For most of us, the sensation of needing to breathe is associated with a lack of oxygen. However, a deeper look into human physiology reveals that it's not oxygen deficiency that
triggers the urge to breathe, but rather the buildup of carbon dioxide (CO2). This crucial
distinction has profound implications for endurance athletes who are seeking to optimize their performance.
Why Do We Really Breathe?
Try this simple experiment: hold your breath for as long as you can (but not too long; let's be
safe, people!). While you were holding your breath, you likely experienced an intense urge to breathe. But did you notice what happened the moment you stop holding your breath? Most (if not all) of us first let out a big exhale before taking a breath in. This is because we still had plenty of oxygen in our blood and our bodies were eager to release the CO2 that has been accumulating in our lungs. The body was prioritizing the removal of CO2, not the intake of oxygen.
Breathing is often thought of as a mechanism to bring oxygen into the body, but in reality, our bodies are much more sensitive to CO2 levels. As CO2 accumulates in the blood, it increases the acidity (lowers the pH) of the blood, signaling the body to expel the excess gas. This buildup of CO2—rather than a lack of oxygen—is what ultimately triggers the urge to breathe.
Oxygen Saturation: More Than Enough Oxygen
For most of us, blood oxygen saturation remains well above the threshold necessary for optimal performance people, even during strenuous physical activity. Fitness trackers often show that blood oxygen levels are around 97% at rest, and they only dip slightly if at all during intense exercise. What's happening during physical exertion is that the body is producing more CO2 as a byproduct of metabolism. This increase in CO2, rather than a decrease in oxygen, triggers the urge to breathe. What this means is that after a hard interval, we breathe rapidly because we need to get rid of the excess CO2 more than we need to bring in more oxygen.
Interestingly, this is why CO2 tolerance plays a major role in endurance performance. If our
bodies can tolerate higher CO2 levels, they can perform better under physical stress. The body's ability to manage CO2 more effectively leads to better oxygen utilization, thanks to a
phenomenon known as the Bohr effect.
The Bohr Effect: CO2's Role in Oxygen Delivery
The Bohr effect describes how rising CO2 levels cause hemoglobin to release oxygen more readily to tissues that need it, such as muscles during exercise. Essentially, CO2 is not just a waste product; it plays a crucial role in enhancing oxygen delivery to working muscles. The higher the CO2 level, the more oxygen is released from hemoglobin to fuel intense physical activity. Training the body to tolerate higher CO2 levels, therefore, directly improves our ability to perform at higher intensities. It enhances the delivery and use of oxygen in the muscles, which is essential for endurance athletes pushing their limits (think VO2max).
Training CO2 Tolerance: Why Breathwork Matters
Most endurance athletes understand the importance of improving cardiovascular fitness and/or VO2max through a variety of on-the-bike workouts. An often-overlooked factor in improving performance is the body's ability to manage and tolerate CO2 buildup during those training sessions. This is where breathwork becomes an important tool. By training the body to handle higher CO2 levels, we can effectively improve our endurance while sitting at a desk or watching TV.
Breathwork techniques that expose the body to higher CO2 concentrations (known as
hypercapnic training) can teach the body to tolerate and even thrive under these conditions. Over time, this type of training helps athletes improve overall efficiency by optimizing oxygen delivery to muscles and improving stamina.
Breathwork Practices for CO2 Tolerance
The most effective method for improving CO2 tolerance is centered around nasal breathing and exercises designed to expose the body to both low oxygen (hypoxic) and high CO2
(hypercapnic) conditions. To train the body to tolerate higher levels of CO2, we need to intentionally engage in practices that encourage its buildup.
How to Start Today
The first and most important step is to start breathing through your nose, all day, every day. Whenever you're not talking or eating, try to breathe only through your nose, especially when sleeping, and even while working out in lower training zones (Zones 1, 2, 3 and even Sweet Spot). Nasal breathing adds a layer of resistance to the breath, making it physically harder, which naturally engages the diaphragm and improves breathing mechanics.
