Lactate Threshold 1 (LT1): Why you should care and what to learn from it

Lactate threshold 1 (LT1) – also known as the aerobic threshold – is not to be confused with LT2 or anaerobic threshold, as LT1 occurs at a lower exercise intensity. LT1 may not get as much attention as LT2, but it’s still a popular ingredient of famous cycling, running, and triathlon training methods. This includes Polarized training, Pyramidal training, and the Norwegian Method. This article covers everything you need to know about the first lactate threshold.

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What is LT1 (lactate threshold 1)?

There are several LT1 definitions, but the following is probably the most common.

LT1 is the lowest exercise intensity at which there is a measurable increase in blood lactate concentration, compared to resting lactate concentrations. LT1 is the first lactate threshold and should not be confused with LT2 (lactate threshold 2, or anaerobic threshold).

In practice, it’s not always easy to determine the lowest intensity at which there is a measurable increase in lactate. This is especially true when you don’t measure blood lactate continuously. Therefore, LT1 is sometimes defined as the intensity at which blood lactate concentration equals 2 mmol/l.

However, this supposes that the resting lactate concentration is just below the 2 mmol/l value, which is not the case in many individuals. Furthermore, a fixed value of 2 mmol/l might create false results for athletes with a lactate concentration relatively low as well as athlete with a relatively high resting lactate concentration. A more appropriate methodology to determine LT1 will be discussed later in this article.

When you’re at rest, your lactate concentration is low. However, it’s not zero. This means that there is a small lactate production and a small lactate combustion (clearance).

When you start to exercise at a (very) low intensity, your aerobic energy system becomes more active. This aerobic energy system combusts lactate. As a result, lactate concentrations can decrease below resting values at low exercise intensities. For example, lactate levels will be lower during a cycling warm-up – compared to lactate concentrations at rest.

When you increase the intensity a bit further, your glycolytic energy system becomes more active too. This glycolytic energy system produces lactate. As a result, lactate concentrations will rise at some point. The exercise intensity corresponding to this first lactate increase is the first lactate threshold, LT1.

It’s important to note that although LT1 is called the first lactate threshold, you could argue whether it is actually a threshold. Nothing significant happens when you exercise slightly above LT1; lactate concentrations remain in steady state. So it is actually a gradual change in lactate production and combustion – not really a threshold.

How to determine LT1?

By fitting a curve through your lactate measurements, you’ll get a lactate curve (see image below). This curve shows your LT1. However, there are 3 things to keep in mind:

The intensity of LT1 is highly determined by the test protocol. Longer steps can result in lower lactate concentrations at low intensities, which affects the intensity at which the lactate concentration reaches 2 mmol/l. Moreover, less trained athletes can show an increase in lactate straight from the start, which means LT1 is directly determined by the starting intensity of the protocol.
The lactate concentration that you measure depends on several other factors, such as exercise intensity, body composition, and measurement errors. If you don’t take these into account, you can get flawed results. The INSCYD performance software takes all these factors into account and creates a lactate curve from your data.
Although the theoretical definition of LT1 is clear, determining LT1 turns out to be challenging in practice. Literature shows that visual detection of LT1 leads to relevant differences between observers. That’s why it is suggested to standardize the determination of LT1.

Rise above the lowest value? Increased compared to the lactate concentration of the previous load step in the incremental test (which makes it again more dependent on the test protocol used)? Or an increase above resting lactate levels?

And then there is the question of what actually is a valid increase of blood lactate concentration? For example: is 0.1 mmol/l increase for real – if your handheld lactate meter has an error of measurement of +/- 0.2 mmol/l?

So besides the already mentioned gender, body composition, test protocol, etc, a real increase of lactate in the blood also depends on the device one is actually using to measure lactate. Because if the error of measurement is 0.3 mmol/l than an increase of lactate smaller than this number can not be interpreted as a real increase with high certainty. It could well just be an error of measurement.

The theoretical definition of LT1 is clear, but in practice it can become challenging to determine LT1 based on a lactate curve. Here is an example of 3 lactate curves of the same athlete – just using different step duration and resting lactate concentration. Where do you mark the LT1 intensity? Literature suggests to standardize the method of determining LT1 but does not offer a precise definition. INSCYD does so for you – read below.

One scientifically solid way to determine LT1!

INSCYD now offers a standardized method to determine LT1, eliminating protocol-related inconsistencies and taking into account factors such as device measurement error, gender, and body composition. Most importantly: the way LT1 is determined is consistent – no matter what your test protocol is. It doesn’t even matter how you test: a field test or lab test with lactate or even a remote test using power or speed only: you will always get one reliable LT1.

How is that possible?

INSCYD creates a metabolic profile of your athlete, which includes data points such as VO2max, VLamax, gender, body composition, lactate production and clearance, and many more. With all these data points it becomes possible to actually calculate the lactate concentration of an athlete in a standardized incremental load test and therefore determine LT1. So thereby LT1 becomes independent of your actual test protocol!

You just use whatever your test protocol is, INSCYD creates the comprehensive metabolic profile and with that, calculates a lactate profile curve for a conventional incremental test.

