2 diabetes, cardiovascular disease, metabolic syndrome, cancer, and Alzheimer’s disease)

Pacing your Z2 right is crucial to get the full effects. Too fast, and you end up in Z3, which won’t be sustainable; too slow, and you won’t get the full benefits. Let’s check together why training at Z2 is beneficial and how to do it.

Let’s define Zone 2. It’s the second zone of a 5-zone system.

  • Zone 1: Awkwardly slow/active recovery pace

  • Zone 2: Endurance

  • Zone 3: Tempo/grey zone

  • Zone 4: Anaerobic Threshold

  • Zone 5: Vo2max and above fast pace

The limit between zones 2 and 3 is defined by the Ventilatory Threshold 1, also known as the Aerobic Threshold. This is the point during exercise when your body transitions from relying primarily on fat oxidation (with oxygen) to an increased reliance on carbohydrates for fuel. In simpler terms, it's when you switch from moderate to more intense effort/exertion.

How to assess it?

At VT1, your breathing becomes noticeably heavier as your body starts to produce more carbon dioxide from metabolizing carbohydrates, but you can still speak in full sentences. It’s a conversational pace. As for breathing, when you're below the aerobic threshold, you can maintain a steady, controlled breathing pattern. But as you approach and surpass this threshold, you'll notice your breathing becomes faster and deeper. This is your body's way of trying to get more oxygen to your muscles and to get rid of carbon dioxide, which builds up as a result of increased metabolism.

Zones could also be assessed by lactate levels in the blood. Lactate testing took the same hype train as Z2 and polarized training and has become more mainstream and accessible. Soon, some real-time lactate meters will probably democratize even further, but there are some limits, as testing lactates right can be tricky. The threshold commonly referred to as the equivalent of the aerobic threshold/VT1, aka LT1 (lactate threshold one), is commonly set to 2 millimoles per liter of blood (mmol/L), but there are variations depending on the way to measure it and body composition differences.

Also, if you measure Z2 via an all-out test or FTP test, for example, the result will be derived from a percentage of the Anaerobic threshold, and if you are more of a petrol head than a diesel engine, your lower zone with that type of zones calculation can be very much overestimated.

Considering all of these, my go-to method to make sure my athletes train at the right intensity is to test and teach them to assess it by feel, via nose breathing, conversational or breathing pattern. The definition of VT1 is an increase in CO2 production, and you need to breathe more heavily to evacuate it. That's what a lab tests with a Gas Exchange Test (GET), using the ventilatory markers rate and depth of breathing. If you pay attention to it, it’s easy to feel that moment where nose breathing won’t be enough, where you can still conduct a conversation, but it becomes harder.

I use power (watts) and HR (bpm) as side data that will show progress in the long term. For the same Z2 feel after months of training, you will see an increase in watts for that same pace. And that's what we want: to be as fast as possible within Z2. Fast as possible by metabolizing fat rather than too many carbs. That's the name of the game for long-distance athletes. Training at VT1 is crucial as it closely aligns with race pacing, especially for longer distances. Your ability to improve your speed/force at this intensity is directly correlated to your future race performances.

So, no FTP test to assess Z2 (I only use "All out" tests or "FTP tests" to assess VT2/Anaerobic threshold) but a nose breathing ramp-up test and a one-hour validation test.

Concretely, I ask my athletes to start from a conservative pace, way below what they think is their current Z2 limit, and to ramp up by 3-min steps little by little (3 watts or 0.5 km/h per step) until they feel this change in the breathing pattern or that nose breathing is not sustainable easily anymore. Then later, they will perform a validation test as a second way to make sure we nailed that Z2 limit. This validation test is very simple: one hour at the Z2 limit you find during the first test, you record HR, and if you see afterward that between the first and second half-hour of this validation test, your HR was decoupling or drifted less than 5%, we are good. If it was more than 5%, then it needs to be readjusted; repeat the previous step until we nail it. It’s a low-tech but science-based method that has proven its accuracy. I had athletes having very similar results with this method and with fancy lab GET tests.

So now that you know more about how to get into Z2 and why you should do it, I hope you will think twice before pushing hard on each session. Sure, pushing hard on key sessions is also beneficial, but 80% of the workload should be performed at Z2 and below. So, no excuses anymore; your time is precious, so by making sure you train at the right intensity, you will make the best out of it.

Looking for More Tips?

Check out some of my past articles that are in connection with this article:

Thank you for reading and see you next week!


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