For years, Functional Threshold Power (FTP) has served as the default method for setting training intensities in endurance sports. Appealing for its simplicity, FTP is derived from a relatively easy field test—bypassing complex lab procedures. Over time, widely adopted training frameworks tied specific “zones” to percentages of this single metric. One such framework defines Zone 5, the so-called “VO2max Training Zone”, at roughly 106–120% of FTP, intending to provoke adaptations that boost maximal oxygen uptake.

However, FTP doesn’t take each athlete’s unique metabolic profile into account . Athletes with similar FTP values can differ significantly in their aerobic capacity. And athletes with similar aerobic capacity can have drastically different FTP values. The result? A “cookie-cutter” training intensity for your HIT intervals that in almost all cases, deliver sub-par training effects for VO2max development. This basically means: the scarce training time is not well spent and performance might plateau and not increase. Sounds familiar?

Before diving deeper, here’s a quick video breakdown of the main issue with using FTP to set Zone 5 (VO2max) training intensity. This explanation shows why two athletes with similar FTPs might respond completely differently to the same high-intensity intervals—and why relying solely on FTP can stall your progress.

How FTP Emerged

During the 1980s, lactate profile testing became the go-to method for pinpointing an athlete’s anaerobic threshold, or maximal lactate steady state (MLSS). This provided clear insight into endurance capabilities, but it required specialized lab equipment. As power meters spread—especially in regions without easy access to lab testing—coaches needed a simpler solution.

Enter Functional Threshold Power (FTP), a performance-based proxy for the popular anaerobic threshold. By replacing direct lactate measurements with a field test, FTP made it easy to roughly estimate the popular anaerobic threshold. Yet the term “functional” hints that this power output isn’t a pure physiological marker—just an approximation.

For most athletes, the real question isn’t whether FTP truly mirrors lactate threshold, but whether a single FTP number can capture all the metabolic nuances that drive high-intensity performance.

The Real Problem: “Cookie-Cutter” High-Intensity Zones (Zone 5)

Using fixed percentages of FTP implies that every athlete responds the same way when working at intensities well above their FTP. However in reality, individuals have their VO2max at various fractions of their maximum aerobic capacity (VO2max). 

And to make it even more complex: this fractional utilization is a moving target, as it can be changed by training (even without the intention to change it).  By relying solely on a standardized fraction of FTP to define “VO2max intervals,” athletes and coaches risk overlooking the real physiological variance among athletes.

To illustrate, let’s look at two typical amateur endurance athletes. Although they share similar body weight and comparable FTP values, they exhibit distinct aerobic (VO2max) and glycolytic (VLamax) 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

Let’s see what FTP based “VO2max training” does to them…

The Purpose of VO2max (Zone 5) – HIIT Intervals

The Purpose of VO2max (Zone 5) - HIIT Intervals

Zone 5, commonly known as VO2max training, consists of short, high-intensity intervals with incomplete recovery. The primary goal is to increase VO2max, which is one of the most critical determinants of endurance performance. In fact, many coaches and athletes refer to these sessions simply as VO2max intervals, highlighting their main purpose.

To effectively stimulate VO2max adaptation, intensity must be high enough to push oxygen uptake above 90% of an athlete’s VO2max. This ensures that the cardiovascular and muscular systems are operating near their limits, forcing adaptations that increase maximal oxygen delivery and utilization. The typical 4-minute interval at 125% FTP is a well-established prescription designed to achieve this.

However, just as in Threshold Training, VLamax must be considered. While Vo2max training naturally involves a significant glycolytic contribution, the key is to prevent excessive activation of this anaerobic metabolism—especially if the goal is to improve endurance rather than increase sprint power. If VLamax utilization exceeds 20%—or ideally, stays below 10%—this session is more likely to serve its intended purpose of boosting VO2max without increasing glycolytic capacity.

In short, the ideal HIIT intervals should:

  • Push VO2max utilization above 90% for a strong aerobic stimulus.
  • Keep VLamax utilization below 10% to prevent unwanted glycolytic adaptations.
  • Be short enough to sustain high-intensity efforts but long enough to fully engage aerobic capacity.

