I used to get out of breath when cycling, running or swimming and a friend mentioned that it is important to fully breathe out when feeling slightly out of breath, as it is due to a build up of carbon dioxide in the lungs, rather than a lack of oxygen. Is this true? I have found that her advice works and I rarely get out of breath now, but I've been trying to find scientific literature to better explain why this is.

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    due to a build up of carbon dioxide in the lungs, rather than a lack of oxygen It's a lot more. Do an all-out effort that blows through your body's entire anaerobic capacity in less than a minute and throws your entire metabolism into a massive O2 deficit. If you do that, you WILL be gasping for air like a fish out of water no matter how you breathe. Heck, good luck trying to stop yourself from breathing fully out - or in for that matter - after you do that. In that case, your body is trying to flush the byproducts of your effort Jan 8, 2022 at 21:15
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    @AndrewHenle: Yes, I also think that our breathing instinct is quite optimized and efficient (at least when it comes to maximum effort) and I don’t think one can “magically” improve it with some conscious effort. But I don’t have any sources to support this gut feeling. On a bicycle posture (seating position) probably plays an important role.
    – Michael
    Jan 9, 2022 at 7:48

2 Answers 2


This is a huge topic. It is true that the breathing reflex is controlled by CO2 in your body (not just in the lungs). The key is the acidity of the cerebrospinal fluid. If you reduce your CO2 level, you will not feel you need to breathe that much but that does not make you to have more oxygen for your exercise. So you will not be faster.

Divers know how to reduce the need for breathing by hyperventilation. That reduces the CO2 content in the body even if it does not raise the O2 level too much. This makes it possible to hold your breath longer. The danger is that you may actually pass out due to insufficient level of oxygen even if you are a few centimeters below the water level. If there is no one to save you, you will die.

To actually get faster, you need to get enough oxygen into your lungs and then your VO2max determines how much you are able to use for your activity. VO2max is the maximum amount of oxygen (measured in volume of O2 per kg of body mass and a unit of time - ml.kg⁻¹.min⁻¹) that your body is able to use for an aerobic activity. It determines the power your body is able to deliver aerobically - that means in a sustained effort, not in a short sprint. Top athletes have values of VO2 max reaching 90, normal people will have smaller values depending on their fitness, their age and their sex. There are many explanations on the internet and also tables of values telling you how good you are with a certain value for your age and sex.

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Menaspà, Sassi, Impellizzeri (2010) https://doi.org/10.1249/mss.0b013e3181ba99bc

Just getting rid of the CO2 will not let you produce more power. Also, quite obviously I think, if you do an effort that is hard enough, you will be out of breath no matter what you do. Just observe professional athletes after they cross their finish line.

  • Can you explain what you mean by VO2max?
    – marcelm
    Jan 9, 2022 at 10:38
  • Thanks for your great answer. I have accepted it, but I was actually thinking from a slightly different perspective. I'm not at a high-end athlete level, I just enjoy doing sports. I do find that when I'm feeling a bit out of breath and I fully breathe out, I can more quickly recover than from panting. My question is more about why does fully breathing out have benefits over panting? Is it due to getting rid of the build up of CO2 more efficiently? For example atmospheric CO2 is ~420 ppm, but can build up higher in non-ventilated areas or when doing exercise, with danger level at ~5000 ppm.
    – Julliet
    Jan 9, 2022 at 14:36
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    Well as a caver I know the danger of elevated CO2 level. 5000 ppm is still quite fine but several per cent is ugly. Mainly headache or even dizziness. But the concentration of CO2 in the air you normally breathe out is even higher, about 4% (40000 ppm). Breathing deeply enough will indeed get you more the gases you need and will help you to get rid of the gases you need to remove. Jan 9, 2022 at 14:52
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    To be honest, "panting" is a new word for me, I am not a native English speaker. But do not confuse fast shallow breathing and fast breathing. You can breathe very quickly and still deep enough, that will lead to hyperventilation, if you continue with it long enough. You can then reach abnormally low CO2 concentration in your body. Jan 9, 2022 at 14:52

I'll post an alternate take on the question.

In exercise science, there are two notable ventilatory thresholds. VT 1 is the level of effort where breathing rate begins to increase noticeably. You won't be able to give a speech comfortably. You will need to talk in short bursts. I think that VT 1 in cycling onsets at about endurance pace, maybe 55-60% of your threshold power (for those not familiar, that's your maximum sustainable power, you're mainly producing aerobic energy, your blood lactate levels are about stable).

VT 2 onsets at a higher power level, when your blood lactate levels are elevated. I think that VT 2 typically onsets around threshold power. Speaking is difficult. You should be able to get a few words out at most.

I'm not sure what ventilatory threshold the OP was at, since we can't observe them directly. I'm not sure their friend's advice was correct, however. Untrained individuals will hit both ventilatory thresholds earlier than trained individuals, possibly much earlier depending on their base fitness. So, if the OP was untrained and trying to keep up with a friend, even if the friend wasn't that fit, their cycling-specific fitness might still have been a lot higher than the OP.

Exercise technique may also part of this. When I was first starting to run in my early teens, I couldn't modulate my power well. That is, I could only run hard or walk - in part beacuse my neuromuscular system was also first adapting to the movements of running. I had the same thing with swimming in my late teens, and I probably still have some of it because I don't swim often. I don't believe I experienced the same thing when I started cycling, but my base fitness was high and cycling isn't as technique intensive as swimming. In short, if this was the issue, then I think the only solution is to persist until you are used to the sport and you can modulate your power well. If you had gross errors of technique, you would want to correct those, and I think this would affect swimming more than cycling or running.

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