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Why doesn't zone 2 work if your VT1 is incorrectly identified?

Zone 2 has become a staple of endurance training. It's presented as the zone of fundamental endurance, mitochondrial development, and "easy but useful" work. But in reality, zone 2 only makes sense if your VT1 is correctly identified .


If your VT1 is misplaced — too high or too low — then your entire zone 2 becomes incorrect : inadequate intensity, diminished adaptations, unnecessary fatigue… and stagnant progress.


Here's why.


1. Zone 2 comes from VT1, not from some magic power or heart rate value


Many define zone 2 as a percentage of FTP, VO₂max, or heart rate. However, the true physiological marker for zone 2 is ventilatory threshold 1 (VT1).


VT1 = first ventilatory break → the moment when your ventilation starts to increase to eliminate the extra CO₂ produced by carbohydrate metabolism.


👉 Zone 2 is simply the area around and just below VT1 .

If VT1 is incorrectly identified → zone 2 loses all physiological meaning.



2. If your VT1 is placed too high, you no longer have zone 2


This is the most common trap:

💥 Many athletes train too close to VT2 thinking they are in zone 2.


Consequences :

  • You're switching to carbohydrate metabolism too early.

  • The ventilation system drifts rapidly.

  • The heart rate increases.

  • the session becomes a tiring and unproductive “grey area”.


👉 Result: you are not developing the aerobic base that zone 2 is supposed to strengthen.


3. If your VT1 is set too low, your sessions become too easy


Less frequent, but just as problematic.

If your Zone 2 is overestimated downwards:

  • Mitochondrial adaptations are slower,

  • Your aerobic system is underutilized.

  • You are lacking the minimum intensity necessary to progress.


👉 You are accumulating unnecessary hours, with low physiological output.



4. The problem with classical methods (FC, FTP, RPE)


Heart rate fluctuates according to:

  • the heat,

  • stress,

  • fatigue,

  • dehydration.


Power or pace does not reflect internal load , only mechanical effort.


Result :

🔥 You think you are in zone 2… but physiologically, you are above or below .


5. Why breathing provides an accurate (and reproducible) VT1


Studies show that:

  • Ventilatory thresholds are highly reproducible (r > 0.90),

  • Ventilation reacts immediately to metabolic transitions.

  • VT1 is an excellent indicator of the carbohydrate/lipid switchover.


Unlike lactate testing, there is no blood test, no strict protocol. Breathing reflects physiology in real time .


With ZoneX, VT1 is detected:

✔ automatically,

✔ Indoors as well as outdoors,

✔ on any type of session.


6. In summary: zone 2 only exists if VT1 is correct


For a Zone 2 session to truly deliver:

  • mitochondrial adaptation,

  • improved fat burning,

  • progression of aerobic metabolism,

  • respiratory stability,


VT1 needs to be measured precisely , and not estimated via approximate formulas.


👉 A zone 2 based on a poorly identified VT1 = a zone 2 which… is not a zone 2 .



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