CO2 Tolerance Training Part 1 | Freedive Earth

CO2 Tolerance Training Part 1

If you’ve done any sort of freediving course, or you’ve been keeping up with any of Dr Otter’s Physiology series, you’ll probably already know that Carbon Dioxide (CO2) is a very important gas for freedivers. Not only is it the major cause of the urge to breathe but, as long as you can stay relaxed, when it reaches high levels in the body, CO2 is very very useful (see Alkaline Diet) . Tolerating high levels of CO2, though, can be difficult and uncomfortable, so training for CO2 tolerance is an essential part of every Freediver’s skill set, even if you just want to spend more time underwater for fun. In this short article we’ll look briefly at some of the science behind CO2 training and then talk about some ways to put it in to practise.

The Science of CO2 Training

CO2 is produced in the body as part of the process called respiration in which energy is produced from glucose by reacting it with oxygen:


It’s a waste product, and causes acidity in the blood, which we’re very sensitive to. We get rid of CO2 from the body when we breathe out, which is why CO2 is so effective at stimulating the urge to breathe. Some people are born with a low sensitivity to CO2 and they will probably end up as better freedivers, all other things being equal. Like everything in the body, though, the receptors sensitive to the effects of CO2 in the blood can adapt over time. The more time you spend with high levels of CO2 in your blood, the less sensitive those receptors will become, and the longer you’ll be able to hold your breath. This is the basis of CO2 tolerance training. Another important fact about this process is that the harder you work under water (the higher your work-rate or the lower your level of relaxation for example), the more CO2 you’ll produce. This is because you need more energy to supply the demands you’re making on your muscles (CO2 is the waste product of this process, remember). Over short distances and times you can build up quite a lot of CO2 in the blood without seriously impacting the level of oxygen, particularly if you keep your rest intervals short enough that you don’t have time to get rid of it all between dives.

CO2 tolerance training

Basic Principles

The first thing to understand about CO2 training is that it really works. Being able to relax whilst experiencing the urge to breathe is a key component of better freediving. As a beginner, CO2 training, particularly in static apnea, will give you big results in a relatively short time providing that you approach it in the right way and stick with it. That said, CO2 training can be quite unpleasant, especially in the beginning. If you’re new to freediving and are still working on your technique, you should keep your CO2 training light because the more stressed you get under the water, the worse your technique will become and the more you’ll get into bad habits. In the end, if you’re not careful, you’ll end up hating the sport altogether. Like everything in Freediving, start slowly and build up over time. Focus on technique at all times. Whatever you’re doing, if you’re doing it in the water, you need the supervision of a competent buddy. Never, Ever Freedive Alone. Although you might think that CO2 training allows you to avoid hypoxia and blackout, there’s always a chance it can happen, particularly with the more advanced exercises shown later on. During your recovery phase, you have a choice about how to breathe. When your rest intervals are long (e.g. 20 seconds or more), you’ll get the most benefit by controlling your breathing rate. Try taking 5 seconds for the inhale and 10 for the exhale. In the long term, though, you’ll get the most benefit of all by decreasing your rest interval to the point of maximum intensity and breathing as you need to. It will take time for you to get to this stage though. When you’re ready, check out our ‘advanced CO2 training’ article (coming soon) for more information on this. If you’re happy that you understand all the points above, and your technique is good enough to allow you to move efficiently through the water, even under a little bit of stress, then you’re ready for some CO2 training. Good luck!

CO2 Tables

First, let’s get to grips with some of the language we use when talking about CO2 training. Terminology: Dive In this case, a dive means a single breath-holding episode with a specific duration. This can be a distance (e.g. in dynamic apnea), a time (e.g. in static apnea) or a depth (e.g. in Constant Weight). Table A ‘Table’ is a series of dives of varying lengths and/or with varying rest intervals which are designed to target a specific gas or a specific part of diving physiology. Rest Interval The length of time spent resting between dives. Turnaround Time The length of time taken for a particular dive including the rest interval. Breakpoint The moment where the level of CO2 becomes so high that you feel you must breathe and so you surface. This is different from the hypoxic limit where you run out of oxygen and black out. This should never happen during CO2 training. Typically, CO2 tables involve a large number of dives with short duration and short rest intervals. This ensures that the level of CO2 in the blood increases throughout the table whilst the level of oxygen (in theory at least) remains pretty constant.


CO2 training for Dynamic Apnea

An easy introduction to CO2 training might look like this:


  Notice that after each set of 2x25m dives, the rest interval decreases by 5 seconds up to the middle of the set, then decreases again. This is called a pyramid because the intensity of the dives increases up to a peak and then decreases again:


          You might choose to vary the intensity by having more or less repeats, decreasing or increasing the rest interval or adding more steps to the pyramid here’s a much harder one: 


Notice that the thing you don’t do is increase the duration of the dive. If you do this, you’ll no longer be focussing on just CO2 alone, the longer swim will almost certainly begin to make you hypoxic. Watch this space for articles about O2 or Hypoxic training techniques.

CO2 training for Static Apnea

For static, the same principles apply - you can just substitute a time for the distance in the tables above:


 Notice that this will take quite a bit longer than your dynamic table, and as you get better and better, it will take longer still. A great variation for static CO2 training is a ‘one breath’ table where you hold for, say, 1min, take a single breath and repeat as many times as you can up to a maximum of, say 10 or 15 dives. This produces a short but very intense training session for which you absolutely must have a buddy!

You can do this for dynamic apnea too, if you’re a real glutton for punishment. Another great benefit of static apnea CO2 training is that you can do it on dry land. When you’re doing this, you don’t need a buddy, just a stopwatch to keep time for yourself so it’s great if you don’t have a training partner for that day. That’s it for this introduction to CO2 training. Stay tuned for advanced concepts and techniques in the very near future, including how and why to incorporate CO2 tolerance into your constant weight training. See you again soon!

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