The metabolic profile, which respects values like actual lactate production and clearance as well as body composition and gender, is used to calculate LT1 with these normalized conditions:

Load-increment (step) duration: 6 minutes.
Resting lactate concentration: 1.25 mmol/L.
Lactate analyzer typical error: 0.35 mmol/L
Lactate analyzer resolution: 0.1 mmol/L.

Under these conditions, an increase of 0.45 mmol/l above the minimum lactate concentration is likely a real increase in lactate which cannot be attributed to the device measurement error.

This 6-minute duration aligns with INSCYD’s recommended step size for lactate testing, as it closely reflects steady-state conditions—a critical aspect when applying LT1 in training scenarios.

And for this month only, if you become an INSCYD user, you will get LT1 completely free.

Lactate threshold 1 (LT1) vs Lactate threshold 2 (Lt2): Understanding the Differences

Lactate Threshold 1 (LT1), also known as the aerobic threshold, occurs at a lower exercise intensity than Lactate Threshold 2 (LT2), or the anaerobic threshold.

In the context of endurance training, LT1 is marked as the first rise in lactate concentration. This should not be confused with the second lactate threshold, LT2, which occurs at higher exercise intensities.

LT1 is called the first lactate threshold, because it occurs at lower intensities than LT2, the second lactate threshold.

LT1 (or aerobic threshold) occurs at a lower exercise intensity than LT2 (or anaerobic threshold). LT2 is marked in this graph, but not derived from this graph.

As mentioned earlier, LT1 is not really a threshold. Nothing dramatically changes when you exercise slightly above LT1: lactate concentrations will remain in a steady state. LT2, on the other hand, is a “threshold” because it clearly differentiates two intensities from each other:

Above LT2, lactate concentrations will rise over time. There is no more lactate steady state.
Below LT2, lactate concentrations will not increase over time. There is a lactate steady state.

With LT1, there is no such clearly visible difference between below- and above LT1.

Lactate thresholds and training zones

Lactate Threshold 1 (LT1) and Lactate Threshold 2 (LT2) are often used to create training zones, such as a simple three zones training model:

Training zone 1: below LT1
Training zone 2: between LT1 and LT2
Training zone 3: above LT2

However, you’ll soon notice that creating zones like this only allows for limited possibilities.

For instance, if you want to train at FatMax, should you aim for Zone 1 or Zone 2? It’s not only much easier but also more precise to use the INSCYD training zones, that tell you exactly at which intensity maximal fat combustion occurs.

One of many examples of training zones created with the INSCYD training zone builder. In this running example, fat and carb combustion is added to the training zones. There is also a dedicated FatMax Training Zone, which refers to the intensity at the maximum fat oxidation rate.

Another aspect is that the zones are too large. Zone 3 is everything from a ~1 hour time trial intensity to a 5-second sprint. On the other side of the spectrum, zone 1 can be so easy that runners need to walk and swimmers struggle to remain horizontal because of the speeds that are too slow to run or swim efficiently.

Contrary to the 3-zone training model, the INSCYD training zone builder helps to create training zones that exactly fit your needs.

Understanding the difference between LT1 and LT2 is key in any sports training regimen. It’s the difference between a lactate steady state and a continuous rise in lactate concentration. Yet, it’s not enough to create an effective, individualized training program.

What if you want to train at FatMax? How do you know how much fuel you need when exercising in zone 1? The traditional 3-zone model leaves many questions unanswered and opportunities untapped.

This is where INSCYD comes into play. Our sophisticated training zone builder enables you to see how much carbohydrates you burn at every exercise intensity and in every training zone. It helps to create training zones that fit your needs exactly, from maximal fat combustion to the intensity of a 5-second sprint.

No more guesswork, no more one-size-fits-all.

Aerobic threshold, ventilatory threshold and FatMax

In this last part of the article, we look at metrics that are often associated with the first lactate threshold.

LT1 vs Aerobic threshold

The first lactate threshold is often associated with the aerobic threshold (AeT). Some even use these terms interchangeably.

You could say that LT1 describes a way to measure the aerobic threshold, namely with lactate measurements.

Whether LT1 and the aerobic threshold are the same depends on the definitions you use.

Both LT1 and the aerobic threshold aim to mark a point at which anaerobic (glycolytic) energy contribution shows a measurable increase. Note, though, that (as always during an incremental test) the aerobic energy contribution is still dominant.

Common misunderstandings about both LT1 and aerobic threshold:

They don’t mark the start of anaerobic energy contribution. As we mentioned, even at rest there’s an anaerobic energy contribution.
They don’t mark a shift from fully aerobic to fully anaerobic energy supply.
They also don’t mark the start of carbohydrate combustion. Even at rest, carbohydrates are burned.
They don’t mark a shift from 100% fat combustion to 100% carbohydrate combustion.
They don’t mark a limited oxygen availability.

In steady state conditions, there’s always a combination of aerobic AND anaerobic energy contribution.

