The “Aerobic Threshold” (AeT) Explained For Cyclists
In this post, we’ll look to understand what the term aerobic threshold (AeT) means in the context of cycling. We’ll look at why it’s an important value to develop and monitor, how to improve it and answer some key questions about this often-ignored metabolic marker.
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It’s fair to say that a great deal of attention is paid to thresholds in the field of cycling training and performance, but a disproportionate amount of that attention is directed at what we might call the “second threshold”…
The second threshold is the intensity range where the body transitions from a state of metabolic control to instability. This phenomenon is referred to by numerous different terms depending on which ‘lens’ this threshold is being examined through.
The most common name is Functional Threshold Power (FTP). You may also be familiar with the term ‘Critical Power’ (CP). These terms aim to define the second threshold based on real-world performances.
We can also define the second threshold by looking ‘under the hood’ at what’s going on metabolically. In this case, the second threshold can be defined as the ‘Lactate Turnpoint’, ‘Lactate Threshold’, ‘Maximal Lactate Steady State’, ‘second ventilatory threshold’ or ‘SmO2 breakpoint 2’.
All these terms are subtly different, but are intended to pinpoint the maximum intensity where metabolic processes are in steady state (i.e. are sustainable for extended periods). Above this intensity, fatigue hits much more rapidly than below this intensity.
Unlike the second threshold, the “first threshold” is comparatively ignored in literature, training software and discussions between cyclists.
This is despite the fact that the first threshold is arguably a more important value to improve and measure for cyclists competing in many popular disciplines such as sportives, long gravel/marathon races and ultra-distance events!
It’s also a more appropriate metric for just about any cyclist to measure when assessing their foundational fitness development in the “base” or early/pre-season training phase(s).
What Is The Aerobic Threshold?
The first threshold is commonly referred to as the aerobic threshold or AeT. Other terms include the first lactate threshold (LT1), the first ventilatory threshold (VT1), or SmO2 breakpoint 1.
Again these terms have subtly different definitions, but broadly all relate to the intensity at which the body begins to use carbohydrate as a significant fuel source to supply energy when riding [1].
It sits beneath the second threshold; typically somewhere between 60-85% of the second threshold, or between 50-75% of VO2max. In the Critical Power model, the aerobic threshold would mark the transition from the moderate exercise domain to the heavy exercise domain. In a polarised model, it defines the transition between Zones 1 and 2.
Contrary to what the name might suggest, the aerobic threshold is not an intensity where you switch between aerobic or anaerobic energy systems!
As a quick physiology refresher, the body has several mechanisms through which it can produce energy. The two main ones that are used to during long-duration, aerobic exercise are the break-down of fats and carbohydrates.
The break-down of carbohydrates (also known as ‘glycolysis’) is a really efficient system that can supply energy quickly. However, the body only has sufficient carbohydrate stores (in the form of muscle and liver glycogen, and blood glucose) to fuel all-out exercise lasting roughly 90-minutes. At most race-like intensities, even if you consume carbohydrates while riding, there will almost always come a point where the body’s glycogen stores are substantially depleted, and you’ll find yourself ‘bonking’ or ‘hitting the wall’. Indeed, glycogen depletion is thought to be the key cause of fatigue in endurance cycling [2].
In addition, glycolysis produces metabolic byproducts that are linked with fatigue. While the body is capable of using oxygen to help ‘hoover' these byproducts up to a certain extent, there is still some ‘leakage’ of these metabolic byproducts, and so broadly speaking, the more you rely on carbohydrates, the more fatigue you accrue, and the harder the effort feels.
In contrast, the break-down of fats is a preferable system for the body during lower-intensity activity, firstly because this system is very sustainable (even the leanest cyclist has ample stores of fat to fuel exercise for days!). Secondly, unlike carbohydrates, the fat oxidation system does not produce any fatiguing byproducts; just water and carbon dioxide, which the body can easily remove if in excess.
The key draw-back of fat oxidation is that it’s a slower process, so as you ramp up your power output, your metabolism is forced to shift to rely more on carbohydrate oxidation in order to supply energy quickly.
The aerobic threshold, is the power output where you begin to need to draw on your carbohydrate stores to a notable extent.
In effect, below your aerobic threshold, exercise is sustainable ‘indefinitely’ (in practice, this isn’t true because factors other than glycogen depletion come into play).