This sets the stage for analyzing how Athlete A and Athlete B actually respond to FTP-based HIIT training.

How FTP-Based HIIT Intervals Affect Our Two Example Athletes

Now, let’s examine what happens when Athlete A and Athlete B perform a 4-minute interval at 125% of FTP—a common high-intensity interval (HIIT) prescribed to increase VO2max. 

The goal of this session is clear:

  • Drive VO2max utilization above 90% to stimulate aerobic capacity improvements.
  • Keep VLamax utilization below 10% to avoid increasing glycolytic power unless sprint power development is a goal.

Athlete A: 338W (4min @ 125% FTP)

  • VO2max Utilization: 79% → Well below the 90% target for VO2max adaptation.
  • VLamax Utilization: 7% → No significant risk of increasing glycolytic power.
Athlete A: HIIT Trining Zones

For Athlete A, this FTP-based prescription completely fails to deliver the intended VO2max adaptation. With an average VO2max utilization of only 79% during the 4min effort, he is simply not pushing his aerobic system hard enough to maximize the training effect in the precious time the athlete invests. This means that despite the session being classified as a “VO2max interval”, it fails to engage the key adaptation mechanism it was designed for.

On the positive side, his VLamax utilization is only 7%, meaning that he is not at risk of increasing glycolytic power. However, this also suggests that the training intensity is too low to recruit the necessary fast-twitch muscle fibers and stress his oxygen delivery system sufficiently.

What should Athlete A do?

He needs to increase intensity beyond 125% FTP to push his VO2max utilization closer to 90% and actually benefit from this type of training. Otherwise, he is spending time in an intensity zone that isn’t delivering the expected adaptations.

Athlete B: 90% VO2max, 21% VLamax (4min @125% FTP)

  • VO2max Utilization: 90% → Hits the target for VO2max training.
  • VLamax Utilization: 21% → Exceeds the safe threshold, making this a high-risk session for increasing glycolytic power.
Athlete B: HIIT Trining Zones

For Athlete B, the intensity successfully engages VO2max, reaching the 90% utilization level (average for the full 4 minutes effort). If the goal was purely to increase VO2max, then this interval would be very effective.

However, there is a major problem:

  • His VLamax utilization is 21%, which is almost guaranteed to increase glycolytic power.
  • This means that, rather than simply improving VO2max, this session is also likely stimulating unwanted glycolytic adaptations, leading to higher lactate production at submaximal intensities, and therefore decreased threshold power instead of increased endurance performance.

What Should Athlete B Do?

If his goal is to improve VO2max without increasing glycolytic power, this type of interval is simply the wrong tool. Instead of HIIT training, he would be better off focusing on Zone 4 – aka threshold training, which still engages VO2max improvements but without excessive glycolytic activation.

Why FTP Alone Can Undermine VO2max Gains

Relying solely on FTP to set high-intensity zones reveals a deeper flaw: FTP isn’t a physiological entity by itself – it doesn’t represent a measure of a metabolic pathway. It’s merely a power output that an athlete can sustain for a certain length of time, which doesn’t necessarily map to how their body uses oxygen or produces lactate.

If the real objective is to boost VO2max—or to control the glycolytic contribution that can derail endurance performance—then focusing on FTP alone will in almost all cases miss the mark of the most effective way to spend precious training time. After all, increasing FTP can only be accomplished by training the two underlying physiological system that create it:

  • Maximize aerobic power (drive VO2max higher)
  • Manage lactate production by controlling glycolytic (VLamax) activation

Prescribing HIIT intervals or “VO2max-intervals” off an arbitrary fraction of FTP ignores these distinct metabolic realities. Two athletes with identical FTP values may need significantly different intensities to reach a true 90%+ VO₂max effort—or to avoid pushing glycolysis too far. By treating FTP as a catch-all indicator, we risk misaligning high-intensity sessions and compromising the very adaptations (stronger VO₂max, better lactate management) those workouts are supposed to deliver.