LT1 vs Ventilatory Threshold 1 (VT1)

Another metric often associated with LT1 is the ventilatory threshold 1 (VT1).

Contrary to LT1, the ventilatory threshold 1 is measured with a metabolic cart (breathing gas VO2 analyzer) instead of with lactate samples. VT1 is the intensity at which ventilation starts to increase at a faster rate than oxygen consumption.

Similar to LT1, you could say that VT1 is a way to measure the aerobic threshold. Although their measurement techniques are different, it will not surprise you that some studies show that LT1 and VT1 mark similar exercise intensities.

There is another similarity: the intensity at which ventilation starts to increase at a faster rate than oxygen consumption (VT1) is at least as hard to spot as the lowest intensity at which there is a measurable increase in lactate (LT1). Therefore, VT1 is also not clearly defined nor easy to detect.

LT1 vs FatMax

Lastly, many cyclists, runners and their coaches think LT1 is the intensity at which fat combustion peaks (FatMax). They use LT1 and FatMax interchangeably, for instance to determine the zone 2 training intensity described by Iñigo San Milan.

It is true that both LT1 and FatMax occur at an intensity below the anaerobic threshold. It is also true that you can increase LT1 and FatMax by increasing the aerobic energy contribution, all other things equal. As a result, you will find a correlation between LT1 and FatMax. However, they don’t necessarily occur at the same exercise intensity.

Moreover, due to methodological challenges in determining Fatmax and LT1, correlations vary between appr. 0.3 (weak correlation) to appr. 0.8 (strong correlation). So, therefore, it is correct to mention that LT1 is not marking the same intensity as FatMax or maximum fat combustion rate.

Moreover, if you want to answer questions like: “How to fuel for a training session at or below LT1?” or “How much energy, fat, and carbohydrates do I burn at or below LT1?”, you still need more information besides an LT1 or FatMax intensity.

To answer these questions, simply use the INSCYD fat and carbohydrate combustion graph, combined with the LT1 exercise intensity.

The fat and carbohydrate combustion graph shows exactly how much energy you burn at a given exercise intensity, like at or below LT1.

Should you want to increase LT1?

We learned that LT1 does not mark a sudden change in physiological processes, hence it’s not really a threshold.

So why would you want to increase LT1?

Because an increase in LT1 can be an indicator of an increase in the performance of the aerobic energy system. Or maybe an indicator of an increase in fat combustion.

Therefore coaches and exercise physiologists like to measure and monitor LT1.

But keep in mind that it is neither a direct measurement of the aerobic power of an athlete – this would be VO2max. Nor is it a direct measurement of maximum fat combustion zone – which would be FatMax or the maximal fat oxidation rate (MFOR).

INSCYD allows you to measure and improve what matters, it provides you not only with LT1, but also with precise measurements of the two “master metrics” that can both influence LT1: FatMax and VO2max.

How to increase Lactate Threshold 1 (LT1)?

In order to increase the power or speed at LT1, one needs to either increase lactate combustion or decrease lactate production (or do both). If your goal is to decrease lactate production and at any given exercise intensity, you should decrease your anaerobic energy supply.

This will also result in an increase of LT1. Here are 5 practical tips on how to do that: 5 Training tips to decrease VLamax. You can implement these tips in running, cycling, swimming and all other (endurance) sports.

If your goal is to increase lactate combustion (clearance) at any given exercise intensity, you should increase your aerobic energy system. This will also result in an increase of LT1.

There are several ways to do this. They range from “long slow distance” endurance training to high intensity VO2max intervals. Learn more about this topic via this link.

Democratizing Lactate threshold 1: Determination of LT1 is now possible without taking lactate samples!

Previously, determining LT1 required blood lactate measurements taken at several exercise intensities. In a lab setting, this could be accomplished using equipment such as a cycling trainer, treadmill, or rowing ergometer. Field tests, like running on a track or repeatedly cycling up a hill, were also common methods.

As described above, INSCYD calculates LT1 for one defined testing scenario using the metabolic profile of an athlete. There are two ways to get a precise metabolic profile of an athlete:

1. Using lactate tests (lab or field-based)

2. Using INSCYD’s popular “Power-Peformance Decoder”.

The Power-Performance Decoder allows for remote testing, using the power data (cycling) or GPS data (running) only. Therefore also, coaches who don’t own a lab or do lactate testing but want to provide LT1 to remotely coached athletes can do this now with INSCYD.

Wrap up and next step

LT1 – also known as the aerobic threshold – is the first lactate threshold. It marks a measurable increase in lactate concentration compared to rest.

We learned that:

LT1 does not mark a significant change in physiological processes, hence it’s not really a threshold.
LT1 is not a good way to determine training zones or substrate (fat vs carbohydrates) utilisation.
LT1 is an indicator for performance changes, but it makes more sense to focus on the actual end goal (e.g. increasing the performance of your aerobic energy system) instead.

Want to Explore LT1 in Even More Detail?

If you’d like to dive deeper into the technical specifics of LT1—complete with scientific references, the underlying research, and how INSCYD’s approach can be applied in real-world training—feel free to download our free white paper, where we: