Endurance Athletes: Why You NEED To Know Your Training Zones

Training zones can be expressed in a number of formats and in a number of variations. Some people prescribe 3 zones, others 5, 6, 9 or even 11 zones. At the end of the day, it doesn’t really matter how many zones you use, as long as you know what adaptation(s) each zone is aiming to achieve.

This article will take you through the basic, but essential characteristics of training zones, how they are calculated, and what they physiologically aim to achieve in a training program.

First things first, though, we need to cover off some physiological science of how the body actually adapts to get fitter in the first place.
There are three energy systems which the body uses to create energy: the ATP-PC (or phosphocreatine system), the anaerobic glycolysis system, and the aerobic glycolysis system. Glycolysis is simply the process of breaking down glycogen (carbohydrate) to create energy with oxygen (aerobic) or without oxygen (anaerobic). As endurance athletes, we are primarily focused on aerobic and anaerobic glycolysis, so we will largely ignore the ATP-PC system in this article.

As a quick overview, the ATP-PC system provides energy for short sprints lasting less than 15 seconds, after which it exhausts and contributes very little to energy production. It creates energy VERY quickly, but has a very limited supply. The only way to replenish this system is through complete rest, which is why we are not too interested in this system for endurance performance (because we essentially run out of this fuel within 15 seconds of exercise, and as endurance athletes we don’t have long periods of complete rest to replenish it).

Anaerobic glycolysis metabolises carbohydrates in the absence of oxygen to create energy. This system kicks in to its full capacity in the 20-60 second time frame of exercise, while our oxygen system is still ‘warming up’. It also increases its energy contribution when exercise intensity is high and we can no longer supply energy solely through the aerobic system. It creates energy quickly, but produces lactic acid as a by-product. Lactic acid is made up of a lactate molecule (measured in lab tests) and a hydrogen ion, which causes muscular fatigue. Hydrogen ions are acidic, causing the pH of the blood to drop (pH = potential hydrogen). The pH scale is a range between 1-14 indicating the acidity or alkalinity of a fluid, with 1 being acidic, 14 being alkaline, and 7 being ‘neutral’. Human blood is tightly regulated and functions optimally in a range between 7.35-7.45. When an influx of hydrogen is delivered into the muscle (as a result of anaerobic glycolysis), the pH of the blood drops below 7.35 (becomes more acidic). Dropping blood pH below 7.35 causes glycolytic enzyme inhibition. In other words, the enzymes responsible for creating energy begin to not function properly, and if blood isn’t returned to normal levels, we will be forced to slow down significantly to clear out hydrogen in order to return to a resting state. Why do we have to slow down? Our mitochondrial enzymes (explained in detail below) literally stop creating energy. No energy = no muscular contractions.

The aerobic glycolysis system kicks in predominately from around 75 seconds into activity, and is the major contributor to energy production in endurance events. This system involves extracting oxygen from the blood stream and transporting it into the muscle, where an organelle called mitochondria use it to create lots of energy. This system creates energy relatively slowly, but provides at least 16x more energy than the anaerobic system, and has a much greater supply than the anaerobic system (you can, obviously, train, race, or exercise for very long periods of time due to an efficient aerobic system). The major benefit of the aerobic system (other than creating 16x more energy than other systems) is that it doesn’t create any fatiguing metabolic by-products. Oxygen goes in, we metabolise it through the muscle’s mitochondria, and we release lots of energy, heat (which eventually will contribute to some fatigue, but nowhere near as much as hydrogen ions do), carbon dioxide, and water (remember this!). The sheer fact that no fatiguing by-products are created with the aerobic system (other than a little heat) is why we can use it for such a long period of time.

So, to sum up the roles and differences of aerobic and anaerobic glycolysis, think of it like this: The aerobic glycolysis system is the predominant energy system used by endurance athletes in endurance events. It creates energy relatively slowly, but in huge proportions and for a long period of time. When exercise intensity increases (whether due to a surge in a race, starting or chasing down a breakaway in cycling, or simply because you’re racing a sprint or Olympic distance triathlon and the intensity is high from the get-go), the anaerobic glycolysis system picks up the slack and assists with energy production because it creates energy quickly. We need it to assist the aerobic system so we have enough energy to fuel our muscles to pick up the pace or maintain a high workload. As a result of this, we create lactic acid, of which the hydrogen component causes fatigue if not metabolised out.

But how do we metabolise hydrogen? With oxygen! The only difference between the aerobic and anaerobic system is that one uses oxygen, while the other doesn’t. We mentioned before that a by-product of aerobic glycolysis is water… What’s the chemical formula for water? H₂O! 2 hydrogen ions attach to 1 oxygen molecule to create water. The aerobic system creates water, while the anaerobic system creates an accumulation of acidic hydrogen ions because there is no oxygen to attach to, which causes blood pH to fall below 7.35, inhibits our glycolytic enzyme activity, and forces us to slow down. What does slowing down do? Reduces the exercise intensity so we no longer require such a high demand from our anaerobic system, allowing oxygen to circulate, bind to hydrogen ions, and convert them to water. We have now recovered and can go again.
Note this process is very complicated and multifaceted, so for anyone with a high-level chemistry or sport science background realising there are more factors that play a part, I get it, but this is all the average individual or athlete needs to understand.

Enough science. Let’s get into why training zones are important.

These are the training zones identified at METS, but you can use any zones as long as you have a clear definition of what they are and the goals of each.

