03/04/2026
Contrary to what many people assume, building training plans is only a relatively small part of developing a hybrid training system like EnduCloud. A much larger share of the work goes into more fundamental questions: what fitness really is, how external load, such as running a given distance at a certain pace, translates into the internal stress the body experiences, how to measure changes in fitness over time, and how to adjust training difficulty so it continues to provide an effective stimulus.
Even that description does not fully capture the complexity of one of the hardest problems in endurance training – a problem we are still far from defining clearly, let alone solving. As athletes improve, their physiological thresholds shift. What was easy yesterday can eventually become too easy. If reference points are not updated, training stops being challenging, and progress stalls.
To address this, EnduCloud evaluates training load against a continuously updated physiological profile for each athlete. At the same time, we still need to communicate fitness in familiar terms such as VO2max-related thresholds, lactate threshold, and training intensity zones. Because athletes come from different training backgrounds, and because zones are more of a descriptive tool than a fundamental one, the system allows users to work with different intensity metrics, be it heart rate, pace, or power, and to choose their preferred zone model.
At first glance, this may seem like a purely technical, even cosmetic, feature. In practice, though, trying to align different models reveals a deeper issue: systems that look almost identical on the surface, with the same “five zones” and similar labels, can actually represent very different intensities.
To understand why, it helps to step back. Intensity is not something we can measure directly. There is no intensity meter attached to the body. Instead, we rely on proxies such as heart rate, pace, and power. Each captures a different aspect of effort: cardiovascular response, external output, mechanical work. But none of them directly measures metabolic intensity. So when we talk about intensity, we are really talking about an approximation.
To make that approximation useful, we divide it into zones. And that is where the problems begin. The way these zones are defined is not consistent. It depends on the anchor chosen, and on how the zones are structured relative to that anchor. Zones can be based on maximum heart rate, threshold heart rate, threshold power, or physiological markers such as VT1 and VT2. Each approach creates a different system.
In many consumer systems, for example, the division is based on percentages of maximum heart rate. Zone 2 is often defined as 60–70% of HRmax, Zone 3 as 70–80%, and Zone 4 as 80–90%. This is simple, intuitive, and easy to apply. But it rests on a strong assumption: that there is a fixed relationship between a percentage of HRmax and a specific metabolic state. In reality, that relationship varies significantly between individuals and across fitness levels.
Other models, such as the Norwegian approach, anchor zones to physiological thresholds instead. VT1 marks the point at which lactate production and ventilation begin to rise more noticeably, while VT2 marks the point at which the system can no longer maintain steady-state balance. The zones are defined relative to those points: below VT1, between VT1 and VT2, around VT2, and above it. In this model, there are no fixed percentages, because the thresholds themselves shift as fitness improves.
This highlights something that sports science has long understood, but that has not fully made its way into coaching practice, consumer devices, or even popular science discussions: the “aerobic zone” does not sit at the same percentage of HRmax for everyone. For a beginner, VT1 might occur around 65% of HRmax. For a trained athlete, it might be closer to 75–80%. For an elite athlete, it may be even higher. In other words, the zone itself moves as fitness improves.
When you try to map one system onto another, the mismatch becomes obvious. “Zone 2” on a watch, defined as 60–70% of HRmax, does not necessarily align with “Zone 2” in a threshold-based model, which is the model most discussions of Zone 2 are actually referring to. Sometimes it is lower, sometimes it overlaps partially, and sometimes it sits entirely below the intended aerobic training range. That means two athletes can both be training in “Zone 2” while doing fundamentally different workouts.
This is not just a theoretical issue. It directly affects how training programs are built. A large part of the professional and popular discussion around endurance training is framed in zone language: how much time to spend in Zone 2, how much in Zone 4, and what the balance should be between low and high intensity. But if the zones themselves are not defined in the same way, those recommendations lose much of their meaning.
Zone 2 is the most obvious example, but the same problem exists at higher intensities as well. In Joe Friel’s widely used 5- or 7-zone heart rate model, “Zone 4” refers to work around threshold — hard, but still relatively sustainable. In the Norwegian 5-zone model, “Zone 4” sits above threshold, in the range where anaerobic contribution becomes more significant and fatigue accumulates quickly. Same label, very different physiological demand.
When we chose to let users select their preferred intensity model in EnduCloud, the goal was not simply convenience. It was to align the language people use with the underlying physiology. At the same time, that flexibility comes with a responsibility: understanding that numbers and labels are not absolute. They are context-dependent, and the quality of their connection to physiology varies.
First, it is important to choose a consistent anchor. If you are using maximum heart rate, you need to understand its limitations. There is substantial research showing that heart-rate-based zones do not reliably map onto physiological thresholds. That means one athlete’s Zone 2 could be below VT1, while another athlete running at exactly the same heart rate might already be above it. If you are working with thresholds, on the other hand, you need to make sure they are current and not based on a test from a year ago, or five years ago, as someone once suggested.
Second, you should be cautious about transferring recommendations from one system to another. A guideline like “spend 80% of your time in Zone 2” is meaningless unless you know how Zone 2 is defined. It can mean very different things depending on whether you are using watch-based zones or a threshold-based model, even if both systems have five zones.
The differences between these systems are not mistakes. They are different ways of simplifying a complex reality. The problem begins when we treat the simplification as if it were the reality itself. When your watch says Zone 2, it is using one definition. When a coach talks about Zone 2, they may be using another. Sometimes they overlap. Sometimes they only partly overlap. Sometimes they barely overlap at all.
If there is one takeaway, it is this: do not focus on the name of the zone. Focus on what it actually represents. Without understanding the anchor behind it, “Zone 2” is just a label. Its real meaning comes from the reference point it is tied to, and that is what ultimately determines whether it becomes a precise and useful training tool.
