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

HIIT vs Norwegian 4x4 Part 2: How VLamax determines the glycolytic training stimulus during interval training for different athlete profiles

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).

Table of Contents

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.

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.

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:

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.

  1. 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.
  2. 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.
  3. Evaluate the resulting %VO2max at each duration to find the optimal balance.
  4. 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:

  1. 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.
  2. Set the target utilization. Define the threshold for the chosen metric (e.g., VLamax utilization at 10%).
  3. 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.

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

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.

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.

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.

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.

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.

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 - Founder Of INSCYD
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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