Is Norwegian 4×4 better than HIIT for improving VO2max?

It depends on the athlete's metabolic profile. Both protocols can improve VO2max, but the effectiveness depends on the athlete's VLamax (glycolytic capacity). An athlete with high VLamax may receive a significantly lower aerobic training stimulus from threshold-based HIIT than an athlete with low VLamax — even if both have the same VO2max.

Why do two athletes with the same VO2max respond differently to the same interval training?

Because their glycolytic capacity (VLamax) differs. A high-VLamax athlete covers more energy demand anaerobically, resulting in lower oxygen uptake during intervals. At the same threshold-based intensity, a low-VLamax athlete may reach 90–92% of VO2max while a high-VLamax athlete only reaches 84–86%. That 20% difference in fractional VO2max utilization means a fundamentally different aerobic training stimulus.

What is VLamax and why does it matter for HIIT prescription?

VLamax (maximum glycolytic rate) measures how much energy an athlete produces anaerobically. It determines where the anaerobic threshold sits relative to VO2max. When HIIT intensity is prescribed as a percentage of threshold, athletes with different VLamax values end up at different fractions of VO2max — making threshold-based prescription unreliable for targeting aerobic adaptations.

How should HIIT intensity be prescribed instead of using threshold?

Prescribe intensity based on a percentage of maximum aerobic power (VO2max) rather than threshold. This ensures that the target VO2max utilization is consistent across athletes regardless of their VLamax. INSCYD's Training Zones Builder calculates these zones from the athlete's full metabolic profile.

What are the key differences between 40/20 HIIT and Norwegian 4×4 protocols?

The 40/20 protocol uses 40-second work intervals at 130% of threshold with 20-second recovery at 50% threshold. The Norwegian 4×4 uses 4-minute intervals at 90–95% of HRmax with 3–4 minute active recovery. The shorter 40/20 format never allows VO2 to reach steady state, while the Norwegian 4×4 allows VO2 to plateau during each interval.

Is Norwegian 4×4 better than HIIT for improving VO2max?

It depends on the athlete's metabolic profile. Both protocols can improve VO2max, but the effectiveness depends on the athlete's VLamax (glycolytic capacity). An athlete with high VLamax may receive a significantly lower aerobic training stimulus from threshold-based HIIT than an athlete with low VLamax — even if both have the same VO2max.

Why do two athletes with the same VO2max respond differently to the same interval training?

Because their glycolytic capacity (VLamax) differs. A high-VLamax athlete covers more energy demand anaerobically, resulting in lower oxygen uptake during intervals. At the same threshold-based intensity, a low-VLamax athlete may reach 90–92% of VO2max while a high-VLamax athlete only reaches 84–86%. That 20% difference in fractional VO2max utilization means a fundamentally different aerobic training stimulus.

What is VLamax and why does it matter for HIIT prescription?

VLamax (maximum glycolytic rate) measures how much energy an athlete produces anaerobically. It determines where the anaerobic threshold sits relative to VO2max. When HIIT intensity is prescribed as a percentage of threshold, athletes with different VLamax values end up at different fractions of VO2max — making threshold-based prescription unreliable for targeting aerobic adaptations.

How should HIIT intensity be prescribed instead of using threshold?

Prescribe intensity based on a percentage of maximum aerobic power (VO2max) rather than threshold. This ensures that the target VO2max utilization is consistent across athletes regardless of their VLamax. INSCYD's Training Zones Builder calculates these zones from the athlete's full metabolic profile.

What are the key differences between 40/20 HIIT and Norwegian 4×4 protocols?

The 40/20 protocol uses 40-second work intervals at 130% of threshold with 20-second recovery at 50% threshold. The Norwegian 4×4 uses 4-minute intervals at 90–95% of HRmax with 3–4 minute active recovery. The shorter 40/20 format never allows VO2 to reach steady state, while the Norwegian 4×4 allows VO2 to plateau during each interval.

INSCYD

[Webinar] Implement metabolic performance testing into a profitable service

Igor Makarov — Tue, 30 Jun 2026 10:20:56 +0000

Webinar

Turn metabolic performance testing into a profitable service

Whether you are thinking to add performance testing to your business or already running it, this is the business case for labs, longevity clinics, gyms, personal trainers and coaching businesses: the workflow, the investment, and the ROI, from someone who built one of Germany’s largest performance lab businesses and advises hundred of similar operations around the world

About the Webinar:

Metabolic performance testing is a key element in sports performance but also in health and longevity. That part is proven.

The harder question to answer is how to make it work as successful business

If you are thinking about adding performance testing into your coaching business, lab operation, gym or clinic – this webinar is likely for you. It will cover protocol, workflow, customer experience and needed equipment to get started. But it will also cover monetization, upselling and return of investment.

If you already offer performance testing as a service, you will learn how to streamline your processes, how to save time creating reports and how to turn one test into follow-up tests, programs, and ongoing revenue. Sebastian Weber walks through the full operation, from the first appointment booking to the end of the client visit, and the numbers behind it.

Demand is moving in your favor. From longevity clinics to large fitness operators, more businesses are adding and expanding performance and metabolic testing. The businesses that run it well, and can prove the value, are the ones clients choose.

You will leave knowing how metabolic performance testing can be implemented as a structured service, what it takes to operate it, and what kind of ROI it can create.

Is this me?

Built for two kinds of operator

If you are adding testing:

You run a gym or studio and want to add testing as a premium, paid service, not just another class.
You run a longevity or health clinic and clients ask for performance and metabolic insight you do not yet offer.
You are starting a performance lab and want it to pay from day one.
You coach privately and want to add a higher-value service clients will pay for.

If you already test:

You want more revenue from every client, through follow-up tests, programs, and retesting.
You want to test more people in less time, without adding cost.
You want better, faster reports you can put your name on and hand a client.
You want more from your data than your current software or spreadsheet gives you.

What you'll learn

THE KEY PARTS OF THE BUSINESS, IN ONE WEBINAR

Workflow

The full customer experience - from appointment booking to when a happy client walks out the door and cannot wait to come back. This will cover the exact steps how to most efficiently integrate testing into your business.

Upsell & Retesting

Turn one test into a relationship. How testing leads into follow-up tests, training programs, coaching, and ongoing client services, the recurring revenue most operations miss.

Protocols, Hardware & Investment

What protocols, equipment, and investment it takes, whether you are starting from zero or adding to what you run today. Including how to offer VO2max and substrate metrics with or without a metabolic cart or VO2 Analyzer.

ROI & Monetization

The numbers. How much labour time the service needs, what it costs, and how much revenue and profit it can generate.

What you'll walk away with

Not theory. A practical picture you can act on.

A clear view of the end-to-end testing workflow, from booking to the end of the visit
The protocols and equipment to start, and the realistic initial investment
An ROI picture: the time it takes, what it costs, and the revenue and profit it can generate
A plan for turning one test into follow-up tests, programs, coaching, and recurring revenue

EXTRA: Not just education but practical implementation: all participants will receive checklists and guidelines which we explain in the webinar.

Presenter

Sebastian Weber

Founder of INSCYD & World-renowned Sport Scientist

Sebastian Weber brings practical experience from building and operating Human Performance Labs, including Germany’s largest Human Performance Lab businesses with multiple locations.
He has worked with several hundred Human Performance Labs and coaching facilities worldwide.

He has worked with several hundred Human Performance Labs and coaching facilities worldwide and will share best practices from real-world implementation.

In this webinar, he shares best practices from real-world implementation: what really works when you offer performance testing as a business, not just theory.

book a free demo

The webinar gives you the business case in general. A demo makes it specific to you. Our specialist shows you how the business case and the implementation apply to your own business, and walks you through the product itself: the testing workflow, the metrics, and the branded, client-ready reports you can put your name on and sell.

The post [Webinar] Implement metabolic performance testing into a profitable service appeared first on INSCYD.

[Webinar] From raw test data to training plan: Join This Live Metabolic Profiling Session

Igor Makarov — Mon, 08 Jun 2026 14:29:43 +0000

Webinar

FROM RAW TEST DATA TO TRAINING PLAN: JOIN THIS LIVE METABOLIC PROFILING SESSION

Join Sebastian Weber, INSCYD founder. Watch live how to build a complete metabolic profile of an athlete, including VO₂max, VLamax, FatMax, MLSS using real data. The results will then be used to project possible performances by changing specific physiological markers and turned into a training strategy.

ABOUT THIS SESSION

Conventional lactate curve testing provides us with an outcome = the lactate concentration, but not with a view on the physiology that produced it. In this session we will close that gap in real time.

Sebastian Weber, INSCYD’s founder, will start with a conventional lactate profile test to build a complete metabolic profile live: VO₂max, VLamax, FatMax, MLSS, carbohydrate utilization and many more. No new protocol, no new equipment, no extra time. Same raw data as you already capture but a very different, much more differentiated outcome.

And it could be your athlete on the screen. Submit your data by June 15 and we may build their profile live.

We walk the full circle, from raw test to training:

Data capture: testing protocols that can be used, and what determines whether those protocols will provide usable data.
Processing: how decode the raw data into a complete metabolic profile of an athlete
Interpretation: interpretation of the metabolic profile in the context of the goals of the athlete aka what the athlete is trying to achieve. We will identify single metrics that limit the performance of the athlete. Then we will create a digital physiological twin of the athlete and change its physiology to understand and quantify how aerobic and glycolytic metabolism shape the performance.
Translation: we will build custom training zones based on the individual physiology of the athlete. Say goodbye to cookie-cutter zines which extrapolate training intensities from threshold numbers.

Presenter

Sebastian Weber

Founder of INSCYD & World-renowned Sport Scientist

Sebastian Weber, a renowned sports scientist and creator of INSCYD, has significantly influenced elite sports training with his expertise in exercise physiology and metabolic profiling. His innovative methods have propelled multiple world champions to success.

Sebastian Weber has consulted the German Swimming Federation, the USA Triathlon Federation and others, showcasing his diverse skills. Athletes under his guidance have notably won 9 gold medals in cycling time trials over a decade, underscoring his exceptional coaching impact in high-performance sports.

Want to Go Deeper?

Book your 45-minute personalized demo with an INSCYD performance expert and discover how coaches and performance labs just like you use INSCYD’s 360° metabolic profiling to unlock hidden performance potential and design laser-focused training programs.

The post [Webinar] From raw test data to training plan: Join This Live Metabolic Profiling Session appeared first on INSCYD.

[Webinar] How to Create a Full Metabolic Profile of Any Athlete — And Use It to Design Training That Actually Works

Igor Makarov — Mon, 13 Apr 2026 13:16:45 +0000

FREE WEBINAR

How to Create a Full Metabolic Profile of Any Athlete — And Use It to Design Training That Actually Works

Watch this free masterclass with Sebastian Weber — the sport scientist behind Olympic champions, Tour de France winners, and world-class federations.

In this webinar, you'll discover:

How to create a full 360° metabolic profile of any athlete in 30 minutes
Why training zones based on % of FTP or threshold don’t trigger the right adaptation
How an athlete’s phenotype determines whether “long & easy” or “short & hard” training is more effective
The mechanisms behind fat metabolism — and what actually improves it
How to design recovery intervals that specifically increase or decrease anaerobic power
Why aerobic capacity (VO2max) has limits — and what determines those limits
Real-world athlete cases: from metabolic testing to training program design

Presenter

Sebastian Weber

Founder of INSCYD & World-renowned Sport Scientist

Sebastian Weber has consulted the German Swimming Federation, the USA Triathlon Federation and other, showcasing his diverse skills. Athletes under his guidance have notably won 9 gold medals in time trials over a decade, underscoring his exceptional coaching impact in high-performance sports.

Want to Go Deeper?

This webinar covers 7 key takeaways. The full 4-Day Hands-on Camp covers 24+ sessions with live athlete testing, real-world data interpretation, and direct mentorship from Sebastian Weber.