Above the aerobic threshold (but below the second threshold), exercise is still sustainable for several hours, but the point of fatigue will be strongly determined by muscle glycogen stores, and at some point you’ll hit a wall where exercise cannot be continued due to glycogen depletion.
Importance Of The Aerobic Threshold
The aerobic threshold is important for cyclists because it indicates the ability to use fats for fuel, and is a very strong indicator of endurance abilities.
Having a higher first threshold power means that you’ll be able to sustain higher power outputs for multiple hours.
Indeed, long endurance events such as sportives, long gravel and marathon races, and ultra-distance events will be carried out close to the aerobic threshold, and shifting this threshold upwards will help improve the power you can average in these events.
Even among disciplines that are shorter and/or more stochastic in terms of power output, it can be important to have a well developed first threshold.
This is firstly because, in many disciplines, the ability to produce high-power efforts after several hours of racing is a key requirement, and having a higher first threshold means that you’re better able to conserve muscle glycogen for these determinative parts of the race.
Secondly, the ability to use fats efficiently for fuel strongly impacts the location of the second threshold, because it leads to a lower rate of lactate production for a given power output. Thus spending time developing your first threshold is a necessary part of raising your second threshold (e.g. FTP).
This is often why endurance cyclists of all disciplines undertake a “base” training phase, where a key goal is to lift the aerobic threshold.
Aerobic Threshold Tests
There are several methods that can be used to determine the location of the aerobic threshold. We’ll start out with the some more traditional lab-based methods, and then move on to describe methods that can be more easily be performed at home.
1. Lactate Testing
The breakdown of carbohydrates (i.e. the process known as glycolysis) produces a substance known as lactate. Lactate can be detected in the blood, and is an indicator of the extent to which the body is relying upon carbohydrates to produce energy.
Some readers may be familiar with the lactate curve below, which shows the relationship between power output and lactate levels in the blood (for more information on lactate, its role in energy production, and implications for training, check out our cycling physiology guide).
The aerobic threshold is often defined as the point at which we first see an uptick in lactate levels (designated LT1 in the chart above).
For consistency and ease of identification, we often define the aerobic threshold as the power output at 2mmol/L of lactate, or the power at which lactate rises by 1 mmol/L above resting values.
In the chart above we can also see the location of the second threshold (designated as ‘LT2’), which is the point where lactate levels begin to increase in a non-linear fashion.
Lactate testing can be performed either in a lab, or at home by purchasing some lactate testing equipment. We describe some key lactate testing protocols for cyclists in our article here.
2. Gas Analysis
The aerobic threshold can also be detected through changes in the composition of expired air, as the increased utilisation of carbohydrates results in an increase in carbon dioxide production.
In this assessment method, we’re looking for a step change in the ratio of oxygen to carbon dioxide in the air you breathe out relative to the power you’re producing on the bike.
This type of testing can only be performed in a lab, and you’ll need to wear a mask while riding on a stationary bike in order to measure the composition of the air you’re breathing out.
Quite often, the aerobic threshold will be identified as part of a standard VO2max test, but it’s worth checking with the lab that this threshold will be measured and reported, as it’s not always included.
3. The Talk Test
A result of the increased production of carbon dioxide when you step over your aerobic threshold is that breathing rate and volume also changes quite markedly.
Below your aerobic threshold, breathing will be very comfortable, and you’ll be able to hold a conversation easily. Above the aerobic threshold, there is a notable shift in ventilation, which means it becomes noticeably harder (but not impossible) to talk.
We can use this principle to test the approximate location of the aerobic threshold as follows:
Begin by riding at a very low intensity that feels subjectively easy (e.g. no more than a 2/10). If you already have a rough estimate of your FTP, you can aim for around 55% FTP.
Hold this power for 5-mins and then recite the alphabet out-loud. Ask yourself whether talking feels comfortable, and record either ‘yes’, ‘no’ or ‘don’t know’.
Being able to recite the alphabet comfortably means that you can do it without having to take frequent heavy breaths. This intensity is often referred to as being a ‘conversational’ effort i.e. one where you could easily hold a conversation with a friend while riding, where your speech is not too dissimilar from talking at rest.
