Most cyclists train the legs and the lungs. They also track every number on the head unit. Yet one variable rarely gets attention, and it decides how hard breathing feels at threshold. That variable is your tolerance to carbon dioxide.

What CO2 tolerance really controls

When you ride hard, your muscles produce more CO2. As a result, blood CO2 rises and the urge to breathe follows. That urge is not triggered by a shortage of oxygen. Instead, it comes from rising CO2 and falling pH. Chemoreceptors in the brainstem and the carotid bodies pick this up.

So if you are very sensitive to CO2, the urge arrives early. Your breathing rate climbs, and much of your effort goes into moving air rather than the pedals. Many endurance athletes also breathe through the mouth by default. This makes each breath bigger and faster, and over time it reinforces the sensitivity.

Why nasal breathing is the first lever

Raising CO2 tolerance keeps your respiratory centre calm at a higher CO2 level. So ventilation rises more gradually as the intensity climbs. Nasal breathing is the simplest lever here. The nose adds resistance, slows the breath, and keeps CO2 slightly higher. It also releases nitric oxide, a gas that widens the airways and blood vessels and improves the match between airflow and blood flow in the lungs.

Short breath holds after a normal exhale add a second lever. These air-hunger drills are central to the Oxygen Advantage method. They expose the body to brief, controlled rises in CO2. As a result, the point at which breathing feels urgent gradually shifts. Newer cyclist-focused research on nasal-only and breath-hold training points the same way. In short, better CO2 tolerance tends to travel with a higher functional threshold power.

How we train it at Atemstark

At Atemstark we start every athlete with a BOLT score. It is a simple measure of how long you can comfortably hold your breath after a normal exhale, taken on waking. A BOLT under about 25 seconds usually signals easy performance to reclaim from breathing mechanics alone.

From there, we build nasal breathing into easy rides and recovery spins first. At that intensity you can keep the mouth closed. Then we add structured air-hunger drills two or three times a week. The aim is not to gasp. Instead, you sit with a light, manageable shortage of air and let the nervous system adapt.

Try this on your next ride

On your next easy ride, breathe only through your nose for the whole session. Drop the pace if you have to, so the mouth stays closed. Does nasal-only feel impossible even at low intensity? If so, that is your starting signal, not a failure.

If you want a structured way to build this, try one of our guided breathing exercises and do them regularly. Then retest your BOLT score after a few weeks and get back onto your bike breathing through your nose!

FAQ

Is nasal breathing on the bike realistic at high intensity?
Not at first, and not at threshold or above for most riders. Start on easy rides and recovery spins where you can keep the mouth closed. The comfortable range then expands as your CO2 tolerance improves.

What is a BOLT score and what should mine be?
BOLT is the comfortable breath-hold time after a normal exhale, measured on waking. Below about 25 seconds suggests room to improve. For endurance athletes, 30 to 40 seconds is a reasonable target.

Does breath-hold training drop oxygen to a harmful level?
No. The short air-hunger holds cause only a small, brief dip in oxygen saturation, and it recovers within a breath or two. You do them at rest or low intensity, never in water or anywhere a blackout would be dangerous.

How long before CO2 tolerance work shows up in performance?
Most riders notice easier breathing at a given power within three to four weeks. Changes in threshold power then build over a longer block, alongside normal training.

Can I combine this with structured interval training?
Yes. Keep nasal breathing and air-hunger drills on easy and recovery days. During hard intervals, breathe however you need to.