The post [Webinar] How to Create a Full Metabolic Profile of Any Athlete — And Use It to Design Training That Actually Works appeared first on INSCYD.

HIIT vs. Norwegian 4×4: The Forgotten Metric That Decides Your Interval Training Outcome (Part 2)

Igor Makarov — Thu, 19 Mar 2026 17:28:10 +0000

Part 2 of the HIIT vs. Norwegian 4×4 Series | By Sebastian Weber, Founder of INSCYD & WorldTour Cycling Coach

Key Takeaways

HIIT and Norwegian 4×4 intervals do not only train the aerobic system. They also stimulate the glycolytic (anaerobic) system, with potentially unwanted consequences for endurance performance.
VLamax utilization (the fractional use of glycolytic power during an interval) determines whether a session will increase, maintain, or decrease an athlete’s glycolytic capacity.
Below 10% VLamax utilization, glycolytic power tends to decrease. Above 15-20%, it tends to increase. This benchmark is based on more than 20 years of testing and training athletes worldwide.
Two athletes with identical VO2max but different VLamax receive vastly different glycolytic training stimuli from the same interval session, with direct consequences for threshold, fat combustion, and carbohydrate utilization.
The solution: anchor interval intensity to %VLamax (to control the glycolytic side) and use interval duration to optimize the aerobic stimulus (%VO2max).

Live Webinar: HIIT vs Norwegian 4×4 — The Science Behind Interval Prescription

Join Sebastian Weber for a deep dive into the physiology behind this article. For coaches, athletes, and human performance labs

What We Discovered in Part 1

In Part 1 of this series, we compared the aerobic metabolism’s response during HIIT intervals and Norwegian 4×4 intervals.

We used two representative interval formats:

Norwegian 4×4: 4 minutes at 100% of threshold (MLSS) with 4-minute recovery
40/20 HIIT: 40 seconds at 130% of threshold with 20-second incomplete recovery

We demonstrated that HIIT (40 seconds on / 20 seconds off) produces a higher percentage utilization of VO2max but for a shorter duration at high VO2. In contrast, Norwegian 4×4 intervals reach a significantly lower %VO2max, but with longer exposure time at that level.

These findings about how both training methods work to increase VO2max are well established in the scientific literature and may even be common knowledge among exercise physiology experts.

However, this was not the main finding.

The main finding of Part 1 was that this general concept is not the same for every athlete. Even for athletes who have the same VO2max. Two athletes with identical maximum aerobic capacity but differently developed glycolytic capacity receive a vastly different training stimulus on the aerobic system. Very different than expected. The inter-individual differences can be huge.

We found that the solution is to prescribe training intensity not as an extrapolation of threshold intensity (FTP, LT2, or similar proxies), but to base the intensity on the very metric the training aims to improve: VO2max.

Read the full Part 1 analysis here: HIIT vs Norwegian 4×4: How VLamax Determines Your Aerobic Training Response.

Missing the Bigger Picture: Why Focusing Only on VO2max Is Not Enough

Both HIIT and Norwegian 4×4 are well researched in the scientific literature for improving VO2max. Both are evaluated in terms of aerobic adaptation. And if your goals are to increase fat combustion rates, spare carbohydrates, or raise threshold values, both methods can achieve this via increasing VO2max.

So how is it that not all athletes receive the same benefit from these intervals, and some athletes do not seem to improve at all? What we are talking about here is not the comparably smaller difference in efficacy because of the inaccurate intensity definition we disclosed in Part 1. What we are talking about is that some athletes show no improvement at all from one of these methods, or in some extreme cases even show decreased overall performance. The latter can happen particularly with highly endurance-trained athletes doing HIIT.

What are we missing?

The overlooked component is this: neither HIIT (40/20 in our case) nor Norwegian 4×4 are workouts which only involve and therefore affect the aerobic metabolic system (VO2). Just because we want to use these workouts primarily to increase VO2max does not mean that they do nothing else to any other system in the body. By focusing entirely on VO2max, it was perhaps easy to overlook that there might be other training effects happening that we did not anticipate and which we may not even want.

For example, think about the massive lactate buildup during these sessions, especially during the 40/20 HIIT intervals. This lactate can likely act as a signaling molecule with positive effects on substances such as BDNF (Brain-Derived Neurotrophic Factor), which helps protect nerve cells in the brain (El Hayek et al., 2019). Both workouts are also prone to increase buffering capacity in the muscles, enabling athletes to handle high-intensity efforts better. A side effect that might not be needed if the training goal is to spare carbohydrates and increase fat combustion, but it does not really hurt either.

But there is something that can potentially hurt reaching the athletic goals.

And this something is likely the reason why some athletes do not seem to get any improvement from one or both of the training methods we discuss here. And it is very likely the reason why some athletes even decrease their performance for long endurance events by doing HIIT.

Hiding in Plain Sight: The Glycolytic Stimulus You Did Not Plan For

It is well established that both HIIT and Norwegian 4×4 can lead to significantly elevated lactate levels. This is especially true for HIIT intervals.

If lactate concentration is elevated, lactate production must also be elevated. We already established in Part 1 that VO2 is very high during these sessions, in the HIIT case close to VO2max. Lactate is combusted in the aerobic metabolism as a fuel (Brooks, 1985), and the higher the VO2, the higher the lactate combustion rate. So if VO2 is near maximal, a large fraction of the produced lactate is being consumed as fuel.

Yet the concentration still rises.

This means the production rate of lactate has increased by a substantial margin to outpace even the elevated combustion.

Now apply the same training principle we discussed in Part 1 for VO2max. The basic idea: the more we use a system, the higher the stimulus for that system to adapt. This is the entire basis of the concept that higher %VO2max utilization leads to greater VO2max improvements.

If this holds true for the aerobic metabolism, it also holds true for the glycolytic (anaerobic) metabolism.

A significant increase in lactate production, as seen in both HIIT and Norwegian 4×4, is a clear sign of elevated glycolytic activity and therefore a valid and powerful training stimulus on the glycolytic system. Scientific research has confirmed that high-intensity training can lead to a positive adaptation of the anaerobic metabolism (Abe et al., 2015; Torma et al., 2019).

Why a Stronger Glycolytic System Can Hurt Endurance Performance

In contrast to increased buffering capacity or BDNF signaling, a structural and persistent improvement of glycolytic capacity is maybe exactly not what you want. A higher glycolytic power, measured as VLamax, means:

The muscle will ‘learn’ to produce more lactate
Maximal Lactate Steady State (threshold) decreases, and with it FTP and LT2 (Poffé et al., 2024)
Carbohydrate combustion increases because lactate is produced from carbohydrates
Higher carbohydrate combustion means the athlete runs out of glycogen faster
Fat combustion decreases

For most endurance athletes, none of the above is appreciated.

How to Measure the Glycolytic Training Stimulus: %VLamax Utilization

So the question becomes: how large is the glycolytic stimulus during a given interval session? This is the same fundamental question we asked for the aerobic system in Part 1.

For the aerobic system, we used VO2 (oxygen uptake in ml per minute) as a marker of how intensely the aerobic system is being used. We expressed it as a percentage of VO2max to get fractional utilization.

The equivalent for the glycolytic system is the lactate production rate (in mmol per minute). We can express the actual glycolytic rate relative to the maximum lactate production rate (VLamax) to get the %VLamax utilization. This tells us, in relative terms, how hard the glycolytic system is working during a given effort.

There is one important difference in kinetics. Oxygen uptake responds slowly and either builds toward a steady state or climbs toward VO2max over time. Lactate production rate reacts faster, but instead of climbing toward maximum during prolonged high-intensity efforts, it actually decreases due to muscle acidosis and declining pH values.

The 10% VLamax Benchmark

From more than 20 years of experience testing and training athletes with VLamax metrics, working with athletes from recreational to Olympic level across all endurance sports worldwide, we have identified a practical benchmark:

Below 10% VLamax utilization: The training stimulus is insufficient to maintain or grow glycolytic power. With repeated exposure, VLamax will likely decrease.

Above 10% VLamax utilization: A mild glycolytic stimulus. May or may not lead to VLamax changes, depending on the broader training context.

Above 15%, especially above 20% VLamax utilization: A potent stimulus to increase glycolytic power. With regular training at this level, VLamax will likely increase.

This benchmark allows coaches to evaluate any interval protocol not just for its aerobic merit, but for its glycolytic consequences.

Get complete view of your athletes performance

INSCYD provides VLamax alongside VO2max, MLSS, LT1, fat and carbohydrate combustion rate, and more. You can test using lactate data, power meters, GPS, or even just a stopwatch. No costly equipment or protocol changes required. Book a free demo to see how it works with your athletes.

Book a Free Demo

Norwegian 4x4: Glycolytic Training Stimulus by Athlete Type

Now we can apply the %VLamax utilization framework to our two interval protocols. We use the same two example athletes from Part 1: both have an identical VO2max of 55 ml/min/kg, the same body weight, the same body composition. Everything is the same except for VLamax. One athlete has a low VLamax of 0.4 mmol/l/s (the endurance specialist). The other has a high VLamax of 0.75 mmol/l/s (the more explosive, sprinter-type athlete).

Both perform Norwegian 4×4: 4 minutes at 100% of their individual threshold, 4 minutes recovery at 50% of threshold.

At the controlled effort of threshold (MLSS) intensity, glycolytic activity actually forms a steady state. By definition, the metabolism is in steady-state conditions: no significant drop in pH, no acidosis-driven inhibition of glycolysis. The glycolytic rate stays relatively constant throughout each interval.

As we can see in the graph, the training stimulus on the glycolytic system is vastly different for the low vs. the high glycolytic athlete during threshold intervals:

The high VLamax athlete (0.75 mmol/l/s): VLamax utilization reaches only approximately 4%. This is well below the 10% benchmark. If this athlete performs Norwegian 4×4 as a dominant and regular part of their training program, this training has the potential to decrease glycolytic power over time.

If it was the goal of this athlete to sacrifice anaerobic power to gain more endurance, great. But if the goal of the athlete is to become more endurant without losing explosiveness, this training has the potential, depending on how the other training sessions are designed, to jeopardize this goal.

The low VLamax athlete (0.4 mmol/l/s): VLamax utilization sits at approximately 10-11%. This is right at the boundary. There is no strong stimulus to increase VLamax, but there is also no significant stimulus to decrease it further. The training is essentially glycolytically neutral for this athlete.

This helps explain why Norwegian 4×4 became popular in the first place. The method was derived from and popularized by elite endurance athletes in triathlon and distance running. These athletes, by nature of their sport and training history, tend to have a low VLamax. For them, Norwegian 4×4 at threshold does not trigger an unwanted glycolytic adaptation. It provides aerobic stimulus without glycolytic risk.

But that does not mean it works the same way for every athlete.

In this part of the video, Sebastian walks through the VLamax utilization data for both athlete types during Norwegian 4×4:

HIIT 40/20: Where the Glycolytic Picture Changes Drastically

With the 40/20 HIIT format, the picture changes. Not only is the glycolytic flux (lactate production rate) much higher than during Norwegian 4×4, it also does not reach a steady state during the short 40-second intervals. Moreover, it actually decreases shortly after the start of the interval session because of the onset of acidosis, which cannot be cleared in the short 20-second recovery periods.

The high VLamax athlete (0.75 mmol/l/s): VLamax utilization stays below 10% utilization of his big anaerobic engine. Based on this workout alone, it is unlikely that VLamax will increase. There is still a potential signal to decrease it.

The low VLamax athlete (0.4 mmol/l/s): Everything changes. Even taking into account a reduced glycolytic stimulus because of the onset of acidosis, the VLamax utilization barely drops below 25% throughout the whole session. Based on what we know about trainability of the anaerobic system, this is a very strong stimulus to increase VLamax. Especially if done regularly, it should come as no surprise if this athlete increases anaerobic power.

