Interval Training Fails

For nearly two decades, many endurance coaches have relied on Functional Threshold Power (FTP) as the main tool for setting training intensities and measuring progress. This approach, originally intended to approximate the maximal lactate steady state (anaerobic threshold), caught on largely because establishing FTP doesn’t require laboratory-based lactate or gas-exchange testing—just a straightforward field assessment. Its simplicity and accessibility quickly made FTP the backbone of numerous training plans.

A particularly influential model, as described 2019 in the book: “Training and Racing with a Powermeter” from Allen, Coggan, and McGregor divides training intensities into multiple “zones,” keyed to percentages of FTP:

  • Zone 1 – Recovery (<55% FTP)
  • Zone 2 – Endurance (56–75% FTP)
  • Zone 3 – Tempo (76–90% FTP)
  • Zone 4 – Threshold (91–110% FTP)
  • Zone 5 – VO2max (106–120% FTP)
  • Zone 6 – Anaerobic Capacity (121–150% FTP)
  • Zone 7 – Neuromuscular Power (>150% FTP)
Allen, Coggan & McGregor's FTP Training Zones

In reality athletes – no matter if they have the same similar or very different FTP values – will differ vastly in their metabolic responses and physiological performance values when working above or below FTP.  

In other words, tying all training intensities and goals to one threshold power (or speed= figure is a “one-size-fits-all” system that neglects the reality of individual physiology—ultimately leading to suboptimal or mismatched training and in most cases: plateauing performance development. Sounds familiar?

History of FTP: A Practical Stand-In for Lab Testing

Back in the 1980s, incremental lactate tests were the gold standard for pinpointing an athlete’s anaerobic threshold (often called the lactate steady state). This lab-based method gave coaches a direct look at endurance capabilities, making it a powerful prescription tool. 

However, as power meters became widely available—particularly in places with fewer lactate-testing facilities—athletes and coaches needed an easier alternative.

That’s where Functional Threshold Power (FTP) stepped in as a field-friendly substitute for full lab work. Its very name—“functional”—signaled that it wasn’t a perfect physiological measurement but rather an estimate grounded in real-world performance. 

For most athletes, the precise accuracy of FTP versus lactate threshold mattered less than its practicality. And indeed, for a practical application FTP is in most cases – at least for non professional athletes – a good and accurate measure to approximate maximal lactate steady state. 

The problem isn’t the accuracy of the FTP value itself, but deriving training zones from it…

The Core Issue: “Cookie-Cutter” Training Zones

An FTP value is nice to know for a coach or an athlete. But the real question people want to answer is: “how should I be training?”. And this is where FTP based zone provided a quick and easy answer – but unfortunately a too easy answer. This is because: an FTP value is individual to an athlete, but training zones are derived using fixed percentages of this FTP value. And those fixed percentages are the issue:

Using fixed percentages of FTP implies that every athlete responds the same way when working at a percentage of their FTP—especially in the threshold intervals at or slightly above threshold (roughly 90–110% of FTP). 

In reality, athletes have very different physiological responses when exercising above FTP, even if their FTP values look similar on paper. This mismatch can lead to suboptimal training for those who need a slightly different workload to truly improve threshold performance.

To illustrate the point, consider two example athletes with fairly typical—but distinctly individual—physiological profiles:

Comparing FTP of 2 athletes
Metric
Athlete A
Athlete B
Body Weight
70 kg
70 kg
FTP
270W
260W
VO2max
66 mL/kg/min
54 mL/kg/min
VLamax
0.75 mmol/L/s
0.30 mmol/L/s

The Purpose of Threshold Training

Threshold training in endurance sports is primarily aimed at increasing MLSS (Maximal Lactate Steady State) and thereby improving threshold power. One of the key mechanisms behind this adaptation is an improvement in VO2max, as the intensity in this zone is high enough to utilize a significant fraction of VO2max—one of the main drivers of aerobic capacity improvements. However, it is still low enough to be sustained for several minutes, allowing for sufficient training duration to reinforce these adaptations.

A crucial aspect of threshold training is the involvement of fast-twitch and intermediate muscle fibers. While these fibers are typically associated with higher-intensity efforts, threshold intervals represent the lowest intensity at which a significant number of them are recruited. The aim is to stimulate aerobic adaptations in these fibers, increasing their oxidative capacity without excessively triggering adaptations of the glycolytic system.

This is where the fundamental assumption behind FTP-based threshold training comes into play—it is perceived as a training intensity with minimal anaerobic metabolism involvement. However, if the intensity is miscalibrated and the activation of the anaerobic glycolytic pathway is too high, it will provoke adaptations in the glycolytic metabolism. This presents a risk:

  • An increase in glycolytic capacity means greater reliance on carbohydrate metabolism and higher lactate production at submaximal intensities.
  • More lactate production means a lower MLSS, which is the opposite of what threshold training is intended to achieve.