A Better Way: Base High-Intensity Work on Real Physiology

If FTP isn’t the best anchor for High Intensity Interval Training, what is? Rather than building off one power metric, it’s far more accurate to key training directly to the physiological systems you want to develop—especially VO2max and glycolytic power (VLamax).

HIIT Training → Prescribe Based on VO2max Utilization

Instead of defaulting to “~125% of FTP” for VO2max work, target roughly 90% of the athlete’s actual VO2max capacity—since science indicates that’s where you’ll get the most potent aerobic stimulus. From there, monitor glycolytic response (VLamax) to ensure you don’t push lactate production too far. If VLamax gets too high, you can adjust the workout structure or intensity to maintain the right balance.

By shifting your approach—from a single percentage of FTP to a data-driven strategy—you can deliver exactly the training load needed to raise VO2max without unintentionally overloading the anaerobic system.

The Huge Learning Athletes Would Never Get From FTP-Based Training

Consider what happens when Athlete A and Athlete B each tailor HIIT Training according to VO2max and glycolytic data, rather than a rigid percentage of FTP:

  • Athlete A sets a precise intensity that pushes him to the correct level of oxygen uptake. He reaps optimal VO2max gains without overshooting into excessive anaerobic activation—an ideal balance for building high-end aerobic power.
  • Athlete B discovers that blindly following a “125% FTP” formula would actually hurt his long-term endurance. By dialing in his intervals to avoid excessive glycolytic spikes, he prevents an unwanted jump in VLamax, which can undercut submaximal performance if left unchecked.

This is a crucial realization neither athlete would have reached by relying solely on FTP-based percentages for “VO₂max intervals.” Had Athlete B stuck to a standard FTP prescription, he could have spent months working at an intensity that spiked lactate production—negatively affecting the very endurance he aimed to enhance.

In short, FTP-based HIIT Training prescriptions assume a single power number can guide an extremely individual metabolic process. The only reliable way to ensure those tough intervals truly target VO2max is to assess and prescribe intensity based on each athlete’s unique physiological metrics.

HIIT Training: FTP-Based vs. Physiology-Based Approach

This isn’t an isolated issue with HIIT training—it’s part of a much broader pattern. We’ve already demonstrated how FTP-based training zones also fail in Zone 2 and Threshold training, 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:

These articles further validate why moving beyond FTP and toward individualized, physiology-based training is not just helpful—but essential.

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

Bridging the Gap: Turning Theory into High-Intensity Training

We’ve established that FTP-based intervals can fail HIIT training by relying on a single metric rather than an athlete’s genuine metabolic systems. The logical solution is to match high-intensity sessions to VO₂max engagement and glycolytic control, ensuring each workout hits the adaptation target.

But how do you apply this day to day?

That’s where INSCYD steps in. By providing a 360° profile of an athlete’s physiology—from aerobic power to glycolytic capacity—coaches and athletes can finally break free from generic FTP percentages. Instead, you’ll know exactly how to dial in HIIT so they truly push oxygen uptake without overshooting into unwanted lactate production.

INSCYD: Elevating Performance Beyond FTP

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.

However, INSCYD goes beyond data collection—its real power lies in translating an athlete’s physiology into actionable training prescriptions

This solves the long-standing problem of ineffective training zones that assume “one size fits all.” With INSCYD, each session is set to the right metabolic demands—letting you raise VO₂max on your own terms, without risking unwanted side effects. In other words, you can train with precision, not guesswork, and ensure that every workout meaningfully advances your fitness goals.

Train with Precision, Not Guesswork

For years, many have relied on FTP-based zones, often not realizing they were only seeing part of the picture. INSCYD changes that dynamic by providing a complete metabolic profile, enabling coaches and athletes to tailor each workout to real physiological data—ensuring more effective, individualized, and goal-focused training.

This shift marks the evolution of endurance preparation: no more “cookie-cutter” approaches. Instead, it’s about evidence-based, science-backed optimization that aligns perfectly with each athlete’s unique needs.

You can keep guessing with FTP—or choose the precision and clarity INSCYD delivers.

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.

If you’re already an INSCYD user, you can try the Recovery Index for free.

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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