Additionally, the nose provides 30 beneficial functions, including adding nitric oxide (which acts as a vasodilator) and warming, filtering, and humidifying the air before it enters the lungs. These different functions all ensure better gas exchange and overall lung health, making it an essential tool for endurance athletes. Even though you will be taking less breaths per minute, the air volume of those nasal breaths will be higher than shallow rapid breaths through your mouth, resulting in more air reaching your lungs for oxygen absorption to occur.
How to Practice CO2 Tolerance Off the Bike
The key to improving CO2 tolerance is to focus on breathing light, low, and slow. Let’s
break these down:
Breathe Light
Focus on taking light breaths through your nose. This will gradually increase the sensation of "air hunger," a sign that CO2 levels are rising in your bloodstream. This will stimulate the body to adapt to higher CO2 levels over time. Watch the video below for a light breathing exercise.
Breathe Low
Focus on breathing deeply through your nose into the lower portions of your lungs by engaging your diaphragm. This allows for better gas exchange and helps manage CO2 buildup more effectively. Watch the video below for a low breathing exercise.
Breathe Slow
Focus on slowing down your breathing cadence to 6-8 breaths per minute. This will not only increase your air hunger, but also engage your parasympathetic nervous system; improving heart rate variability (HRV). Watch the video below for a slow breathing exercise.
Gears of Breathing on the Bike
If you've joined our Tuesday and Thursday public Zwift workouts this winter, you may have heard the coaches talking about the gears of breathing. Hopefully, you've been trying this during your recent workouts. What you may notice when you feel the need to switch from Gear 1 (nose in, nose out) to Gear 2 (nose in, mouth out) is the first alarm bell going off, signaling that your CO2 levels are rising.
Working harder requires more metabolic energy, which creates more CO2, and CO2 lowers the blood pH, making it more acidic. The brain doesn't like this, so it sets off the "warning alarm," telling the body to increase the respiratory rate and start expelling some of this
built-up CO2. But as we've learned, we don't want to get rid of too much CO2, because keeping it in the system helps deliver more oxygen to the working muscles. That's why we shift from Gear 1 to Gear 2.
As we start working even harder and producing even more CO2, the brain sets off the "main
alarm." At this point, the brain forces you to shift from Gear 2 to Gear 3 (mouth in, mouth out). This allows the body to expel CO2 at a faster rate, helping to bring our blood pH to a more manageable and safer level.
Our goal is to get the body to sound those alarms and make those shifts later in our power progression. If we can do that, we've taught our bodies to become less sensitive to CO2.
It is possible for some athletes to reach Zone 4 and into the Sweet Spot while remaining in Gear 1! Yes, you read that correctly; you can be working at around 90% of your FTP while breathing in and out through your nose. That is crazy! It’s not that those athletes are producing less CO2; it's just that they can tolerate more of it.
Conclusion: The Importance of CO2 in Endurance Training
Rethinking the urge to breathe can unlock new levels of performance for endurance athletes. Breathlessness is not caused by a lack of oxygen, but by the body's need to clear excess CO2. It's important to understand that CO2 is a byproduct, not a waste product. This is similar to how blood lactate was also once considered a waste product but is now recognized as a valuable fuel source that athletes train to tolerate in order to enhance performance. By training our bodies to tolerate higher levels of CO2, we can improve oxygen delivery to muscles, boost performance, and break through performance plateaus.
Next time you feel the urge to gasp for air, remember it's not about needing more oxygen; it's about clearing CO2. With the right breathwork training, you can push your limits further than you ever thought possible.
At BaseCamp, we believe that every cyclist has the potential to achieve greatness, no matter where they start. Our mission is to create a community-driven training environment where cyclists and triathletes of all levels can train together, support each other, and grow stronger, faster, and more confident in their abilities. Our cycling training programs are expert driven and tailored to your needs. Whether you're a seasoned pro or just getting started, BaseCamp is where you belong.
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