And remember what an increased VLamax means: it reduces FatMax, decreases threshold power, and increases carbohydrate utilization. Likely not what the athlete wished for by doing HIIT training.

In this part video, Sebastian compares %VLamax utilization for both athlete types during 40/20 HIIT:

See this in action with real athlete data

Join our live webinar where Sebastian Weber demonstrates how VLamax changes both the aerobic and glycolytic response to the same interval. Open to coaches, athletes, and human performance labs.

Why Some Athletes Stagnate or Regress: The Two Sides of the Coin

Now we can see why some athletes might not improve their endurance with these types of training:

High anaerobic (high VLamax) athletes: They do not receive the optimal stimulus on their VO2max, as we discovered in Part 1. The intervals will not harm their performance, but based on the threshold-based intensity setup, neither training format is as efficient as it could be. On the other hand, these athletes risk losing their edge a bit when it comes to sprinting and short high-intensity efforts.

Low anaerobic (low VLamax) athletes: They receive a much better aerobic stimulus, so they potentially improve the system which is, relatively speaking, already stronger. The lower-intensity Norwegian 4×4 can work well for these athletes. But doing HIIT training carries a very high risk of increasing VLamax.

Increasing VLamax would counter any improvements of VO2max.

An improved VO2max would mean the potential for higher lactate combustion and less production resulting in higher threshold power and an increased ability to use fat as fuel.

But the parallel increase in VLamax offsets those gains. Higher VLamax means higher lactate production, higher carbohydrate utilization, and therefore lower threshold power and lower FatMax.

The result in many cases will appear, at least in conventional performance testing, as a stagnation in performance, even though all metabolic systems might have actually adapted. To understand how VO2max alone is not enough, and why the full metabolic profile matters, see our detailed analysis.

The Solution: Set Intensity Based on VLamax, Set Duration Based on VO2

We just discovered that two athletes doing the same VO2-based intervals can have a significantly different training stimulus and therefore trigger adaptations in their glycolytic system, which can lead to unwanted consequences that actually hamper performance.

What do we do about it?

The answer is straightforward, but it requires a shift in perspective.

Until now, the primary goal of interval design was clear: maximize fractional utilization of VO2max. But we need to introduce a constraint: we must keep the glycolytic stimulus in check. Because if glycolysis receives a sufficiently high stimulus, it will trigger adaptations, such as the expression of more glycolytic enzymes, thereby increasing glycolytic capacity.

So instead of anchoring intensity to threshold, or even to VO2max, we shift the anchor point.

We anchor it to VLamax, the marker for the glycolytic capacity of an athlete.

Controlling the Anaerobic Side

By prescribing intensity as a percentage of VLamax, we directly control the glycolytic contribution of the effort. This ensures that the intensity is sufficiently low to not trigger a significant increase in VLamax.

In other words: we prevent the interval from turning into a high glycolytic stimulus session.

And here you might ask: but I wanted VO2max improvement in the first place!

Yes, and if you are comfortable with possibly increasing VLamax at the same time, you do not need to worry here. But if you want to keep VLamax at its current level, or even decrease it, you need to take its utilization into account when creating the interval.

Using Duration Instead of Intensity to Drive the Aerobic Stimulus

To make sure you still create the VO2max stimulus you were looking for, the answer is relatively simple:

VO2max adaptation is not driven by intensity alone.

It is driven by two factors:

The %utilization of VO2max
How long this high %utilization is maintained

This brings us back to physiology. At intensities above steady state, VO2 continues to rise over time, a phenomenon known as the VO2 slow component.

This means: even if the initial intensity results in a moderate percentage of VO2max, VO2 will continue to increase the longer the effort is sustained.

So instead of increasing intensity, we increase duration.

By extending the duration of the interval:

VO2 climbs progressively
Higher fractional utilization is reached over time
The aerobic stimulus is achieved by spending more time at high %VO2max

This allows us to achieve the same goal, a high VO2 stimulus, but through a different mechanism.

How to Apply This in Practice

Practically, this approach is simple.

Start by defining intensity based on VLamax: A 10% threshold has been proven to work best here. Because the glycolytic ceiling is fixed, the target power output remains the same regardless of whether the interval lasts 40 seconds or 6 minutes.
Design intervals of increasing duration: e.g., 2 min, 4 min, 6 min, 8 min. What changes with duration is not the intensity, but the aerobic response: longer intervals give VO2 more time to ramp up.
Evaluate the resulting %VO2max at each duration to find the optimal balance.
Optionally monitor additional markers: heart rate and lactate concentration during training to verify the athlete stays in the prescribed zone.

In this part of the video, Sebastian demonstrates this exact workflow using the INSCYD Training Zones Builder:

How the INSCYD Training Zones Builder Makes This Possible

The INSCYD Training Zones Builder is the practical tool that executes this workflow. It allows coaches to:

Select any physiological metric as the ‘master metric’ that defines the zone. For the aerobic problem from Part 1, that metric is %VO2max. For the glycolytic problem from Part 2, it is %VLamax. Other available options include fixed lactate concentration, percentage VO2max, substrate utilization, FatMax, and more.
Set the target utilization. Define the threshold for the chosen metric (e.g., VLamax utilization at 10%).
Read the output across multiple metrics. Once the zone is defined by VLamax, you can select whatever output metric you are interested in. In the example from this article, we check the %VO2max utilization and the percentage of anaerobic threshold. You could also select lactate concentration to verify during actual training whether the athlete stays in the prescribed zone.

This approach moves coaching from generic percentage-of-threshold prescription to physiology-based interval design. It ensures that you trigger the right system that you want to trigger with the right training stimulus.

The result: effective high-intensity interval training where both sides of the metabolic equation are controlled, individualized to each athlete’s unique profile.

Stop guessing interval intensity

NSCYD builds individualized training zones based on each athlete's metabolic profile, including VLamax, VO2max and more. Whether you want to maximize aerobic gains, control glycolytic adaptation, or both, the Training Zones Builder gives you the physiological precision to do it

Book a Free Demo

Full Circle: Truly Individualized Interval Training

This is where everything comes together.

In Part 1, we showed that:

The same interval does not produce the same stimulus for every athlete
Threshold-based prescriptions can misrepresent the actual aerobic load in terms of %VO2max utilization

Now we take the next step.

Instead of forcing the athlete into a predefined intensity, we design the interval around the athlete’s physiology.

We control the training intensity based on VLamax, to ensure we do not trigger unwanted adaptations. We then modify the duration of the interval to reach the desired VO2 stimulus.

This is what truly individualized training looks like.

It ensures that the athlete receives the best possible training stimulus and therefore spends their limited training time in the most efficient way. Not generic extrapolation from FTP. But a targeted intervention based on the actual metabolic profile of the athlete.

Go beyond the theory. In our intensive 4-day camps for coaches and lab professionals, Sebastian teaches the complete framework behind metabolic profiling, diagnostics, nutrition, and training adaptation, including the VLamax and VO2max concepts covered in this article.

You will work with real athlete data, perform hands-on testing, and leave with the skills to apply physiology-based training prescription with your own athletes. Open to coaches, sports scientists, and performance lab operators.

Guided by Sebastian Weber — the scientist and coach behind multiple Olympic champions, 9 World Championship titles, and Tour de France victories.

Frequently Asked Questions

Does HIIT increase or decrease VLamax?

Whether HIIT increases or decreases VLamax depends on the athlete’s existing metabolic profile. For athletes with low VLamax (low glycolytic power), short HIIT intervals like 40/20 at 130% of threshold can produce VLamax utilization above 25%, which is a strong stimulus to increase glycolytic power. For athletes with already-high VLamax, the same session may produce comparatively low utilization levels, providing little glycolytic stimulus or even contributing to decreased VLamax over time. The response is determined by the ratio of the athlete’s aerobic capacity (VO2max) to their glycolytic capacity (VLamax). INSCYD quantifies both metrics to predict the glycolytic training effect before the session is performed.

Why do some endurance athletes get slower from HIIT?

Athletes can regress from HIIT because the training simultaneously improves the aerobic system (VO2max) while also strengthening the glycolytic system (VLamax). For endurance athletes with low baseline VLamax, the high glycolytic stimulus of short HIIT intervals can increase VLamax. A higher VLamax lowers the anaerobic threshold, increases carbohydrate combustion, and decreases fat oxidation. These negative downstream effects can fully offset the VO2max improvement, causing performance stagnation in performance. Learn more about managing VLamax in our guide on tips to decrease VLamax.

Is Norwegian 4x4 better than HIIT for endurance athletes?

Norwegian 4×4 is glycolytically safer for most endurance athletes because threshold-level intensity keeps VLamax utilization low (approximately 4% for high VLamax athletes and around 10% for low VLamax athletes). This means the risk of unwanted glycolytic adaptation is minimal. However, ‘better’ depends entirely on the individual athlete’s metabolic profile and training goals. For athletes with high VLamax who want to maintain their anaerobic capacity, Norwegian 4×4 may actually erode their glycolytic power over time. The optimal protocol depends on knowing both VO2max and VLamax.

How can I control the glycolytic stimulus during interval training?

To control the glycolytic stimulus, you need to know the athlete’s VLamax and then prescribe intervals based on %VLamax utilization rather than %threshold. Using INSCYD’s Training Zones Builder, set VLamax as the ‘master metric’ and define a utilization ceiling (e.g., 10%). The tool calculates the corresponding power/pace for different interval durations and shows the resulting %VO2max utilization. Coaches can then adjust interval duration to maximize aerobic stimulus while keeping the glycolytic stimulus within the desired range. Lactate measurements during training can validate that the athlete stays in the prescribed zone.

Can VO2max improve while threshold stays flat?

Yes. This is exactly the stagnation pattern described in this article. When VO2max intervals also produce a strong glycolytic stimulus, both VO2max and VLamax can increase simultaneously. The higher VO2max should raise threshold, but the higher VLamax pushes it back down through increased lactate production. The net result is that the anaerobic threshold remains unchanged or even decreases despite measurable VO2max improvement. This pattern is especially common in endurance athletes with low baseline VLamax who perform regular short HIIT sessions.

Don’t miss the live webinar: HIIT vs Norwegian 4×4

Sebastian Weber will walk through real athlete data, show how VLamax changes the aerobic stimulus, and demonstrate the Training Zones Builder live. Whether you’re a coach, athlete, or run a human performance lab — this session is for you.

References

Brooks, G.A. (1985). Lactate: Glycolytic End Product and Oxidative Substrate During Sustained Exercise in Mammals – The “Lactate Shuttle.” Comparative Physiology and Biochemistry: Current Topics and Trends, Vol. A, Respiration-Metabolism-Circulation. Springer, Berlin.
El Hayek, L. et al. (2019). Lactate Mediates the Effects of Exercise on Learning and Memory through SIRT1-Dependent Activation of Hippocampal Brain-Derived Neurotrophic Factor (BDNF). Journal of Neuroscience, 39(13), 2369-2382.
Helgerud, J. et al. (2007). Aerobic High-Intensity Intervals Improve VO2max More Than Moderate Training. Medicine & Science in Sports & Exercise, 39(4), 665-671.
Laursen, P.B. & Jenkins, D.G. (2002). The Scientific Basis for High-Intensity Interval Training. Sports Medicine, 32(1), 53-73.
Poffé, C., Van Dael, K. & Van Schuylenbergh, R. (2024). INSCYD Physiological Performance Software Is Valid to Determine the Maximal Lactate Steady State in Male and Female Cyclists. Frontiers in Sports and Active Living, 6, 1376876.
Abe, T. et al. (2015). High-Intensity Interval Training-Induced Metabolic Adaptation Coupled with an Increase in Hif-1α and Glycolytic Protein Expression. Journal of Applied Physiology.
Torma, F. et al. (2019). High Intensity Interval Training and Molecular Adaptive Response of Skeletal Muscle. Sports Medicine and Health Science, 1(1), 24-32.

