Over the past two decades, Functional Threshold Power (FTP) has become the cornerstone for determining training intensities and tracking performance progression in cycling, triathlon and endurance sports. At its core, FTP is an attempt to approximate the physiological phenomena of the maximal lactate steady state, which is usually derived from a lactate profile test.

The appeal and widespread usage of FTP is grounded in its simplicity: unlike laboratory-based lactate or gas exchange tests, it can be established with a relatively straightforward field test, making it widely accessible to cyclists and runners and their coaches. This ease of measurement – only using a powermeter or GPS device – has fueled its widespread adoption, turning FTP into a central reference point in cycling and running training to monitor and benchmark performance development during a training process, but most importantly to create training zones.

The latter has become so deeply ingrained in some endurance sports that they are rarely questioned. The widespread acceptance of FTP based training intensity zones has led to a situation where athletes and coaches take them as a given, often without considering their limitations or whether they truly lead to the most efficient training possible.

Want a quick, clear explanation of why a training zone based on FTP can backfire for Zone 2? Watch our short video below. Once you’ve watched, continue below to see how this plays out in real athlete examples and learn about a smarter alternative to the one-size-fits-all FTP method.

Now, let’s dive deeper.

The widespread acceptance of FTP has led to the development of standardized training zones, primarily based on the zones given by Allen, Coggan, and McGregor in their 2019 Book “Training and Racing with a powermeter”. These zones segment intensity levels based on percentages of FTP, providing a structured way to guide workouts and monitor effort. While different models exist, the most commonly used system divides training into the following zones:

  • 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)

These zones provide structure, making training plans easier to design and to follow. However, their dominance is not just a minor limitation—it represents a significant barrier to progress. By assuming that a default set of percentages of an FTP value makes universally applicable training zones, this method of %FTP overlooks a crucial reality: athletes’ physiology is unique, and therefore their training has to be too. 

Two athletes with identical FTP values will – in almost all cases – differ drastically in their metabolism and physiological conditions when exercising above or below FTP. Athletes with different metabolic profiles demand different training intensities to improve performance, yet they are often confined to the same rigid zones. This is not just outdated thinking—it’s a fundamental flaw.

In this article, we’ll focus only on the shortcomings of Zone 2, where many athletes rely on a generic FTP percentage to guide their endurance training. 

The Origins of FTP: A Proxy of a Proxy

To understand why FTP has limitations, it’s important to examine its origins. In the 1980s, physiologists identified the concept of the maximum lactate steady state (MLSS). Incremental tests with lactate sampling became a widely used method to approximate this physiological marker, known as the anaerobic threshold. Lactate testing provided a direct insight into an athlete’s endurance capabilities, making it a valuable tool for training prescription.

However, with the rise of power meters—especially in regions like the USA where lactate testing was less common—coaches and athletes sought an alternative. The solution? A proxy of a proxy. Since lactate threshold testing wasn’t widely accessible, Functional Threshold Power was introduced as a practical substitute, estimating threshold performance based on power output rather than direct metabolic measurement. 

The term “functional” itself was a recognition that FTP does not represent a true physiological value but rather an approximation of performance. For an athlete however, it isn’t relevant if the functional threshold power is an accurate proxy of a lactate threshold or maximum lactate steady state. The real issue for an athlete lies in something else…

The Core Problem: FTP-based zones create the false sense of individualized training intensities but result in creating "Cookie-Cutter" style training zones

Most training plans assume that every athlete responds similarly within FTP’s predefined endurance zone. In reality, e.g. fuel utilization is highly individual—at a given % of their FTP, some athletes operate at or below their maximum fat combustion rate, while another athlete will be training significantly above their FatMax zone. 

The mismatch between standardized FTP zones and real physiology results in inefficient training prescriptions.

In order to prove this point and make it tangible, we use two example athletes who are not exceptional in their physiology but represent typical endurance athlete profiles. We will examine the most common training set defined by FTP-based zones for Zone 2:

Zone 2 (Endurance) Training: steady pace for >1h, focusing on aerobic metabolism

Here are the two athletes we are going to use to prove the point. For the sake of simplicity we used cycling as a sport, but the example below apply also to running and triathlon as you are going to see:

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

With these two example athlete profiles in mind, let’s see what FTP-based Zone 2 training does to them.