Therefore, while threshold intervals are aimed at pushing the lactate balance point right and upward, a too high glycolytic activation during the training could shift this balance in the wrong direction— limiting improvements in MLSS rather than enhancing them. Or in other words: the possible improvements in aerobic capacity are eaten up by an increased lactate production.

How FTP-Based Threshold Training Affects Our Two Athletes

Now, let’s examine our two example athletes: what happens when Athlete A and Athlete B execute an 8-minute interval at 108% of their FTP—a typical threshold training designed to improve threshold power. The expectation is that this training should drive adaptations that improve MLSS/FTP, mainly by increasing VO2max while keeping glycolytic contribution low enough to avoid raising VLamax.

Athlete A: 291W (108% FTP)

  • VO2max Utilization: 71% → Moderate aerobic stimulus, but not ideal for VO2max adaptation.
  • VLamax Utilization: 4% → Strong likelihood of decreasing or maintaining VLamax.
  • Lactate Concentration After 8 Min: 6.1 mmol/L
Athlete A: Threshold Training

For Athlete A, the prescribed FTP-based Zone 4 intensity results in a suboptimal aerobic stimulus. His 71% VO2max utilization is within an endurance training range, but it is not efficient to drive the strong VO2max adaptations that are expected from this type of session. As nicely demonstrated in the scientific literature, a higher VO2max engagement would be needed to make meaningful improvements in his aerobic capacity.

However, the most critical factor here is VLamax utilization—only 4%. This suggests that his glycolytic system is barely being taxed, meaning his VLamax is likely to decrease over time. This isn’t necessarily a bad outcome—reducing VLamax can be a valid strategy for improving endurance performance. A lower VLamax means less lactate production at submaximal intensities, which could lead to a higher MLSS and FTP.

But here’s the catch:

If Athlete A believes that this training is increasing his FTP by improving VO2max, he is likely mistaken. The real reason his MLSS might improve is because his glycolytic capacity is being suppressed—not because he is pushing his aerobic ceiling higher. Without separately measuring VO2max and VLamax, he remains completely in the dark about what is actually driving his improvements.

This leads to a major risk: If Athlete A wants to maintain anaerobic power (e.g., for sprinting, accelerations, or breakaways), this training is counterproductive. His sprint power will likely decline as his glycolytic system downregulates. If his goal is to improve MLSS while keeping sprint ability intact, this training is setting him on a path to failure—because it is fundamentally not achieving the adaptation he expects. Plus, as described above, his adaptations of the aerobic system  – aka VO2max – are subpar. 

Athlete B: 281W (108% FTP)

  • VO2max Utilization: 83% → Strong aerobic stimulus, well-positioned for VO2max gains.
  • VLamax Utilization: 11% → Right on the threshold where VLamax could start increasing.
  • Lactate Concentration After 8 Min: 7.1 mmol/L
Athlete B: Threshold Training

For Athlete B, the FTP-based threshold training prescription appears to work much better from an aerobic standpoint. His 83% VO2max utilization ensures he is getting a much better VO2max adaptation, which will positively impact MLSS.

However, the problem lies in his 11% VLamax utilization. This is right on the edge of stimulating positive glycolytic adaptations. If the intensity is slightly too high, it risks increasing his VLamax, which could lead to greater lactate production at submaximal intensities— lowering his MLSS instead of improving it.

What Should Athlete B Do?

The solution for Athlete B is simple: reduce intensity slightly to stay below the threshold where glycolytic adaptation might occur. If his goal is to improve MLSS without increasing anaerobic capacity, he should lower the target wattage slightly to ensure he remains under 10% VLamax utilization. This would preserve the aerobic gains while avoiding unwanted glycolytic adaptation.

What About Lactate Concentration?

At first glance, the lactate concentrations of Athlete A (6.1 mmol/L) and Athlete B (7.1 mmol/L) seem quite similar. One might assume that because both athletes are at comparable lactate levels, they are experiencing similar metabolic stress.

However, this is not the case. The lactate concentration alone does not provide meaningful insight into whether the session is driving the desired adaptations.

  • Athlete A has low VLamax activation (4%), meaning his lactate is mostly a result of a slow but continuous lactate accumulation, without stimulating the glycolytic system.
  • Athlete B has much higher VLamax activation (11%), meaning his lactate concentration reflects a greater glycolytic contribution, which could lead to unintended increases in anaerobic capacity.

The Problem? FTP is A Power Metric, Not a Complete Physiological Measure

FTP-based Training vs. Physiology-based Threshold Training

Looking at how differently two athletes can respond to the same threshold training reveals the deeper issue with FTP-based zones. Although VO2max and VLamax are crucial considerations, the real problem is simpler: FTP itself isn’t an actual metabolic capacity—rather, it’s just an output an athlete can sustain for a set duration. Relying on a fixed percentage of FTP assumes that one number accurately reflects the right physiological stimulus for everyone.