About the Author

Sebastian Weber is a sports scientist, founder of INSCYD, and creator of the first test to measure glycolytic power (VLamax) in 2003. His work in exercise physiology and metabolic profiling has helped WorldTour cyclists win 9 World Championship titles, Olympic medals, and Tour de France victories. Sebastian consults for the German Swimming and Skiing Federations, USA Triathlon, and is a teacher for the German Triathlon Federation and Olympic S&C coaching program. Want to learn directly from Sebastian? Join his 4-Day Hands-On Camp on Physiology, Diagnostics, Nutrition, and Training Adaptation.

Sebastian Weber

Founder of INSCYD & World-renowned Sport Scientist

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The post HIIT vs. Norwegian 4×4: The Forgotten Metric That Decides Your Interval Training Outcome (Part 2) appeared first on INSCYD.

HIIT vs Norwegian 4×4: How VLamax Determines Your Aerobic Training Response

Igor Makarov — Fri, 13 Mar 2026 15:08:26 +0000

Part 1 of the HIIT vs. Norwegian 4×4 Series | By Sebastian Weber, Founder of INSCYD & WorldTour Cycling Coach

Key Takeaways

Norwegian 4×4 intervals (4 min at 100% threshold / 4 min rest) and 40/20 HIIT (40 sec at 130% threshold / 20 sec rest) produce fundamentally different VO2 responses due to oxygen kinetics.
Two athletes with identical VO2max but different VLamax (glycolytic power) will experience significantly different aerobic training stimuli from the same interval prescription.
Prescribing interval intensity as a percentage of threshold is flawed because it ignores individual differences in metabolic profile. Intensity should be prescribed based on %VO2max to ensure the intended aerobic adaptation.
Solution to quantifies both VO2max and VLamax, enabling coaches to prescribe individualized interval intensities using the Training Zones Builder.

Live Webinar: HIIT vs Norwegian 4×4 — The Science Behind Interval Prescription

Join Sebastian Weber for a deep dive into the physiology behind this article. For coaches, athletes, and human performance labs

Why Norwegian 4×4 Is Everywhere Right Now

Right now, Norwegian 4×4 is everywhere.

Podcasts, coaching seminars, social media threads — suddenly everyone is prescribing 4×4-minute intervals around threshold (often described in the literature as MLSS) as if we’ve just discovered the missing piece of endurance training. It’s “evidence-based.” It’s “proven.” It’s “what the Norwegians do.”

We’ve already covered the Norwegian method and how to apply it in training in a previous article. But now we need to go deeper.

Let’s pause for a moment.

A few years ago, it was HIIT. Just recently, Zone 2 became the dominant era.

Endurance training trends move like fashion. What is “in” today quietly replaces what was “in” yesterday.

Yet here’s the important part:

HIIT isn’t wrong. Norwegian-style 4–6 minute intervals around threshold aren’t wrong either.

Both methods are supported by a substantial scientific body of evidence. Both improve VO2max. Both improve performance. Both have been repeatedly validated under controlled conditions.

Scientific data does not become invalid just because a new trend appears.

So how can athletes and coaches report massive gains with Norwegian intervals — while others achieve equally compelling results with HIIT?

If both approaches are well researched and effective, the conclusion cannot be that one is “right” and the other “wrong.”

The more useful explanation is this:

Both methods work. But they do not work equally well for every athlete.

The crucial difference lies not in the trend — but in the mechanism.

The crucial difference lies not in the trend — but in the mechanism. HIIT and Norwegian-style intervals stress the system in different ways. They emphasize different physiological pathways. They challenge metabolism differently. And depending on who the athlete is — from a phenotype and training status perspective — one stimulus may produce different adaptations.

This is what we are going to clarify in this article.

We will show that the real answer is not in the hype, but in the physiology of the athlete.

Because the decisive question is not: “What is currently popular?”

It is: “Which physiological profile is in front of me — and which stimulus will move it forward?”

What Are Norwegian 4×4 and 40/20 HIIT Intervals?

Before we move forward, we need to ensure we are on the same page and avoid misunderstandings.

“HIIT” and “Norwegian intervals” are broad terms. Different coaches mean different things when they use them. If we are going to compare these methods meaningfully, we must define exactly what we are talking about.

For this article, we will contrast two representative examples.

Norwegian 4×4 Protocol Explained

4 minutes ON @ 100% of threshold (Maximal Lactate Steady State, MLSS; or FTP as a rough proxy)
4 minutes OFF @ 50% of threshold

The recovery is deliberately low intensity. About 50% of threshold represents very easy work and allows for good recovery in between the controlled intervals.

This 4×4 structure is consistent with the likely most popular version of the so-called “Norwegian intervals” approach.

Classic 40/20 HIIT Protocol Explained

40 seconds ON @ 130% of threshold intensity (Maximal Lactate Steady State, MLSS; or FTP as a rough proxy)
20 seconds OFF @ 50% of threshold

Prescribing such high-intensity intervals as a percentage of threshold is common practice — especially in cycling and triathlon. Threshold serves as a reference anchor from which supra-threshold intensities are extrapolated. Extrapolation from threshold to training zones for VO2max and high intensity intervals per se is an issue that can lead to imprecise training and unwanted adaptations as described here, but this isn’t part of this article.

The 40/20 format is simply one well-established example we use here to prove the point. A 30/30 structure would operate under very similar principles. The exact seconds are less important than the relative intensity and recovery structure.

Norwegian 4×4 vs. 40/20 HIIT: Head-to-Head Comparison

Norwegian 4×4 vs 40/20 HIIT: Head-to-head comparison of interval protocol parameters, VO2 kinetics, and prescription methods.
	Norwegian 4×4	40/20 HIIT
Work interval	4 minutes	40 seconds
Rest interval	4 minutes	20 seconds
Intensity (typical)	~100% of threshold (MLSS)	~130% of threshold (MLSS)
VO2 kinetics	Approaches steady state; rises high then falls between intervals	Oscillates; never fully peaks, never fully drops
Aerobic stimulus	Depends on athlete's VLamax	Depends on athlete's VLamax
Best prescribed by	%VO2max (not threshold)	%VO2max (not threshold)

Norwegian 4×4

Work interval 4 minutes

Rest interval 4 minutes

Intensity ~100% of threshold (MLSS)

VO2 kinetics Approaches steady state; rises high then falls between intervals

Aerobic stimulus Depends on athlete's VLamax

Best prescribed by %VO2max (not threshold)

40/20 HIIT

Work interval 40 seconds

Rest interval 20 seconds

Intensity ~130% of threshold (MLSS)

VO2 kinetics Oscillates; never fully peaks, never fully drops

Aerobic stimulus Depends on athlete's VLamax

Best prescribed by %VO2max (not threshold)

In both sessions, recovery periods are intentionally kept low-intensity. What differs substantially is:

The duration of the work interval (40 seconds vs. 4 minutes)
The duration of the rest interval (20 seconds vs. 4 minutes)
The relative intensity (~130% vs. ~100% of threshold)

Now that the structures are clearly defined, we can start answering the question: why should you (or the athletes that you coach) prefer the one over the other type of interval?

In order to answer this question we need to look at what actually happens inside the body, more precisely the muscles, during this kind of training.

The Aerobic Response: What Happens During HIIT and 4×4 Intervals?

Before comparing athletes, we first need to look at the sessions themselves — and understand how and why these intervals became so popular for increasing aerobic performance.

VO2 as a Proxy for Aerobic Training Stimulus

A practical way to evaluate the aerobic impact of an interval session is to examine oxygen uptake during these intervals. Oxygen uptake may serve here as a proxy for the aerobic training stimulus — the signal to the body to improve aerobic capacity.

Two simple dimensions matter when using oxygen uptake as a proxy for aerobic improvements:

How high does oxygen uptake rise? The fractional utilization of VO2max has been established as an important marker. The importance of accumulated time near 90% VO2max as a potential driver of VO2max adaptation has been emphasized repeatedly in the interval training literature (Laursen & Jenkins, 2002; Rønnestad et al., 2014; Odden et al., 2024). For a deeper look at this, see our article on VO2max intervals.
How long does oxygen uptake remain elevated? Think about exercise duration – what is the total accumulated exposure time at a high aerobic rate. Classic 4×4 interventions have repeatedly demonstrated significant increases in VO₂max (Helgerud et al., 2007; Daussin et al., 2008).

Both dimensions matter but are not equally the same. For example: long low intensity workouts can have a substantial positive effect on growing more mitochondria in the muscle – the powerhouse in muscle cells where the aerobic metabolism actually happens – but less stimulus to increase cardiac output of the heart (HELGERUD, Jan, et al. 2007).

VO2 Response During Norwegian 4×4 Intervals

In the 4×4 intervals, the work phase is long enough for oxygen uptake to approach its destined value — its steady state, or near steady state. Once it reaches that level, it remains elevated for several minutes before recovery begins. Between repetitions, the four-minute recovery allows VO2 to drop substantially before the next interval starts.

VO2 Response During 40/20 HIIT

In contrast, during 40/20 intervals, no true steady state is reached. The work phase is too short for VO2 to fully climb to its potential end value. However, the recovery is also too short for it to return to baseline.

The Critical Difference in Oxygen Kinetics

Now, when we compare our two interval formats — 4×4 and 40/20 — an important difference becomes visible.

The difference comes primarily from oxygen kinetics:

Oxygen kinetics are not the same as power kinetics.

Power can increase or decrease abruptly. VO2 cannot.

When intensity rises, oxygen uptake follows with a delay — similar to the likely familiar heart rate kinetics. When intensity drops, VO2 also declines gradually.

This delay changes the exposure profile of the two sessions:

In 4×4: VO2 rises high and stabilizes, then falls low again between intervals.
In 40/20: VO2 oscillates — never fully reaching its potential peak, but never fully dropping either.

That distinction is central. Because it determines how long oxygen uptake is elevated, how stable that elevation is, and how the aerobic stimulus is distributed over time.

In this video, Sebastian walks through the oxygen uptake kinetics of both protocols, explaining why the 40/20 format never allows VO2 to reach steady state while the Norwegian 4×4 does

Comparison of oxygen kinetics of 10×40” @130% MLSS / 20” @50% MLSS (solid line) vs. VO2 kinetics during 4’ @100% MLSS / 4’ @50% MLSS (dashed line). In the 40” on phase, VO2 neither reaches VO2max (55 ml/min/kg in this example) nor a plateau. In the 20” off phase, VO2 doesn’t decrease fully but remains elevated, starting from a higher level in the subsequent interval.

Now comes the important question: does every athlete respond the same?

Why Athletes With the Same VO2max Respond Differently

Up to this point, we have only looked at the intervals themselves. We established that 4×4 and 40/20 produce different aerobic exposure profiles, both formats are supported by the literature, and both can increase VO2max.

But now we need to ask a much more important question:

Do all athletes receive the same stimulus from the same interval?

To answer that, we must introduce the individual.

The Role of VLamax in Aerobic Stimulus

Let’s use two example athletes. They have the same body weight, the same body composition, the same VO2max — the same everything. All key physiological performance indicators are identical — except for VLamax.

VLamax (maximum glycolytic rate) measures an athlete’s anaerobic glycolytic power — how quickly they produce energy through glycolysis. It is a key metric for understanding how an athlete’s metabolism responds to interval training.

Athlete A — Low VLamax: Lower anaerobic capacity, lower sprint power, lower short-duration peak output
Athlete B — High VLamax: Higher glycolytic capacity, higher sprint power, stronger short efforts

Both have the same VO2max. But they are not metabolically the same.

High VLamax vs Low VLamax: Different Metabolic Profiles

So why does it matter if one athlete is anaerobically stronger?

The interval intensity is based on % of threshold, so it is already individualized to the athlete. So why should a higher VLamax — i.e., higher glycolytic power — affect 4×4 or HIIT training?

The reason is simple:

The energy required for any effort is provided partly by anaerobic energy production — using carbohydrates that are converted into lactate — and partly by aerobic metabolism (which uses lactate and fats as fuel).