Zone 2 (Endurance) Training

Zone 2 training is primarily aimed at improving aerobic capacity and optimizing fat metabolism while keeping carbohydrate combustion in check. Its relatively low intensity allows athletes to accumulate high training volumes without excessive fatigue, making it a fundamental component of endurance training. 

A key aspect of Zone 2 is training at or near FatMax, the intensity at which fat oxidation reaches its peak. Scientific research has shown that training at FatMax not only reduces body fat but also enhances aerobic capacity. The likely mechanism behind this is that fat combustion requires more oxygen than carbohydrate combustion, leading to an elevated oxygen uptake during training. Over time, this may improve training efficiency by increasing mitochondrial density and enhancing the body’s ability to utilize oxygen.

Another critical function of Zone 2 training is managing carbohydrate consumption. Since endurance athletes often train for several hours, keeping carbohydrate utilization low during these sessions helps preserve glycogen stores. This is especially important in the broader context of a weekly training schedule, where Zone 2 rides are often followed by or precede high-intensity interval sessions

Maintaining glycogen availability reduces the risk of bonking, supports recovery, and helps prevent excessive fatigue that could lead to illness or overtraining. By carefully balancing fat and carbohydrate metabolism, Zone 2 training not only enhances endurance performance but also ensures athletes remain adequately fueled for subsequent training sessions.

Now, let’s look at what happens when two athletes train in Zone 2, defined here as 70% of FTP. Athlete A and Athlete B have similar FTP values—270W and 260W, respectively—but their physiological responses to this intensity are vastly different.

Athlete A

At 70% of FTP (189W), Athlete A is utilizing 49% of his VO2max, which is relatively low in terms of aerobic stimulus. More critically, he is burning 84g of carbohydrates per hour. Given that the body can only substitute around 60–90g per hour through oral intake (with 90g being the upper limit requiring specific gut training), this means that Athlete A is very likely operating in a carbohydrate deficit. 

If he started the session with anything less than fully replenished glycogen stores, this could negatively impact the session itself. And even if he gets through the workout, the depletion could compromise subsequent high-intensity training, increasing the risk of under-recovery, illness, or suboptimal performance in the following days. Furthermore, his FatMax intensity is 10% lower than 189W, meaning that instead of maximizing fat metabolism—the primary goal of Zone 2—he is missing the intended adaptation altogether.

Athlete B

In contrast, Athlete B, training at 182W (70% of his 260W FTP), has a very different metabolic response. He is working at 58% of his VO2max, meaning he is likely receiving a better aerobic stimulus. More importantly, he burns only 64g of carbohydrates per hour, which is much easier to keep in check with normal fueling strategies. Even better, 182W aligns almost perfectly with his FatMax intensity, meaning he is actually hitting the primary target of Zone 2 training: maximizing fat oxidation while keeping carbohydrate depletion in check.

This stark contrast highlights a fundamental flaw in prescribing training intensity solely based on FTP. Both athletes are following the same “Zone 2” prescription, yet only one of them is truly getting the intended benefits. Athlete A is missing FatMax, burning through glycogen reserves, and potentially compromising future training sessions. Athlete B, on the other hand, is nailing the session.

This is not just a theoretical problem—it has real consequences. Endurance athletes dedicate countless hours to Zone 2 training, often sacrificing time with family, work, and other commitments. The problem is not that they need to train more—but that they need to train smarter. Increasing the training effect wouldn’t cost more time or effort—it would simply require dialing in intensity correctly.

The Root Cause of the Problem: FTP Is Not a Physiological Metric

Everything analyzed above about Zone 2 Training reveals the fundamental flaw in FTP-based training zones. It’s not just that maximum aerobic and anaerobic power (VO2max and VLamax) need to be considered separately—the deeper issue is why FTP-based training zones fail in the first place:

FTP is not a metabolic entity like VO2max, FatMax, or VLamax. It is simply an intensity marker, a power output that an athlete can sustain for a certain duration. There is no relation between a fixed percentage FTP and these metabolic systems you actually try to improve by training. Yet, training zones based on FTP assume that a fixed percentage of FTP can accurately determine the right intensity to target specific physiological adaptations.