Active Recovery Zone:

Energy system we are improving: None.
Intensity range: <56% VO₂ max.
Goal: Metabolise hydrogen ions and recover from a tough session quicker.
Explanation: Oxygen + hydrogen = water. Increasing blood oxygen circulation at a low intensity means an influx of oxygen can attach to all the circulating hydrogen ions quicker than if we do nothing. This metabolises hydrogen, converts it to water, and brings blood pH back to a resting 7.35-7.45.
Where in a Training Program?: Immediately after hard/intense sessions, or the day after a hard/intense session.

Endurance Zone:

Energy system we are improving: Aerobic Glycolysis.
Intensity range: 56% VO₂ max until the point just before we begin producing lactic acid above resting values.
Goal: Improve VO₂ max by improving the number, size and surface area of mitochondria (the organelles in muscle responsible for aerobic energy production, remember?), increase the volume of the heart (so we can circulate more blood and therefore oxygen each beat), increase capillarization of the muscle (improve blood/oxygen supply at the muscle so the mitochondria can absorb it).
Explanation: It is important for these training sessions to be completed in the absence of lactic acid. Remember how hydrogen/pH below 7.35 shuts down enzymes that are responsible for glycolysis? We don’t want this. We want mitochondrial enzymes firing at 100% in order for them to be stimulated to grow and adapt, not only 40%, 60% or 80% of them working because we a filled them with acidic blood.
Where in a Program?: All stages, really. A high emphasis at the beginning of a program when we are “building an aerobic base”. This is your easy long, slow distance training.

Threshold Zone:

Energy system we are improving: Anaerobic Glycolysis
Intensity range: The point at which you begin producing lactic acid above resting values up until the point that we reach our maximum capacity to clear it out (ie threshold/FTP/max intensity we can hold for 45-60 minutes)
Goal: Improve functional threshold power and bring this closer to 100% VO₂ max by improving the body’s ability to tolerate and clear lactic acid.
Explanation: The average athlete reaches threshold at 70% VO₂ max. Those highly adapt through working at this intensity are able to tolerate larger amounts of it, and also clear it at a quicker rate. As a result, they can withstand intensities of up to 88-94% of VO₂ max before reaching threshold. Train specifically. Want to increase your lactic acid tolerance? Train in the presence of lactic acid. This zone does exactly that.
Where in a Program? Generally, after your base building phase. Build the engine (VO₂ max), then build the proportion of that engine you can functionally use.
Note that the first half of this zone could be considered the “Tempo” zone for those who use that in training.

VO₂ Max Zone

Energy system we are improving: Aerobic glycolysis predominantly, with some anaerobic benefits.
Intensity range: >95% VO₂ max
Goal: Improve VO₂ max. Similar goals and outcomes to endurance zone training.
Explanation: Involves using long interval training (2-4 minutes) to create huge oxygen deficits. Stimulates PGC-1a which is the enzyme responsible for creating more mitochondria, strengthening our muscular endurance etc.
Where in a Program?: In my personal opinion, this should be done in conjunction with endurance zone training during your base building phase. Why? Identical goals and adaptations, a hugely time efficient but effective session, and it boosts VO₂ max far more than endurance zone training alone.


Anaerobic Zone:

Energy system we are improving: Anaerobic predominantly, with some aerobic benefits.
Intensity range: >100% VO₂ max
Goal: Same as Threshold Zone: Improve functional threshold power and bring this closer to 100% VO₂ max by improving the body’s ability to tolerate and clear lactic acid.
Explanation: Involves short bursts of very high intensity, with varied rest. The result is a sudden and sharp influx of lactic acid, followed by a brief recovery period, before creating another sudden influx of lactic acid. This forces the body to tolerate and clear out lactic acid quickly. Again, train specifically. If you want to become good at dealing with lactic acid, complete training which involves flooding the muscle with it, allowing a partial recovery, and repeat the process. The body adapts to the stressors placed upon it.
Where in a Program?: Generally, in conjunction with the Threshold Zone after a base building phase. As with the endurance and VO₂ max zones, anaerobic training has very similar goals and adaptations to the Threshold Zone, so these should be paired together.


  • Understanding energy system interplay is important: the aerobic system and anaerobic system work together to create energy to fuel performance, it is the duration and intensity which determine their relative contributions. Aerobic glycolysis uses oxygen, anaerobic glycolysis does not.

  • Training zones don’t need to be complicated: these zones work on three goals: recovery, VO₂ max/aerobic glycolysis improvement, or functional threshold power/anaerobic glycolysis improvement.

  • Train in stages: Improve your aerobic capacity through Endurance Zone work and VO₂ Max Zone work, then change the focus to Threshold Zone and Anaerobic sessions, while incorporating race-specific sessions. Build the engine, then maximise the proportion of the engine you can functionally use. To learn more about building your aerobic engine, read this article. To learn more about improving your functional capacity, read this article.

  • The body adapts to the stressors placed upon it: train in the presence of lactic acid, get better at tolerating and clearing it. Train in the absence of lactic acid, get better at utilising oxygen.

  • For maximal adaptation and training efficiency, you need to identify YOUR training zones (not the % of max HR rubbish) and learn how to use them. With the number of overuse injuries in endurance sport, it is critical to train smart to avoid unnecessary training load and subsequent injury. Maximal adaptation with minimal workload… Stay healthy, stay happy, be fit.

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Written by Luke McIlroy – Director of Sport Science at METS Performance Consulting