Yet when training for threshold gains, we’re not really trying to boost FTP for its own sake. Instead, we aim to:

  • Improve aerobic performance by tapping into high percentages of VO2max
  • Manage glycolytic power to keep lactate production in check
  • Enhance MLSS (maximal lactate steady state), which is the true “threshold” intensity

By prescribing all threshold work purely from FTP, we’re not targeting these deeper physiological levers. Two athletes with the same FTP can wind up with very different metabolic responses—some under-training, others overreaching—because FTP doesn’t directly show how they use oxygen or produce lactate. 

Ultimately, that’s why FTP-based threshold zones often miss the mark and fail to deliver the desired adaptation.

This isn’t an isolated issue with threshold training—it’s part of a much broader pattern. We’ve already demonstrated how FTP-based training zones also fail in Zone 2, where using fixed percentages leads to missed adaptations or unintended outcomes. If you want to see how these flaws play out in other crucial intensity zones, be sure to check out – The Limitations of FTP-Based Zone 2 Training

Want a quick, clear explanation of why a training zone based on FTP can backfire for Zone 2? Watch our short video below.

A Better Approach: Align Training with Actual Physiology

If FTP doesn’t accurately anchor threshold intensity, where should you turn? 

Rather than working off a single average power value, it makes more sense to pinpoint the specific metabolic demands of an athlete. In other words, look beyond FTP and consider how athlete’s VO2max (aerobic capacity) and VLamax (glycolytic power) actually interact at higher intensities.

Threshold Training → Prescribe Based on Glycolytic Power Control

  • Instead of setting Threshold Training as 108% of FTP, we determine the intensity that results in 8% VLamax utilization.
  • This ensures that glycolytic power stays in check while maximizing the percentage of VO2max engagement.
  • Now we are not guessing—we are directly targeting the physiological adaptations we want

By focusing on these core physiological markers, you can set training intensities that truly challenge and develop your athlete’s lactate steady state. Instead of extrapolating percentages from one number, you determine where an athlete’s aerobic system is driven strongly enough to improve but glycolytic contribution stays in check. This ensures that threshold training targets the right adaptations—raising MLSS in a way that matches the athlete’s unique physiology.

Bringing Theory to Reality: Implementing Physiology-Driven Threshold Training

We’ve seen that a static percentage of FTP misses the mark for threshold training because it ignores the actual metabolic systems that truly drive performance. The logical step is to replace broad assumptions with physiological data. 

But how can this actually be done in practice?

That’s where INSCYD can be transformative. It provides a 360° view of each athlete’s performance including aerobic power, glycolytic capacity, and lactate dynamics, among other metrics. Armed with this information, labs and coaches can set threshold intervals that precisely match the desired metabolic challenge, rather than relying on a single power figure.

In practice, you might discover that 85% of VO2max at a certain wattage is optimal for one athlete, while another needs to back off slightly to keep VLamax in check. Each training can then be calibrated to push MLSS higher without inadvertently shifting too much toward anaerobic development. The result? Threshold training that actually aligns with the athlete’s physiology—making every session count toward meaningful, long-term progress.

INSCYD Solves the Problem

INSCYD equips you with a detailed metabolic profile—all from straightforward field tests using simple power-based (cycling) or GPS-based (running). You can also use field- or lab-based lactate testing to collect data if it’s available.

This eliminates the need for a full lab setup while still capturing data that goes far beyond a single power metric. 

And get metrics such as:

… and many more metabolic insights.

These insights aren’t just analytical—they guide practical adjustments to training prescriptions. With INSCYD, you don’t have to rely on a blanket percentage of FTP. Instead, you can:

By replacing guesswork with measurable data, INSCYD solves the pitfalls of traditional “one-size-fits-all” zones. Athletes and coaches can be confident that every threshold interval is purpose-built for the individual’s physiology—maximizing gains while avoiding unintended setbacks.

Train Smarter, Not Harder

For years, many athletes and coaches have clung to FTP-based zones, unaware that these standardized percentages overlook key aspects of physiology. INSCYD bridges that gap by mapping an athlete’s complete metabolic profile and letting you set training zones according to real physiological responses—resulting in more precise, individualized, and impactful workouts.

This marks the evolution of endurance training: leaving behind one-size-fits-all methods in favor of science-backed, data-driven optimization. 

The choice is simple: Stay with vague FTP assumptions, or train with pinpoint accuracy using INSCYD.

Ready to begin? If you’re not yet using INSCYD, book a free demo to take the first step toward integrating physiology-based training into your performance testing and coaching.

And if you’re an athlete, find an INSCYD-certified coach or lab to get your Recovery Index.

Get 360° View of Your Athletes Performance with Detailed Metabolic Profile at Your Fingertips

Stop guessing training intensity. Start using real physiological data to individualize your athlete’s training and drive consistent progress.

  • Get everything in one test — revealing the full physiological breakdown that truly drives endurance success.
  • Use the same protocols, same equipment, same process—just get 10× more insights without disrupting how you work.
  • Built on deterministic models and validated through peer-reviewed literature, our approach ensures the highest accuracy and reliability.
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