How is this energy distribution between those two systems determined? It is determined by the relative strength of the two systems — aerobic and anaerobic — in relation to each other.

So when two athletes have the same VO2max (same maximal aerobic power) but different VLamax (different glycolytic power):

The athlete with the lower VLamax has, relatively speaking, a stronger aerobic system. This athlete will rely more on aerobic energy production, leading to higher oxygen uptake during the effort.
The athlete with the higher VLamax will rely more on anaerobic energy production. As a result, less energy is produced aerobically, leading to lower VO2 during the same effort.

In short: when both athletes perform the exact same interval session — for example, 4×4 at their individual threshold, or 40/20 at 130% of their individual threshold — they do not receive the same aerobic stimulus.

And the reason is not their VO2max, but their VLamax.

The athlete with the lower VLamax — the one who is relatively weaker anaerobically — tends to achieve higher relative oxygen uptake during the same intervals. The more “aerobic” phenotype receives a stronger aerobic stimulus from the same training.

The athlete with the higher VLamax — the more glycolytic phenotype — tends to generate more anaerobic contribution at the same relative intensity. This means more glycolytic energy production, greater anaerobic contribution, and paradoxically: lower sustained VO2 levels. Even though VO2max is identical.

To learn how to manage this, read our guides on tips to decrease VLamax and how to increase or decrease VLamax in running.

Key Takeaway: Same Prescription, Different Stimulus

At the same VO2max. At the same relative, threshold-based intensity. At the same interval prescription.

The training stimulus is not the same. Because the metabolic contribution is not the same.

The athlete who is already stronger anaerobically may receive a smaller aerobic stimulus from the very interval intended to improve aerobic capacity.
The athlete who is weaker anaerobically may receive a disproportionately larger aerobic stimulus from that same session.

This is why choosing between Norwegian 4×4 and HIIT based on trends or general recommendations misses the point entirely. The right protocol depends on the athlete’s individual metabolic profile.

In this part of the video below, Sebastian compares oxygen uptake kinetics for a high glycolytic vs. low glycolytic athlete performing the same interval sessions. During the 40/20 HIIT at 130% threshold, the low VLamax athlete reaches close to 90% of VO2max as peak VO2, whereas the high VLamax athlete reaches significantly lower VO2 values. The same principle applies to Norwegian 4×4: the low VLamax athlete achieves roughly 20% higher fractional VO2max utilization than the high VLamax athlete — despite identical VO2max and identical threshold-based intensity prescription.

There is another simple way to think about it: The athlete with the higher anaerobic capacity has their threshold at a lower percentage of VO2max. Therefore, prescribing 4×4 intervals at 100% of threshold automatically results in a lower fractional utilization of VO2max for that athlete.

Want to see this in action?

Join our live webinar where Sebastian Weber walks through real athlete data showing how VLamax changes interval response. Open to coaches, athletes, and human performance labs.

The Solution: Prescribing Intensity Based on VO2max, Not Threshold

There is a more direct way to think about this problem.

We say we want to improve VO2max. We design intervals to increase VO2max. We analyze literature about time spent near 90% of VO2max.

And then — instead of prescribing intensity based on VO2max — we prescribe it as a percentage of threshold and extrapolate from there.

That is the conceptual inconsistency. And it’s one we’ve explored before in HIIT training: intensity based on %FTP and its shortcoming.

If two athletes have the same VO2max but different VLamax, their threshold sits at a different percentage of VO2max. That means:

When prescribing 4×4 at “100% of threshold,” the aerobic trigger is not the same.

One athlete may operate at 90–92% of VO2max.
The other may only reach 84–86% of VO2max.

Same VO2max. Same threshold-based prescription. Different aerobic stimulus.

How INSCYD Training Zones Builder Solves This

Now that we recognize the problem, the logic of the solution becomes clear.

If the goal is to stimulate VO2max, then interval intensity should be anchored to VO2max. Not indirectly through threshold.

By prescribing intensity as a percentage of VO2max — and by understanding the oxygen kinetics behind it — we can directly target the intended fractional utilization.

The INSCYD Training Zones Builder allows you to do exactly this: receive the %VO2max for any interval duration and interval intensity. This way it becomes possible to understand the exact aerobic training stimulus in any interval training of interest. Learn more about how it works.

In this part of the video, Sebastian walks you through the Training Zones Builder with both athlete profiles — and shows why prescribing intervals based on %threshold produces fundamentally different aerobic stimuli.

Training zones for Norwegian 4×4 are based on maximum aerobic power for both: a) a high anaerobic (glycolytic) athlete, b) a low glycolytic athlete. Because the intensity is set to 80%, 90%, 100% of aerobic power, the target training intensity is the same for both athletes as they have the same VO2max. This leads to comparable average %VO2max utilization from low 70% to around 85%. However, the training intensity expressed as % anaerobic threshold (similar to LT2, FTP) is significantly different. For example: to achieve 80% VO2max utilization, the low glycolytic athlete needs an intensity of 112% of threshold. In contrast, an almost comparable 77% utilization of VO2max requires 134% of threshold power in the high glycolytic athlete.

This approach ensures that two athletes with the same VO2max but different metabolic profiles truly receive the same aerobic trigger.

Stop guessing interval intensity

INSCYD builds individualized training zones based on each athlete’s metabolic profile — including VLamax and VO2max. See how it works for your athletes.

Book a Free Demo

Learn this in person — directly from Sebastian Weber

This article covers the science behind interval prescription. At the 4-Day Hands-On Camp on Physiology, Diagnostics, Nutrition, and Training Adaptation, you go further: hands-on physiological testing with real athletes, building metabolic profiles, and applying this knowledge to training decisions on the spot.

Over 4 days, you will conduct lactate and VO2 testing at the running track and on the road, learn how to interpret and apply metabolic data to training programs, and answer the question this article raises: how does the physiology of an athlete determine whether to go long and easy or short and hard?

Guided by Sebastian Weber — the scientist and coach behind multiple Olympic champions, 9 World Championship titles, and Tour de France victories.

Frequently Asked Questions

Is the Norwegian 4×4 better than short HIIT intervals for improving VO2max?

Neither protocol is universally better. The effectiveness depends on the athlete’s individual physiology, particularly their VLamax (glycolytic power). Long intervals like the Norwegian 4×4 allow VO2 to ramp to near-maximum levels, while short intervals like 40/20 produce oscillating VO2 responses. The key factor is whether the prescribed intensity actually elicits the intended percentage of VO2max for that specific athlete.

Why shouldn’t I prescribe interval intensity based on threshold alone?

Threshold-based intensity prescription assumes a fixed relationship between threshold and VO2max across all athletes. However, the ratio of threshold to VO2max varies significantly between individuals, largely driven by differences in VLamax. An athlete with low VLamax working at 100% of threshold may operate at 90–92% of VO2max, while an athlete with high VLamax at the same threshold-based intensity may only reach 84–86% of VO2max. Prescribing “100% of threshold” therefore produces very different aerobic stimuli for these two athletes.

What is VLamax and why does it matter for interval training?

VLamax (maximum glycolytic rate) measures an athlete’s anaerobic glycolytic power — how quickly they produce energy through glycolysis. VLamax is critical for interval training prescription because two athletes with identical VO2max but different VLamax values will reach different percentages of VO2max at the same threshold-based intensity. INSCYD is one of the few platforms that quantifies VLamax alongside VO2max to enable individualized training prescription.

How does oxygen kinetics affect interval training effectiveness?

Oxygen kinetics describes how quickly VO2 rises at the onset of exercise. In long intervals (4+ minutes), VO2 has time to approach steady-state values. In short intervals (under 60 seconds), VO2 may never reach its peak before the rest period begins. This means the actual time spent at high percentages of VO2max — which drives aerobic adaptation — can vary dramatically between interval formats and between individual athletes. For more on effective HIIT programming, see our detailed breakdown of work-to-rest ratios.

How can I individualize interval training for each athlete?

To truly individualize interval training, you need to know both the athlete’s VO2max and VLamax. INSCYD quantifies both metrics from field-based testing using the athlete’s own equipment, then uses these values to calculate individualized training zones based on %VO2max rather than threshold alone. The Training Zones Builder generates zones that account for each athlete’s unique metabolic fingerprint.

Don’t miss the live webinar: HIIT vs Norwegian 4×4

References

Laursen, P.B. & Jenkins, D.G. (2002). The scientific basis for high-intensity interval training. Sports Medicine. DOI
Helgerud, J. et al. (2007). Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine & Science in Sports & Exercise. DOI
Daussin, F.N. et al. (2008). Effect of interval vs continuous training on cardiorespiratory control. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. DOI
Rønnestad et al. (2015), Scand J Med Sci Sports. DOI
Odden, I. et al. (2024). European Journal of Sport Science. DOI

About the Author

Sebastian Weber

Founder of INSCYD & World-renowned Sport Scientist

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[Webinar] HIIT vs Norwegian 4×4: The Science Behind Interval Prescription

Igor Makarov — Fri, 13 Mar 2026 13:10:15 +0000

Webinar

HIIT vs Norwegian 4x4: The Science Behind Interval Prescription

The interval you prescribe to improve VO2max could be quietly working against your athlete’s performance. And the reason is not the training — it’s who the athlete is.

About the Webinar:

Both HIIT and Norwegian 4×4 are proven to improve VO2max. So why do some athletes see real gains while others stagnate — or even get worse? Sebastian Weber will show why the same interval prescription produces completely different outcomes in different athletes, even when their VO2max is identical.

The aerobic side

Two athletes performing the exact same threshold-based interval can reach very different levels of oxygen uptake. Same workout, same VO2max, different aerobic stimulus. Sebastian will show the data and explain why.

The side nobody talks about

Neither HIIT nor Norwegian 4×4 only affects the aerobic system. There is a second training effect happening in parallel. For certain athletes, this effect can quietly undo the aerobic gains the training was supposed to deliver.

Real athlete data

This is not a theory session. Sebastian will walk through actual metabolic profiles showing how the same interval produces opposite training effects in two different athletes.

Live demonstration

Sebastian will show how to identify which interval approach fits which athlete, and how to set the right intensity for both the aerobic and glycolytic side.

Both methods are backed by science. But choosing the right one starts with understanding the athlete in front of you.

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Master Data Application

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Utilize Practical Tools

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Presenter

Sebastian Weber

Founder of INSCYD & World-renowned Sport Scientist

Want deeper insights? - Get 360° View of Your Athletes Performance with Detailed Metabolic Profile

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[Live Session] Automated Indoor Metabolic Testing | INSCYD x Velocity

Igor Makarov — Fri, 06 Mar 2026 11:28:18 +0000

Live Session

See Automated Metabolic Testing in Action — Full Session Recording

Watch Sebastian Weber, WorldTour cycling coach and INSCYD founder, guide a live group metabolic test inside Velocity.

See the full protocol in action — how athletes test themselves independently, how a complete 360° metabolic profile can be generated automatically — VO2max, VLamax, MLSS, LT1, Fat and Carbohydrate combustion rates — and what this means for your coaching business or training.

No lab. No manual processing. Testing scales, your effort doesn’t.

