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Altitude & breathing: how VT1 and VT2 change in the mountains (and how to adapt your training)

Altitude profoundly alters the way the body produces energy. Less oxygen, more ventilation, faster fatigue: it is an environment where breathing becomes a central indicator to understand your physiology.

And this change is directly reflected in your ventilatory thresholds (VT1 & VT2) , which do not behave at all like they do at sea level.


1. Why altitude changes your breathing


From 1,500–2,000 m altitude, oxygen pressure decreases. As a result, your body has to breathe more to compensate.


This hyperventilation leads to:

  • a decrease in CO₂ (respiratory alkalosis effect)

  • an increase in the work of breathing

  • an acceleration of the respiratory rate

  • a higher perception of effort


👉 Breathing becomes more costly, more unstable, and less efficient.


2. How does VT1 change with altitude?


🔻 VT1 almost always decreases at altitude

For what ?

  • the body switches earlier to a carbohydrate metabolism

  • the ventilation accelerates more rapidly

  • the ability to remain in an “easy” aerobic zone decreases


In concrete terms:

At sea level, your VT1 can be at 200 W / 5:00/km. At 2,000 m, it can drop to 170–180 W / 5:20/km .


👉 Doing “Zone 2” at altitude is more difficult than you think .

Many athletes believe they are staying in endurance mode… when in fact they are drifting above VT1.



3. How does VT2 change with altitude?


🔻 VT2 drops even more sharply than VT1

Main reason: The ability to buffer acidosis decreases at altitude.

Moreover :

  • CO₂ is expelled with more difficulty

  • higher acidity

  • very expensive ventilation

  • reduced VO₂max


Result :

👉 The intensity you can maintain at the threshold decreases significantly.


Example :

VT2 at 260 W in the plain → 225–235 W at 2,000 m.



4. Why heart rate alone is no longer sufficient for altitude training


Heart rate at altitude is misleading:

  • It increases at rest, upon waking, even without fatigue.

  • it drifts faster

  • It does not reflect the metabolic changes associated with a lack of oxygen.


You may very well be below your usual Zone 2 heart rate , but already above VT1 in terms of respiration.


👉 Breathing becomes the only truly reliable indicator .


5. How to train intelligently at altitude using VT1/VT2


🔹 Below VT1: “real” basic endurance

Goals :

  • improve respiratory efficiency

  • avoid depleting carbohydrate reserves

  • facilitate acclimatization


Very useful for:

  • long outings

  • easy jogging

  • recovery sessions


🔹 Between VT1 and VT2: Controlled active endurance

Use with caution: → it is often more expensive than in the plains.

Goals :

  • accustom the body to a slight oxygen deficit

  • develop ventilatory tolerance

  • simulating the intensity of competition at altitude


🔹 Above VT2: high intensity… in small quantity

Keep this for:

  • short intervals

  • technical exercises

  • power work


Excessive intensity at altitude = rapid fatigue + slower recovery.



6. How ZoneX makes altitude training easier


With a respiratory sensor like ZoneX , you can:

  • measure your new VT1/VT2 in the mountains

  • monitor your ventilation live

  • Adjust your intensity levels according to the actual altitude

  • avoid unnecessary ventilatory drift

  • completely personalize your Zone 2

  • protect your metabolism from overload


👉 Altitude changes your thresholds,

👉 ZoneX allows you to measure them…

👉 and therefore to train correctly.


Conclusion


Altitude is not just a matter of “thinner” air: it is a profound change in physiology.


Through breathing and ventilatory thresholds, you can:

  • avoid pacing errors

  • calibrate your true endurance zones

  • better manage your fatigue

  • optimize your progress

  • to make the most of an internship or stay in the mountains


👉 Altitude destabilizes your metabolism.

👉 Breathing helps you understand it and train better.



 
 
 

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