The title of Dr Andrew Sellars' episode is deliberately provocative: your legs aren't the problem, it's your breathing. For a lot of riders that lands as a relief and a puzzle at once — relief, because the lungs-on-fire feeling on a climb is the most visceral part of suffering on a bike; a puzzle, because we've always been told that breathlessness means we're short of oxygen. Sellars, an anesthesiologist and respiratory physiology researcher, spends the conversation gently turning that assumption inside out. You're not gasping for oxygen. You're trying to get rid of carbon dioxide.
I'll say up front that breathing training is a younger, less settled field than power-based training, and Sellars is careful about that himself. But the underlying physiology he describes is well established, and understanding it changes how you interpret your own body on a hard effort.
It's CO2, not oxygen
Here's the reframe, in his words. "The main reason you breathe faster with higher-intensity exercise is to blow off CO2, which is a byproduct of metabolism," Sellars said. "So the harder you work, the more CO2 you produce, the harder you have to breathe to blow it off."
That single sentence overturns the intuitive model most of us carry. We assume the panting on a climb is the body desperately sucking in oxygen. But under almost all conditions a healthy cyclist's blood stays well saturated with oxygen even at high intensity — you're not actually running out. What's changing is carbon dioxide. Hard work produces CO2 fast, blood levels climb, and your brainstem responds by ramping up your breathing to clear it. The urgent, can't-get-enough-air feeling is your body reacting to rising CO2, not to an oxygen emergency. The gasp is an exhaust problem, not an intake problem.
This matters because it relocates the limiter. If breathlessness were purely about oxygen supply, there'd be little you could do about the sensation beyond getting fitter. But because a large part of it is about your tolerance to CO2 — how high you'll let it rise before your body panics — it becomes a trainable variable. Two riders at the same intensity can feel very differently about it depending on how comfortable their physiology is with that CO2 build-up. It's part of why the same effort that reads as a controlled Zone 2 or threshold session for one rider feels frantic for another.
Training breathing without thrashing yourself
The practical promise Sellars makes is the part that matters for time-pressed riders. Normally, to expose your body to the breathing demands of a hard race, you have to actually ride hard — which is costly in fatigue and limited by how much intensity you can recover from. He describes a way around that.
"If you rebreathe some CO2, you can balance your physiology," he said. "Now you can actually train your breathing for as long and hard as you want, without actually negatively affecting your physiology, and without having to drive the body to levels that would be really hard — to mimic a race without riding really, really hard." In other words, by controlling the CO2 you breathe, you can deliver a strong breathing stimulus while sitting still, decoupling the breathing training from the leg-destroying intensity it would normally require.
It's worth being measured about this. It's an emerging approach, the kind of thing to treat as a complement to your riding rather than a replacement for building your aerobic engine. The legs, heart and lungs still need the miles. But the logic is sound: if CO2 tolerance is part of what makes hard efforts feel survivable, and you can train it without the fatigue cost of more hard riding, that's a low-risk lever worth understanding — particularly alongside the more established respiratory work covered in our breathing training guide and the kind of ventilation monitoring teams like Visma are now using.
The CO2 advantage
Sellars made one aside that nicely captures the whole shift in thinking. Talking about the popular breathing book that's introduced a lot of athletes to this world — The Oxygen Advantage — he said it "really should have been called the CO2 advantage, the carbon dioxide advantage, because the entire book is about this understanding of what happens to your body with higher" CO2.
That relabelling is the lesson in miniature. The whole field gets marketed under the banner of oxygen because oxygen is what we instinctively associate with breathing and performance. But the actual mechanism — the thing that drives your breathing rate, the thing that makes you panic on a climb, the thing you can train — is carbon dioxide. Once you flip that around, your own experience on the bike starts to make more sense. The breathlessness isn't a warning that you're suffocating. It's a tolerance threshold being crossed, and tolerance is something bodies can adapt.
Where this fits with what you already know
This CO2 reframe doesn't overturn endurance training — it sits underneath it and explains some things you've already felt. Think about why nasal breathing feels so restrictive when you first try it on an easy ride: you're limiting how fast you can clear CO2, so your body protests even though your oxygen is fine. Think about why a calm, experienced rider can hold a hard tempo with slow, controlled breathing while a nervous one hyperventilates at the same power: the difference is partly tolerance to the rising CO2, not fitness. The sensation and the physiology have been telling you this all along; Sellars just names the mechanism.
It also clarifies what breathing work can and can't do. It won't manufacture an aerobic engine — that still comes from riding, from the heart and mitochondria and capillaries adapting to months of training. What better CO2 tolerance may do is change how a given effort feels, letting you hold it with less panic and more control. Those are different things, and conflating them is where breathing-training marketing tends to overreach. Treat it as a layer that can make your existing fitness more usable and more comfortable, not as a substitute for building that fitness in the first place.
How to think about it as a rider
So what do you do with this? First, reinterpret the sensation. When your breathing goes ragged on a hard effort, you're not short of oxygen — you're at the edge of your CO2 tolerance. That alone can take some of the fear out of it; the panic is partly a reflex, not a real emergency, and riders who understand that often ride hard efforts more calmly. It's a small mental edge of the kind we explore in the context of pacing and perceived effort.
Second, treat breathing as a system you can work on, with appropriate humility about how new the science is. If you're curious, the structured respiratory work in our breathing guide is the sensible place to start, and CO2-tolerance methods like the ones Sellars builds are an emerging addition rather than a proven shortcut. Keep building the engine through riding; layer breathing work on top, not instead.
If you want to start somewhere simple and free, two habits cost nothing and build awareness. The first is nasal breathing on your easy rides — the ones that should actually be easy: breathing in and out through your nose on Zone 1 and low Zone 2 efforts naturally limits how fast you can clear CO2, which gently nudges your tolerance — and conveniently also stops you riding your easy days too hard, because the moment you push past easy, nasal breathing becomes impossible. The second is simply paying attention: when your breathing surges on a climb, consciously slowing and deepening it for a few breaths, rather than letting it spiral into a pant, often restores a surprising amount of control. Neither is a magic protocol, and the dedicated CO2-tolerance tools Sellars builds go further, but both let you feel the principle in action without any kit.
Beyond that, keep your expectations calibrated. Approach structured breathing work as an experiment layered onto solid training, give it a fair trial, and judge it by whether your hard efforts feel more controlled — not by whether your FTP jumped, because that's not what it's for. That framing protects you from both the hype and the dismissiveness, and leaves you with a useful new lens on a sensation you'll feel on every hard ride for the rest of your life.
Sellars' message isn't that your legs don't matter — it's that the breathing you've always treated as a fixed, helpless response is more trainable, and more misunderstood, than you thought. The next time the air feels like it's not enough, remember it probably is. The problem is the carbon dioxide you're trying to get rid of, and that's a problem with a handle on it.
Hear the full conversation with Dr Andrew Sellars on the Roadman podcast. For the structured side, read our breathing training guide for cyclists, and bring your questions to the community on Skool.