Trusted by Elite Federations, Coaches, Labs and Leading Universities Worldwide

Finnish Sports Lab Jyväskylä

Luxembourg Athletics Federation

German Triathlon Federation

French National Institute of Sport, Expertise, and Performance

Pro Cycling – Team Bardiani CSF

Singapore Swimming Association

Pro Cycling – Team Alpecin Fenix

German Swimming Federation

Pro Speedskating – Team IKO

Belgian Royal Speed Skating Federation

Pro Cycling – CANYON//SRAM

Luxembourg Institute for High Performance Sports

Hungarian Canoe Federation

Belgian Cycling Federation

German Skiing Federation

Belgian Triathlon Flanders Federation

Pro Cycling – Movistar Team

Roojai Cycling Team

Olympic Center Upper Austria

University of California, Santa Barbara

American Canoe Association

Hungarian Canoe Federation

Singapore Swimming Association

Belgian Cycling Federation

Pro Cycling – CANYON//SRAM

Pro Cycling – Team Bardiani CSF

Roojai Cycling Team

Finnish Sports Lab Jyväskylä

Belgian Triathlon Flanders Federation

American Canoe Association

German Swimming Federation

German Triathlon Federation

Pro Speedskating – Team IKO

Pro Cycling – Movistar Team

German Skiing Federation

Olympic Center Upper Austria

University of California, Santa Barbara

Belgian Royal Speed Skating Federation

Pro Cycling – Team Alpecin Fenix

Luxembourg Athletics Federation

Luxembourg Institute for High Performance Sports

French National Institute of Sport, Expertise, and Performance

This Is Now Available for Your Coaching or lab Business

What you just watched is available now — for your coaching business or lab. The INSCYD × Velocity integration gives you everything you need to offer fully automated indoor metabolic testing to your athletes.

Athletes test themselves independently — no supervision required
Complete 360° metabolic profile generated automatically — VO2max, VLamax, MLSS, LT1, Fat and Carbohydrate combustion rates
Individual and group sessions — no lab, no equipment investment
Testing scales, your effort doesn't

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Lactate Profile Curves: a concept used for more than 50 years

Igor Makarov — Wed, 18 Feb 2026 13:29:09 +0000

For more than 50 years, lactate profile testing has been a central tool in endurance training.

Graded exercise tests with incremental increases in speed or power, blood lactate measurements at each stage, and the resulting lactate curve are deeply embedded in the way endurance performance is assessed.

The age of this concept alone does not make it wrong. But…

Newton’s theory of gravity is several centuries old and still correct. Age is not the issue. The issue is how a concept is used — and whether its interpretation has kept pace with advances in science.

For decades, lactate curves have offered something highly attractive to coaches and practitioners: a methodology that appears to provide an objective assessment of endurance performance.

A lactate profile curve provides a speed or power at anaerobic threshold — commonly used as a proxy for maximal lactate steady state — which is then used to create intensity zones for training [1,2].

Curves from the same or different athletes can be compared over time, and a shift of the curve is interpreted as a shift in endurance performance. Lactate profile curves seem to summarise endurance capacity in a straightforward and widely accepted test methodology.

Over time, an implicit belief has emerged from this practice: that the position and shape of the lactate curve reflect aerobic performance itself.

This belief is rarely stated explicitly, but it quietly underpins how lactate data are interpreted. As long as lactate remains low and stable, metabolism is assumed to be aerobic. When lactate begins to rise sharply, an “anaerobic” contribution is assumed to dominate. The power or speed at this transition — often labelled an anaerobic threshold — is therefore interpreted as a marker of aerobic endurance performance, or more precisely “the highest intensity which can be covered aerobically without the accumulation of lactate” [1].

In this logic, the curve becomes the performance. A right-shifted lactate curve is taken to indicate improved endurance and aerobic capacity, as higher speed or power can be sustained without (anaerobic) lactate accumulation. The threshold becomes a proxy for how “endurance-trained” an athlete is.

This reasoning did not emerge by accident. It is rooted in an outdated view of metabolism in which lactate was considered a by-product of anaerobic metabolism occurring when oxygen supply was insufficient [3]. If lactate appeared only under oxygen-limited conditions, its rise would naturally signal the limit of aerobic performance.

That view shaped the interpretation of lactate curves for decades — and it still strongly influences how lactate testing is used today.

If you’re more interested in watching these concepts explained step by step, Sebastian Weber goes deeper into the same ideas in this webinar. He shows why conventional lactate curve interpretation falls short — and how the very same tests can be used to extract VLamax, VO₂max, substrate utilization, and comprehensive metabolic insights, without additional testing, equipment, or time.

Continue reading below for the full explanation.

Something doesn’t add up

If lactate profile curves truly reflect aerobic endurance performance, they should behave accordingly.

However, several observations challenge this assumption.

A fundamental one is the relationship between aerobic capacity and the position of the lactate curve. Aerobic performance is commonly associated with maximal oxygen uptake (VO₂max) and the capacity of the oxidative system. If lactate curves were a direct representation of aerobic endurance, athletes with comparable aerobic capacity should show comparable lactate curve positions.

In practice, they do not.

Athletes with similar VO₂max values can exhibit markedly different lactate profile curves. In some cases, these differences are substantial and are often interpreted as large differences in “aerobic endurance performance.” Yet the underlying aerobic capacity, as quantified by VO₂max, remains essentially the same.

Conversely, significant right-shifts of the lactate curve can be observed without meaningful changes in VO₂max. If aerobic capacity has not changed, improved aerobic performance cannot be the explanation.

This inconsistency becomes even more apparent when comparing athletes from different event specialisations. Historical data from elite runners show that lactate curves of 100 m, 400 m, 800 m, 1500 m, and marathon athletes differ profoundly. While it is tempting to explain this purely by differences in aerobic endurance, this explanation quickly breaks down.

The aerobic capacity of 800 m and 1500 m runners is not dramatically lower than that of marathon runners. In some cases, VO₂max values are comparable. Yet their lactate curves can be shifted far to the left. If lactate curve position were primarily a measure of aerobic performance, such discrepancies would be difficult to justify.

These differences point toward an alternative explanation: lactate curve position is not determined by aerobic capacity alone.

Another important performance determinant that is entirely absent from lactate profile testing, yet strongly influences the results, is economy and efficiency. In running, for example, a better running economy means lower energy demand and therefore lower oxygen uptake at a given speed.

Even if one accepts the idea that an inflection point in lactate concentration marks aerobic capacity, a closer look reveals a problem: the inflection point refers to a speed or power output on the x-axis of the lactate curve. Differences in economy mean that the oxygen uptake at this intensity can vary substantially between athletes, rendering a direct link between the inflection point and aerobic capacity questionable.

Body composition adds another frequently overlooked factor. Lactate concentration is measured in mmol·L⁻¹ — a ratio. The dilution space depends on total body water, which is influenced by muscle mass and fat mass. Changes in body composition — through diet, strength training, or seasonal adaptations — can alter measured lactate concentrations without any change in metabolic performance, thereby shifting the apparent position of the lactate curve.

And then there is the protocol.

Lactate profile curves are highly sensitive to testing protocols. Step duration, workload increments, and sport-specific testing conditions substantially influence measured lactate concentrations. Different protocols — particularly differences in stage duration — can produce different curves in the same athlete, even when physiological capacity has not changed.

This has two important consequences.

First, lactate curves obtained with different protocols are not directly comparable.

Second, any threshold concept or method used to estimate MLSS must be matched to the specific protocol employed.

For example, the well-known 4 mmol·L⁻¹ threshold was validated only in treadmill running tests using five-minute stages with short recovery intervals. Deviations from these conditions alter the lactate curve and render a fixed threshold concept invalid.

Taken together, these observations reveal a core problem. Lactate curves are expected to answer a question they are not designed to answer. Even if the protocol is standardised and the threshold concept is properly matched, fundamental interpretational issues remain unresolved.

If curve position can change without changes in aerobic capacity; if large differences appear between athletes with similar aerobic performance; and if economy and body composition substantially alter the curve, then lactate profile curves may not be the robust tool they are often assumed to be for assessing endurance performance and guiding training decisions.

Something does not add up.

Outcome is not the cause

To understand why lactate profile curves behave the way they do, it is necessary to step back from traditional sport-science interpretations and look at lactate from the perspective of modern biomedical sciences.

Much of the conceptual framework still used in lactate testing originated in the 1970s and 1980s, when lactate was widely considered a waste product of anaerobic metabolism [3].

Since then, the understanding of lactate metabolism has fundamentally changed.

Decades of research in biochemistry, physiology, and cellular metabolism have established that lactate is not a waste product.

Lactate is continuously produced — even at rest and at low exercise intensities — and, crucially, even under optimal oxygen availability [4–6]. At the same time, lactate is continuously oxidised as a fuel, primarily within working muscle.

This concept is often referred to as the lactate shuttle [4,5]. Glycolysis produces lactate, which is transported and oxidised within mitochondria, thereby linking glycolytic and oxidative metabolism. Lactate is therefore not a sign that aerobic metabolism has failed; it is one of the substrates aerobic metabolism actively uses.

From this perspective, a crucial implication follows.

Lactate concentration does not indicate whether metabolism is aerobic or anaerobic. Instead, lactate concentration reflects the balance between two concurrent processes:

Lactate concentration = lactate production − lactate clearance [6,7]

This is not a conceptual model or theoretical assumption. It is a quantitative, physiologically established relationship.

Lactate production is driven by glycolytic flux. Lactate clearance is closely linked to oxidative metabolism and oxygen uptake. Both processes occur simultaneously across all exercise intensities. Changes in lactate concentration therefore reflect changes in the balance between production and oxidation, not the presence or absence of oxygen.

At low intensities, lactate clearance can exceed production, resulting in stable or decreasing concentrations. As intensity increases, lactate production rises disproportionately and eventually exceeds clearance, leading to increasing lactate levels.

The key point is not that lactate rises.
The key point is why it rises.

Once lactate concentration is understood as an outcome, it can no longer be treated as a primary causal variable. It does not explain what limits performance or which physiological systems are adapting. It simply reflects where the system has arrived.

Confusing a result for a cause

Using lactate concentration as a primary control variable in training is a subtle but fundamental mistake — one that is difficult to detect precisely because “everyone does it this way.”

Consider medicine. A physician does not treat fever by targeting body temperature itself. Fever is an outcome — a signal that something else is occurring. Treatment targets the underlying cause, such as infection or inflammation. Temperature is monitored for assessment, not manipulated as the cure.

The same logic applies to engineering.

An engineer tasked with improving fuel efficiency does not optimise fuel consumption directly. Fuel consumption is an outcome of engine design, drivetrain efficiency, aerodynamics, rolling resistance, and many other factors. Engineers intervene at the level of these mechanisms and then observe fuel consumption to assess the result.

Yet in endurance training, lactate concentration is often treated as both assessment and control.

If lactate concentration is the outcome of multiple physiological processes — including oxidative capacity, glycolytic flux, substrate utilisation, economy, and body composition — then targeting lactate values alone provides no information about which of these processes should be changed.

Outcomes are informative.
But outcomes do not tell us what to change.

Using lactate concentration to unlock a holistic performance assessment

At this point, one thing is clear:

Lactate concentration is not useless.
But it is not what it has often been used for.

Instead of asking what a certain lactate value means, we can ask what creates it.

Lactate concentration results from two processes with fundamentally different physiological origins and kinetics: lactate production in glycolysis and lactate clearance via oxidative metabolism.

Across multiple workloads, the differing behaviours of these processes constrain the possible solutions, making it possible to infer how much lactate must be produced and oxidised to generate the observed concentrations.

In other words, lactate concentration can be decomposed into its components.

Glycolytic lactate production is a function of workload and maximal glycolytic flux (VLamax). Because workload is known during an incremental test, VLamax can be calculated.

The same logic applies to oxidative metabolism. Lactate clearance depends on oxygen uptake, which itself reflects workload and maximal aerobic capacity (VO₂max). As a result, lactate profile curves can be used to calculate both VLamax and VO₂max without additional testing and with gold-standard accuracy [8].

Because lactate is produced exclusively from carbohydrate metabolism, glycolytic flux is directly linked to carbohydrate utilisation. Knowing lactate production therefore provides information about carbohydrate oxidation. The remaining energy demand, not covered by carbohydrates, must be supplied by fat oxidation — allowing fat oxidation rates, FatMax, and maximal fat oxidation to be derived.

Together, this forms a coherent physiological chain described in the literature:

External performance determines energy demand.
Energy demand is met through oxidative and glycolytic metabolism.
Glycolytic flux determines lactate production.
Oxidative capacity determines lactate clearance.
The balance between both determines lactate concentration and substrate utilisation.