But when we talk about the actual goals of training, we are never aiming to improve FTP itself. Instead, we are working to:

  • Burn more fat efficiently.
  • Utilize more oxygen to increase aerobic performance (= VO2max).
  • Control adaptation of the glycolytic system to avoid unwanted adaptations (e.g. increase VLamax)

The problem is clear: We are prescribing training not based on the actual physiological systems we want to improve, but rather by extrapolating from FTP, which has no direct link to any of these systems. And that is exactly why FTP-based training fails to deliver optimal results in zone 2. …except if you are lucky and in your current state of training the % of FTP matches your Fatmax zone. But don’t rely on this, because when your performance improves by training the fixed percentage won’t be accurate anymore and then your training isn’t spot on efficient anymore. And then you stop progressing. Sounds familiar?

The Solution: Prescribe Training Based on the Physiological Metric you want to improve!

If FTP is not the right anchor for Zone 2 intensity, what is? Instead of extrapolating training zones up and down from FTP, we should set training intensities directly based on the physiological targets we want to improve.

Zone 2 → Prescribe Based on FatMax

  • Instead of using an arbitrary percentage of FTP (e.g., 70% FTP), we set the intensity at the athlete’s maximal fat combustion rate (FatMax).
  • In our example, Athlete A should train at 172W and Athlete B at 178W.
  • Notice that Athlete A has a higher FTP than Athlete B, but his Zone 2 intensity is actually lower. THIS is individualized training.

By doing so, we are not guessing—we are targeting the adaptation we want: maximizing fat metabolism and controlling carbohydrate burn.

From Theory to Practice: How to Implement This in Training

So far, we have identified the problem—FTP-based training zones fail because they extrapolate intensity rather than prescribing it based on the actual physiological systems we want to improve in Zone 2. We have also outlined the solution—training intensity in Zone 2 should be individualized based on FatMax and real carbohydrate/fat usage data, ensuring that each session is driving the intended adaptation.

But how can this actually be done in practice?

This is where INSCYD provides the technical solution—allowing athletes and coaches to move beyond FTP and get a complete 360° athlete performance analysis.

INSCYD: Your Gateway to Physiology-Based Training

INSCYD enables coaches to build a complete metabolic profile of an athlete using simple power-based (cycling) or GPS-based (running) field tests—no metabolic cart required. 

These tests, which involve a series of short all-out efforts, deliver insights far beyond FTP. INSCYD also supports field- or lab-based lactate testing, offering multiple ways to collect data based on what’s available.

With INSCYD, coaches and athletes gain access to the key physiological metrics that actually determine performance:

… and many more metabolic insights.

However, INSCYD does not just analyze an athlete’s physiology—it allows coaches to apply this knowledge to training prescriptions.

With INSCYD, you can:

  • Create training zones based on the athlete’s unique metabolic profile, not arbitrary FTP percentages.
  • Set Zone 2 intensity at FatMax—ensuring the highest possible fat oxidation rate.
  • Determine Zone 4 intensity by setting glycolytic power utilization at 8%, maximizing aerobic stimulus while controlling glycolytic adaptations.
  • Adjust Zone 5 to 90% VO2max—proven by science to deliver the best VO2max gains, while monitoring whether glycolytic activation is too high.

This completely solves the problem of ineffective training zones or, worse, training in zones that actively work against an athlete’s goals. With INSCYD, athletes and coaches no longer have to guess—they can train with precision and ensure that every session is optimized for progress

The Bottom Line: Train Smarter, Not Harder

For years, athletes and coaches have relied on FTP-based zones, unaware that they were working with an incomplete system. How many hours have been wasted ot training at the most efficient intensity. How many athletes and coaches have scratched their heads asking why performance plateaus? 

INSCYD changes that. By providing a complete, holistic metabolic profile and allowing training zones to be set based on real physiological responses, INSCYD makes training more effective, individualized, and goal-oriented.

This is the future of endurance training: no more one-size-fits-all training zones—just science-backed, physiology-driven performance optimization.

🚀 The choice is clear: Keep guessing with FTP, or train with precision 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.

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.

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