If you’re having to take frequent breaths roughly every 5-8 letters of the alphabet, or thereabouts, then this indicates that talking has become uncomfortable.
Repeat the steps above at increments of around 5% FTP or 10-15W, until you reach a level where breathing quite clearly no longer feels comfortable.
Your aerobic threshold is the highest power at which breathing feels comfortable. If your breathing feels uncomfortable, or you’re not sure, then this power is classified as being above your aerobic threshold.
Testing should be either indoors, or on a quiet section of road where you can easily control your power.
Note that you shouldn’t be increasing the intensity to the point where reciting the alphabet feels extremely uncomfortable (e.g. you’re only able to say 1-2 letters at once, and feel that your breathing is very heavy). At this point, you’ve gone very far beyond your aerobic threshold, and have probably crossed your second threshold!
4. DFA Alpha 1
A fairly new method of determining the aerobic threshold is through measurement of heart rate variability (HRV), and various studies have been conducted looking at the viability of this method.
HRV is the variability in the length of time between heart beats. So for example, at rest, you may have a heart rate of 60bpm. That’s an average of one beat per second.
However, the heart does not beat in perfect regularity. Instead, there will be variability in the length of time between beats, for example, successive beats may have intervals of 0.8 seconds, 1.4 seconds, 0.7 seconds, 1.1 seconds and so on.
HRV is a value that tells us how variable the length of time between heart beats is. It’s more commonly used as an indicator of recovery status.
We can take this HRV analysis a step further to look at whether there are any patterns or ‘correlations’ in the variability of the heart rate over different time-scales. We can measure this using a metric called DFA Alpha 1. This metric effectively tells us how chaotic the HRV data is, with higher values representing less chaotic (i.e. more correlated) data.
Interestingly as exercise intensity increases, heart beats seem to become more chaotic, and several studies have shown that on average, the aerobic threshold is crossed when DFA Alpha 1 falls below 0.75 (and likewise the second threshold or so-called ‘anaerobic threshold’ is crossed when DFA Alpha 1 falls below 0.5) [3].
Unfortunately, delving into recent data in more detail [4, 5], it’s clear that while the threshold of 0.75 can be used for identifying the aerobic threshold at the population level, it doesn’t work very well on an individual level. For example, using DFA Alpha 1 can give rise to an aerobic threshold that’s as much as 10-15bpm above/below the lactate-derived threshold.
Thus, while DFA Alpha 1 is worth mentioning given the attention it’s received lately, it’s not an approach we would endorse.
Aerobic Threshold Workouts
Here are some ideal training methods to use when improving the aerobic threshold is the goal:
1. Long-duration, low-intensity rides
The main type of training that helps to improve the aerobic threshold is longer-duration, lower-intensity riding carried out at an intensity that’s at or slightly below the aerobic threshold.
This is an intensity that promotes many aerobic adaptations that improve the muscles’ capacity to use fats for fuel, including improvements in mitochondrial size and content, increases in enzymes linked with fat oxidation, and increased density of capillaries around muscle fibres.
By riding for longer, this also helps to train slightly ‘higher power’ muscle fibres (which aren’t ordinarily recruited at a low intensity, but which are called upon later in a ride as the primary muscle fibres begin to fatigue). Making these higher-power fibres more aerobically efficient means that higher powers can be sustained while still deriving a considerable portion of energy from fat oxidation.
Longer, low-intensity rides also seem to help improve the efficiency with which oxygen can be used to break-down fats and produce energy.
Ultimately, these rides should be a staple of any endurance cyclists’s training, and ideally should make up around 70-80% of all training sessions.
2. Carbohydrate Restricted Training
There’s some evidence that coupling low-intensity rides with reduced access to carbohydrates can further enhance the benefits of these rides.
Carbohydrate restriction can be achieved in several ways, but the strongest evidence is for a strategy that involves (i) completing a high-intensity workout later in the day, which will deplete muscle glycogen levels, (ii) restricting carbohydrate intake after that session (i.e. sticking to predominantly fats and proteins to refuel after the session) and then (iii) completing a low intensity ride early the next morning, before consuming any foods or drinks that contain carbohydrates.
We write more about carbohydrate restricted training here.