This structure is not speculative. It is grounded in established physiology and supported by decades of research. Interpreted this way, lactate profile testing moves from description toward understanding.

From understanding to application

Lactate testing does not need to be abandoned.
But it does need to be used differently.

When lactate concentration is embedded in a causal framework, the curve stops being the answer and becomes the starting point for asking better questions.

This shift does not change what is measured.
It changes what becomes possible.

How to go deeper

1. Learn the framework in practice (in person)

We offer intensive weekend workshops focused on applying physiology-based analysis in real performance diagnostics. 👉 https://inscyd.com/workshop/

2. Check if your testing qualifies for a 360° performance assessment

Not every lactate test provides the information required for this type of analysis.

👉 Check whether your testing qualifies to be transformed into a holistic 360° performance assessment.

For coaches and labs

For athletes

Book a demo: turn testing into a scalable, client-ready performance service

See how INSCYD helps you move from raw test data to a complete physiological assessment—so you can start testing faster, deliver clearer insights, and grow a repeatable service across athletes.

Start faster: run lactate or remote testing workflows without heavy lab setup—and get structured outputs in one platform.
Scale your service: automate parts of testing and reporting so assessments can run with less manual work.
Deliver clearer value: communicate progress and decisions with client-ready visuals, reports, and education assets that support confident action.

Renowned sports scientist and creator of INSCYD, has significantly influenced elite sports training with his expertise in exercise physiology and metabolic profiling. Sebastian has developed the first ever test to measure glycolytic power aka VLamax back in 2003. He has used his knowledge to some of the best world tour cyclists to win 9 world championship titles, Olympic medals and Tour de France victories.

Sebastian Weber

Founder of INSCYD & World-renowned Sport Scientist

References

Mader A et al. Sportarzt und Sportmedizin, 1976
Jamnick NA et al. PLoS One, 2018
Brooks GA. J Appl Physiol, 1986
Brooks GA. Physiol Rev, 2009
Brooks GA et al. J Physiol, 2022
Stanley WC et al. Am J Physiol, 1985
Poffé C et al. Front Sports Act Living, 2024
Podlogar T et al. J Sci Cycl, 2022

Explore Other Blog Posts

Article

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[Webinar] The “No Lactate Curve” – Lactate Testing

Igor Makarov — Fri, 23 Jan 2026 10:08:45 +0000

Webinar

The “No Lactate Curve” – Lactate Testing

If you do lactate testing, there is a very high chance you are underutilizing it.

Do you create a lactate profile and lactate- or threshold-based training zones for endurance sports?

If yes, you are missing out on most of the value this testing could provide.
And if you offer lactate testing as a commercial service, you may also be missing out on up to 3× the revenue you could generate.

Here’s why:

Lactate is now understood as a central regulator of energy metabolism (1).

It links glycolysis (anaerobic metabolism) to aerobic metabolism, influences substrate utilization, and allows the quantification of maximal glycolytic power (VLamax).

In other words, lactate testing can provide far more insight than it typically does today.

Conventional incremental exercise testing using lactate—most likely the type of test you already perform—can be used to derive:

VLamax – maximal glycolytic power
VO₂max – because lactate concentration is causally linked to it
Fat and carbohydrate oxidation rates, including MFO and FatMax
Training zones based on VO₂max, fat oxidation rates, and metabolic demands
Fueling strategies for training and racing

In this webinar, you will learn how and why conventional lactate testing can deliver all of this information.

Imagine turning your current lactate testing—with no additional equipment, time, or effort—into a full metabolic analysis.

If you want to understand why and how this works, sign up for the webinar.

Presenter

SEBASTIAN WEBER

Founder of INSCYD & World-renowned Sport Scientist

Want deeper insights? - Get 360° View of Your Athletes Performance with Detailed Metabolic Profile

The post [Webinar] The “No Lactate Curve” – Lactate Testing appeared first on INSCYD.

INSCYD 2025 in Review: Making Physiological Testing More Accessible, Profitable, Scalable, and Confident

Igor Makarov — Thu, 18 Dec 2025 11:46:06 +0000

Before we move into 2026, we want to look back at 2025 and summarize the year in one place. Here’s our 2025 wrap-up.

In 2025, we focused on three things: access, scale, and confidence. That meant lowering the barrier to start testing, enabling assessments that can run at scale to grow your business 24/7, and strengthening the scientific + educational foundation—so the insights you deliver are practical, reliable, and easier to act on.

At a glance, 2025 was about helping you:

Start testing easy and faster—and run it with less friction
Scale metabolic assessments with less manual work
Make improvements visible and easy to communicate to your clients
Reinforce confidence with science (and open the door for more research)
Apply insights with confidence through practical education

If your goal is to turn testing data into insights athletes value—and use that clarity to drive real performance gains and grow a stronger performance service—this wrap-up shows what’s now in place and what you can start using today.

Let’s begin.

1. INSCYD × CARDIOWORLD × NOVA: Lower the barrier to lactate testing

In 2025, we partnered with CARDIOWORLD (Europe) and NOVA (US) to make lactate testing easier and more profitable to run. By bringing the meter, consumables, and INSCYD software together in one place, coaches and labs can get started faster and run testing consistently.

NOVA is the direct manufacturer of the Nova Lactate Plus Meter, supplying athletes, coaches, and labs globally.

CARDIOWORLD is Europe’s exclusive importer of the Nova LACTATE PLUS analyzer and consumables, supporting athletes, coaches, and labs across Europe with fast access to lactate measurement technology.

What you can get:

Discount on lactate strips/products (up to 15%) — via NOVA directly in the US, and via CARDIOWORLD in the EU (Europe’s exclusive importer of the Nova LACTATE PLUS analyzer and consumables).
A free lactate meter option to get started fast (Nova LACTATE PLUS).
A simple “one-stop” setup — get everything needed to start or upgrade lactate testing, plus INSCYD software, in a single trusted place.

Why this matters in practice

Make lactate testing more profitable: Partnership pricing helps reduce the ongoing cost of strips, so regular testing can be delivered with healthier margins.
Turn Lactate Testing into lab-level performance assessment: INSCYD turns lactate values into a 360° physiological performance profile, including metrics like VO2max, VLamax, MLSS, LT1, and fat/carbohydrate combustion rates—so each test delivers a full picture of performance.
More reliable inputs: Nova LACTATE PLUS is scientifically validated for accuracy—so the lactate data you bring into INSCYD starts from a more reliable measurement base.

This partnership makes lactate testing easier to start and repeat—and more profitable to offer—by enabling coaches and labs to deliver a complete, lab-level performance assessment in INSCYD from a lactate test alone.

Want to benefit from the partnership? Book a free call with our performance team.

And learn more about what you can get just from lactate tests.

2. INSCYD × Velocity Integration: turn metabolic testing into a fully automated 24/7 revenue stream

We launched the INSCYD × Velocity integration to help cycling and triathlon coaches and labs move metabolic testing from a time-intensive, manual workflow to a fully automated, on-demand service—without losing the scientific depth of INSCYD analysis.

Velocity is a coach-centric virtual training platform built around video-based, coach-led training, including live instruction and group formats.

What the integration unlocks:

Testing that runs without you (24/7): Athletes execute tests anytime—results are delivered automatically to your INSCYD account, even outside your working hours.
On-demand, video-guided protocol: athletes can complete the test any time, following Sebastian Weber’s (Founder of INSCYD) video guidance inside Velocity to support consistent execution—without you needing to be present.
Automatic data transfer (Velocity → INSCYD): once the test is completed, athlete data flows automatically into INSCYD for analysis—removing manual steps and reducing the time between execution and results delivery.
Individual + group testing formats: Velocity supports group sessions so coaches can run multiple tests in the same environment.
The complete performance analysis output: the workflow is built with INSCYD’s scientifically validated Power Performance Decoder (PPD) to to generate a comprehensive metabolic profile from power data (including VO2max, VLamax, MLSS, LT1, fat + carbohydrate combustion rates, and more).

Why this matters in practice

Spend less time on execution and admin: automation reduces the need for protocol supervision and manual data processing—so you can focus on analysis, planning, and athlete consultation.
True on-demand testing: because execution doesn’t require you to be present live, athletes can test when it suits them—including outside office hours and across time zones.
Designed to be scalable and profitable: a way to scale metabolic testing as a repeatable service (including group formats).

Coaches who are not INSCYD users yet and are interested in implementing the INSCYD x VQ Velocity integration can start by scheduling a free demo with us.

INSCYD users can visit our Help Center article here to implement the integration immediately (please ensure you’re logged in to access the article)

If you’re an athlete interested in INSCYD testing, head over to Velocity, create your account, and go to the marketplace to select the INSCYD coaching group (coaches who can provide you with a full INSCYD metabolic profile)

3. 4-Day Hands-on Physiological Testing & Coach Education Camps: Sold Out Across 5 Locations Worldwide

Five stops. Five sell-outs. In 2025, we ran our 4-Day Coaching & Testing Camps, and every date filled up in Park City, Leuven, Frankfurt, Girona, and Manchester—a clear signal that coaches and performance labs are looking for more hands-on testing practice and more practical application, not just theory.

This wasn’t a “sit and listen” format. The camp focused on testing athletes, applying test data to training, running lactate and VO₂ testing in practical environments, and learning how to interpret metabolic data for better training decisions—working directly with athletes, sharpening testing skills, and bringing the learning back into everyday coaching and lab practice.

What participants gained

Hands-on learning with athletes and coaches in the field—including testing athletes, applying test data to training, and understanding individual athlete needs.

Practical experience in metabolic testing—conducting lactate and VO₂ testing at the running track and on the road (including practical sessions using COSMED K5).

Confidence in interpretation and application—learning how to translate metabolic data into training programs with high practical relevance.
Clearer physiological reasoning for training decisions—understanding mechanisms that inform when an athlete benefits more from low-intensity/high-volume work versus more intensive interval sessions.

The camp program included 10 sessions on core metabolism topics, 7 sessions on adaptation to training, 2 practical testing sessions (VO₂ + lactate sampling using COSMED K5), and 5 sessions on interpretation + application (zones, limiting factors, nutrition).

Because feedback was incredible in 2025, we’ll bring these camps back next year—with even more practical application. Stay tuned for the announcement.

4. Power-Performance Decoder Update: Lab-Level Performance Assessment Just From Power/Speed

We shipped a major update to our Power-Performance Decoder (PPD) testing technology—designed to make testing faster, more flexible, and more accurate, while adding built-in validation so you can trust results with less back-and-forth.

What PPD is: PPD is INSCYD’s remote performance test that turns power (cycling) or speed (running) into a structured physiological assessment—without requiring lactate sampling or measured VO₂ during the test.

With the update, running tests can also be executed using distance + time (and cycling with time + average power)—so you can run testing with a power meter, a GPS watch, or even a stopwatch when needed.

What’s new in PPD

Shorter, smarter protocols: PPD v3 uses two short sprints (~16–24s) plus two efforts between 3–8 minutes—with flexibility to choose durations that fit the athlete and environment.
Manual effort input: you can enter distance + time (running) or time + average power (cycling) without uploading a FIT file—useful for track-based or group testing where you just need a stopwatch and measured distance.
Auto-correction of effort selection: when you mark an effort, PPD can scan nearby data to find a more accurate segment—then lets you choose between your marked segment and the optimized one for consistency.
Traffic Light accuracy check: key outputs (VO₂max, VLamax, Critical Power/Speed) are flagged green / yellow / red so you can quickly see whether a test meets expected accuracy or should be reviewed/repeated.
No mandatory file upload: creating and completing a PPD test no longer requires a FIT file—you can complete tests via manual entry when needed.

