Altitude, ventilation and thresholds: what happens above 1500 m?

PAIRFS
3 days ago
3 min read

Altitude profoundly influences breathing, ventilatory thresholds, and the ability to produce sustained effort.

Many cyclists, trail runners and triathletes instinctively feel it: from 1500–1800 m, “the breath leaves before the legs”.

But what is actually happening at the respiratory level?

Why are VT1 and VT2 moving?

And how can ZoneX be used to adapt faster?

Here's what science says — and what you need to know to perform at altitude.

1. From 1500m onwards, your ventilation changes (even without the effort starting).

From an altitude of 1500 m:

the partial pressure of oxygen decreases,
Each breath brings less O₂.
The body immediately compensates by increasing ventilation .

👉 Result: Your breathing accelerates even at rest.

This phenomenon is called the hypoxic ventilatory response .

The higher you climb, the higher the basal ventilation increases → which “compresses” your training zones.

2. VT1 and VT2 move… downwards

Studies show that at high altitudes:

VT1 appears earlier , because ventilation increases to compensate for the drop in O₂.
VT2 decreases significantly because acidosis occurs more quickly.
The athlete reaches their red zone well before the usual intensity.

At 2000–2500 m, the following are frequently observed:

VT1: −5 to −10% intensity
VT2: −10 to −20% (sources: Dempsey 2014, Mazzeo 2008, Lundby 2016)

👉 You are not “less fit”: the ventilatory thresholds are simply shifted .

Measure your thresholds precisely

3. Why does the effort seem more difficult?

Three main reasons:

1) Ventilatory costs increase

Breathing faster = more work for the respiratory muscles → faster fatigue.

2) CO₂ is expelled faster

This alters blood pH → respiratory signals are amplified.

3) The body switches to the carbohydrate zone earlier

Fat burning becomes less efficient in hypoxia → carbohydrates are used earlier → VT2 arrives faster.

4. How to adapt: the importance of breathing

After 24–72 hours at altitude:

Ventilation increases permanently.
The saturation level rises slightly.
The perception of effort decreases,
VT1 and VT2 gradually readjust.

But without monitoring, it's difficult to know when and to what extent you are acclimatized.

This is where ZoneX changes the game.

5. What ZoneX allows you to see at altitude

Thanks to respiratory analysis, ZoneX detects in real time:

✔ Your new VT1 / VT2

→ upon your arrival at altitude (1500–2500 m)

✔ The evolution of your breathing day after day

→ to find out if you are fully acclimatized or still “in debt”

✔ Respiratory drift during ascents

→ key sign that you are exceeding your true threshold too soon

✔ Your ability to recover between efforts

→ often reduced at altitude, but can be improved

✔ On days when hypoxic fatigue affects you more than usual

→ ZoneX detects an abnormally low VT1/VT2 → sign of over-fatigue

In short:

👉 ZoneX transforms an invisible physiological adaptation… into usable data.

Learn more about ZoneX

6. How to train at altitude (guided by your breathing thresholds)

• Days 1–2: low intensity only

Remain on VT1 until ventilation stabilizes. ZoneX should display: “smooth breathing” .

• Days 3–5: Return to the tempo

Small blocks between VT1 and VT2 , on a slight incline. ZoneX should display: “controlled breathing” .

• Days 5–10: reintroduction of “near VT2”

Blocks 3–5 min close to the adjusted ventilatory threshold. ZoneX: “stable ventilatory transition” .

• Days 10+: high intensity possible

Only if ZoneX shows:

VT2 reassembled,
slight ventilatory drift
steady breathing at moderate intensity.

7. Key points to remember

Above 1500 m:

Your ventilation changes immediately.
Your breathing thresholds are decreasing,
the effort seems more intense,
Acclimatization depends on respiratory stabilization.

👉 With ZoneX, you can monitor your thresholds and adaptation live , without a mask, without lactate, without a laboratory.

This is the most precise way to optimize your altitude training — and perform in the mountains.