3. Sustained tempo/sweetspot efforts
Training slightly above your aerobic threshold for longer intervals of time (e.g. 30-60 minutes in total per session) may also help to develop the aerobic threshold, specifically by training those higher-power fibres that are primarily activated when you step above your aerobic threshold. A rough exercise intensity to target is approximately 75-95% FTP.
This type of training will promote adaptations such as increased capilliarisation and mitochondrial size and density within these higher-power fibres, helping improve their capacity for fat oxidation.
It’s important not to include too much of this type of training though, firstly because it can be very stressful and can lead to overtraining. Secondly, it tends to promote glycolysis more so than fat oxidation, and it’s important to have an appropriate balance between sessions below the aerobic threshold, where the rate of fat oxidation is maximised, and sessions above the aerobic threshold, where a higher proportion of muscle fibres can be trained.
We have plenty of workouts designs that work in this intensity range in our workout library.
This style of workout can also beneficially be coupled with carbohydrate-restriction as described above.
4. High-intensity interval training (HIIT)
Finally, a small amount of high-intensity interval training can have a downstream impact on the aerobic threshold.
In particular, intervals that are carried out above the second/’anaerobic’ threshold can help improve the efficiency of mitochondria in the muscles and may also help promote fat oxidation in some cases. Sessions that allow large amounts of time to be spent riding close to your max heart rate may also help to improve cardiac output and thus the delivery of oxygen to the working muscles. These factors can translate to an improved capacity for fat oxidation and elevated aerobic threshold.
Generally-speaking you’ll typically want to include some high-intensity intervals 1-2 times per week at most, and intervals targeting improvements in VO2max are probably the key ones to look for if you’re seeing improvements in the aerobic threshold.
Again, we have plenty of workouts designs that target VO2max in our workout library.
Example Training Weeks
Below we’ve outlined some example training weeks for athletes seeking to develop their aerobic threshold.
8-10 Hours/Week
12-15 Hours/Week
Common Questions
Hopefully this article has provided a good overview of what the first aerobic threshold is, why it’s important, and how it can be determined and developed. We’ll wrap this article up with a few quick common questions.
Is the aerobic threshold the same as ‘FatMax’?
Aerobic threshold is not quite the same as FatMax. FatMax is the exercise intensity at which fat oxidation/fat burning rates are maximised. However, in practice, FatMax and the aerobic threshold sit at very similar powers, so if you know one, you’ll have a pretty good idea of where the other roughly sits.
What’s a ‘good’ aerobic threshold?
Relative to VO2max, a good aerobic threshold would sit round 65-75%. Relative to the second threshold (e.g. FTP), this would generally be around 78-85%. In terms of Watts/kg, professional male cyclists (e.g. World Tour Pros) will generally have an aerobic threshold of around 4-5W/kg, and for females, this would be around 3.5-4W/kg.
Can aerobic threshold change independently of the second threshold?
While the aerobic threshold and the second threshold (‘anaerobic threshold’) are very strongly related, it’s possible to shift your aerobic threshold so that it sits closer to your second/anaerobic threshold by focussing on low intensity training below your aerobic threshold. But in general, if you see an upward shift in your aerobic threshold, then you’ll usually also see an upward shift in your second/anaerobic threshold.
References
1. Spurway, N. C. (1992). Aerobic exercise, anaerobic exercise and the lactate threshold. British Medical Bulletin, 48(3), 569-591.
2. Abbiss, C. R., & Laursen, P. B. (2005). Models to explain fatigue during prolonged endurance cycling. Sports medicine, 35(10), 865-898.
3. Rogers, B., Giles, D., Draper, N., Hoos, O., & Gronwald, T. (2021). A new detection method defining the aerobic threshold for endurance exercise and training prescription based on fractal correlation properties of heart rate variability. Frontiers in physiology, 11, 596567.
4. Rogers, B., Berk, S., & Gronwald, T. (2022). An Index of Non-Linear HRV as a Proxy of the Aerobic Threshold Based on Blood Lactate Concentration in Elite Triathletes. Sports, 10(2), 25
5. Mateo-March, M., Moya-Ramón, M., Javaloyes, A., Sánchez-Muñoz, C., & Clemente-Suárez, V. J. (2022). Validity of Detrended Fluctuation Analysis of Heart Rate Variability to determine intensity thresholds in professional cyclists. European Journal of Sport Science.