Why this matters in practice

Less time spent testing, more time coaching: the updated protocol removes the long effort and keeps the testing workflow shorter.
More flexibility in the real world: you can adapt efforts to terrain, facilities, and athlete preferences without being forced into one rigid duration.
Higher-confidence results: the update is designed to increase accuracy and adds effort correction + validation checks to reduce preventable errors.

PPD is built to deliver a remote, lab-level style performance assessment from power or speed—so coaches and labs can assess athletes without requiring lactate sampling or VO₂ measurement during the test.

With new PPD, testing is shorter, more flexible (including manual inputs), and easier to validate with built-in accuracy checks.

Want to see what the workflow looks like and how the outputs show up in INSCYD? Book a demo.

5. Performance Development Chart Update: Fully Customizable Graphs to Show Progress Over Time

We updated the Performance Development Chart so you can fully customize how you visualize an athlete’s development—easy to compare, and built to communicate progress clearly to athletes and stakeholders.

Performance Development Chart let you track and present development across key performance and physiological metrics—so you can go beyond “single-number” tracking and tell a clearer story of how an athlete is progressing over time.

What’s new (and what it enables)

Compare related metrics side-by-side with time-scaled or evenly-spaced views, and create up to 3 personalized charts per sport.
Multi-metric charts with up to 3 Y-axes—so you can track how different systems develop together (e.g., VO₂max and VLamax, or threshold markers alongside power outputs).
Choose what matters (nothing more, nothing less): select from INSCYD’s physiological metric library, group complementary metrics, and use sport-specific configurations.
Make it usable in real workflows: interactive navigation (hover + zoom) and PDF report integration so your customized charts can show up directly in athlete reports.

Why this matters in practice

Clearer progress stories = higher athlete confidence. When athletes can see and understand what’s changing, they stay more engaged in the process.
Better decisions, with more context. Multi-metric views help you interpret development patterns that single-metric tracking can miss.

Performance Development Chart help you turn physiological development into visuals that are personalized, explainable, and report-ready—so your athletes don’t just see “numbers,” they understand their progression.

Get a guided walkthrough of the Performance Development Chart and discover fully custom-branded reports.

6. Research & scientific commitment: 2 Peer-reviewed Publications + INSCYD for Research Access

In 2025, INSCYD’s scientific rigor showed up in two tangible ways:

peer-reviewed research published using INSCYD in applied performance contexts.
dedicated INSCYD for Research inquiry path—so universities and research groups can explore using INSCYD in their studies with clearer expectations and documentation.

Published research in 2025

1. Buffer capacity: reliability of a non-invasive approach

Published in Frontiers in Sports and Active Living, this pilot study tested the reliability of estimating buffer capacity from capillary blood lactate and pH measured before/after exhaustive cycling. Post-exercise inputs showed good-to-excellent reliability, while pre-exercise inputs were poor, and the authors note validity vs gold-standard methods still needs further study

2.Super-sprint triathlon: physiological predictors in Junior & U23 athletes

The study built physiological profiles from swim/run testing and applied them to predict super-sprint triathlon performance, identifying metabolic steady-state markers as key predictors in their model. The metabolic profile calculations were performed using INSCYDю

What we launched for the research community: INSCYD for Research

Also we introduced INSCYD for Research—a dedicated way for universities, research groups, and scientific teams to request access to INSCYD for academic and applied studies.

It’s designed to make it easier to evaluate fit upfront by clearly outlining what researchers can expect: scientifically validated analytical software, transparent methodological documentation to support reproducible protocols, and support for integrating INSCYD into study design and data workflows (with optional scientific guidance).

What research teams can get:

Access to scientifically validated analytical software for academic/applied research
Transparent methodological documentation to support research protocols
Support integrating INSCYD into study design + data workflows (with optional scientific guidance)

"Our goal is to enable researchers to run high-quality, reproducible studies without barriers created by equipment constraints or methodological capacity."

Sebastian Weber

Founder of INSCYD

How collaboration works (high level):

submit inquiry
scientific team review & follow-up
align scope and get access to the functions/analyses needed.

7. YouTube in 2025: Consistent Video Education Content—driven by what coaches ask for

In 2025, we started publishing educational videos consistently on YouTube—and the response from the community has been a big part of why we’re continuing. Our goal was simple—make key performance concepts easier to learn in a format you can consume quickly, revisit when needed, and share with athletes or colleagues.

A big thank you to everyone who watched, liked, and especially commented. Your questions and feedback help us choose what to cover next and where we should go deeper. We’ll keep publishing regularly in 2026—and we’re already preparing a few special releases.

If you don’t want to miss them, subscribe to the channel.

Three of the most popular videos from 2025:

Zone 2 Training Explained: Best Intensity for FatMax & Endurance

A practical breakdown of why “Zone 2 = X% of FTP” can miss the mark—and why dialing training to fat + carbohydrate combustion is what makes low-intensity work truly individual.

Lactate Clearance Explained: How to Recover Faster & Improve Performance

Why recovery isn’t just “easy” or “hard”: it’s duration + intensity—and lactate clearance helps inform both interval sessions and racing demands.

Why Your VO2max Interval Training Isn’t Working (And How to Fix It)

If you set VO2max intervals as a fixed % of threshold/FTP, different athletes can end up training at very different % of VO2max—so results stagnate even when the workout “looks right” on paper.

Subscribe to our YouTube to stay up to date with new videos (and what’s coming in 2026).

8. Webinar Library: expert sessions you can apply immediately (and our top webinars of 2025)

In 2025, we launched our Webinar Library—a dedicated place where you can explore our expert-led webinars and quickly find the sessions most relevant to you, including filters by sport (cycling, running, triathlon, skiing, and more).

What you’ll find inside

Actionable, coach-ready frameworks (not just theory) you can implement right away.
Sessions grounded in high-performance practice, using scientific methods referenced in the materials as being used in elite environments (Tour de France teams, national federations, Olympic programs).

Top webinars in 2025:

Unf*ck Your Lactate Testing!

Webinar

Built for coaches and labs who already test (or want to start), but feel their protocols or interpretation are “fuzzy.” It breaks down protocol choices, lactate dynamics, common blind spots (including body composition + meter accuracy), and how to extract more than “thresholds” from lactate testing.

Watch the webinar.

Journey to Paralympic Gold: USA Triathlon Coach Parker Spencer’s Strategy

Webinar

A case-based session featuring USA Triathlon coach Parker Spencer and Paralympic gold medalist Chris Hammer, covering how physiology and performance data informed decisions across swim, bike, and run—plus how metabolic profiling with INSCYD supported the training strategy.

Watch the webinar.

Master Race-Day Pacing and Nutrition for Endurance Athletes

Webinar

A practical methodology for building individualized pacing + fueling strategies using metabolic profiling—covering race-demand analysis, converting speed to power requirements, and calculating carbohydrate balance for long events. The webinar materials describe a prediction model with 99% accuracy in race time forecasting (as presented in the webinar).

Watch the webinar.

9. Popular blog reads: moving beyond %FTP for Zone 2, Threshold, and HIIT

In 2025, some of our most-read educational posts focused on a core coaching problem: using one FTP number to prescribe multiple training goals—and why that often leads to mismatched intensities, stalled progress, and unclear adaptations.

The top topics coaches dug into:

The Limitations of FTP-Based Zone 2 Training: Why It Fails Endurance Athletes

Article

The Limitations of FTP-Based Zone

Why “% of FTP = Zone 2” can break down across athlete profiles—and why two athletes with the same FTP can require very different Zone 2 intensities to get the same adaptation.

Read the full blog post.

Why You Don’t get faster Threshold Intervals— And How Physiology Based Training can Fix it for You

Article

Why threshold intervals can plateau when “threshold” is treated as a single number—and how a physiology-first approach changes what you prescribe and why.

Read the full blog post.

HIIT Training: Intensity based on %FTP and its shortcoming

Article

Why prescribing VO₂max/Zone 5 intervals as a fixed % of FTP often produces sub-par VO₂max training effects for some athletes—or unwanted glycolytic activation for others. The post walks through a practical example of two athletes with similar FTP but very different VO₂max/VLamax profiles, and argues for prescribing HIIT based on VO₂max engagement (~90%+) while keeping glycolytic contribution (VLamax utilization) under control.

Read the full blog post.

Closing: What’s ready for you going into 2026

2025 was about making physiological testing access, scale, and confidence—so you can deliver clearer insights to athletes and build a stronger performance service around them.

If you’re heading into 2026 with any of these goals, the building blocks are now in place:

Start faster: get a practical lactate-testing setup (meter + consumables + software) and turn those tests into a complete performance assessment inside INSCYD.
Scale without adding hours: offer remote metabolic testing that can run without live supervision—and reduce manual admin through automation.
Show progress clearly: turn development into report-ready visuals that help athletes understand what’s changing and stay bought into the process.
Reinforce confidence: lean on published research use cases, and a clearer pathway for research teams to access INSCYD.
Keep learning: apply concepts through webinars, videos, and educational articles—built to support real coaching decisions.

If you want to see how this looks in your workflow, book a demo. We’ll walk you through the best starting point for your setup (lactate testing, PPD remote testing, or automation via Velocity), and show how INSCYD turns raw data into a full lab-level performance assessment + branded reports.

And if you want to stay close to what’s coming next, subscribe to our newsletter, follow us on YouTube, and keep an eye on new webinars—we’re continuing the education push in 2026.

Book a demo: turn testing into a scalable, client-ready performance service

See how INSCYD helps you move from raw test data to a complete physiological assessment—so you can start testing faster, deliver clearer insights, and grow a repeatable service across athletes.

Start faster: run lactate or remote testing workflows without heavy lab setup—and get structured outputs in one platform.
Scale your service: automate parts of testing and reporting so assessments can run with less manual work.
Deliver clearer value: communicate progress and decisions with client-ready visuals, reports, and education assets that support confident action.

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Article

The post INSCYD 2025 in Review: Making Physiological Testing More Accessible, Profitable, Scalable, and Confident appeared first on INSCYD.

INSCYD

[Webinar] Implement metabolic performance testing into a profitable service

Turn metabolic performance testing into a profitable service

About the Webinar:

Is this me?

Built for two kinds of operator

If you are adding testing:

If you already test:

What you'll learn

THE KEY PARTS OF THE BUSINESS, IN ONE WEBINAR

Workflow

Upsell & Retesting

Protocols, Hardware & Investment

ROI & Monetization

What you'll walk away with

book a free demo

[Webinar] From raw test data to training plan: Join This Live Metabolic Profiling Session

FROM RAW TEST DATA TO TRAINING PLAN: JOIN THIS LIVE METABOLIC PROFILING SESSION

ABOUT THIS SESSION

Want to Go Deeper?

[Webinar] How to Create a Full Metabolic Profile of Any Athlete — And Use It to Design Training That Actually Works

How to Create a Full Metabolic Profile of Any Athlete — And Use It to Design Training That Actually Works

In this webinar, you'll discover:

Want to Go Deeper?

HIIT vs. Norwegian 4×4: The Forgotten Metric That Decides Your Interval Training Outcome (Part 2)

Key Takeaways

Table of Contents

What We Discovered in Part 1

Missing the Bigger Picture: Why Focusing Only on VO2max Is Not Enough

Hiding in Plain Sight: The Glycolytic Stimulus You Did Not Plan For

Why a Stronger Glycolytic System Can Hurt Endurance Performance

How to Measure the Glycolytic Training Stimulus: %VLamax Utilization

The 10% VLamax Benchmark

Norwegian 4x4: Glycolytic Training Stimulus by Athlete Type

HIIT 40/20: Where the Glycolytic Picture Changes Drastically

Why Some Athletes Stagnate or Regress: The Two Sides of the Coin

The Solution: Set Intensity Based on VLamax, Set Duration Based on VO2

Controlling the Anaerobic Side

Using Duration Instead of Intensity to Drive the Aerobic Stimulus

How to Apply This in Practice

How the INSCYD Training Zones Builder Makes This Possible

Full Circle: Truly Individualized Interval Training

Frequently Asked Questions

References

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