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I recently put all the combat sports athletes I train through the CO2 Tolerance Test and used the results to assign specific breathing exercises to improve their athletic performance as well as their health and wellness.  The results have been astonishing: Improved cardiopulmonary function, reduced state anxiety, and better overall control of the autonomic nervous system!

I first learned about the test from exercise physiology mastermind Brian Mackenzie and immediately fell in love with the assessment for its ability to convey so much information about a person’s lung function at NO cost (all you need is a stopwatch).  Results of the test are indicative of how well a person can tolerate carbon dioxide in their system, how well they can utilize oxygen, and how well they can breathe. In addition, results of the test are highly correlated to state anxiety, that is, a person’s predisposition to feeling stressed, anxious and not in control.  In the present article I will discuss why improving CO2 tolerance is important, how to implement the CO2 Tolerance Test, how to interpret the results, and how to improve based off of those results.  Here we go…



There is a very close correlation between CO2 tolerance and state anxiety; the better control you have over your breathing, the less generalized anxiety you feel.  Whether you’re an athlete or an out of shape banker, the ability to control your nervous system to manage anxiety and arousal is imperative for health and performance.  Low anxiety equals low cortisol. And low cortisol equals less fat, more muscle, and better physical and mental performance.

To go deeper into the specifics of breathing control, and now we’re getting into nerdy sports performance talk, so skip to the next section if you don’t care, high CO2 tolerance can optimize aerobic metabolism.  According to the Bohr Effect, the lower your blood pH and the higher your blood CO2 levels, the easier it is for your body to “absorb” oxygen. Therefore if we inhale too early before truly needing to from a physiological standpoint, we’re putting a cap on our maximum minute ventilation.  In layman’s terms, we’re maxing out our breathing rate before we have a chance to max out our oxygen absorption. Adding to that, poor CO2 tolerance is correlated to poor breathing control. Any and all athletes want to have strong breathing muscles and excellent breathing control so that the pulmonary system (i.e. the lungs and surrounding structures) will never be the weak link; lactate threshold and/or movement economy should always be the weak link in endurance performance.



  1. Take 4 full breaths, 1 breath every 5-10 seconds: a 3-5 second inhale, followed by a 5-10 second relaxed exhale, 1 second pause before beginning to inhale again.
  2. At the top of the 4th inhale (totally full), start a timer and exhale as slowly as possible.  Stretch out the exhale for as long as possible. It’s helpful to close your eyes so that you can more effectively stay relaxed.
  3. Stop the timer when your air runs out, or you need to inhale.



  • >80 seconds: Elite
    • Advanced pulmonary adaptation, excellent breathing control, excellent stress control
  • 60-80 seconds: Advanced
    • Healthy pulmonary system, good breathing control, relatively good stress control
  • 40-60 seconds: Intermediate
    • Generally improves quickly with focus on CO2 tolerance training
  • 20-40 seconds: Average
    • Moderate to high stress/anxiety state, breathing mechanics need improvement
  • <20 seconds: Poor
    • Very high anxiety and stress sensitivity, mechanical restriction possible, poor pulmonary capacity



  1. Nasal breathing during training.  Having your mouth closed while exercising will force your body to offload CO2 at a slower rate than you’re used to.  Give it a shot next time you’re working out, and be surprised by how hard it is to train your pulmonary system for maximum efficiency!
  2. Counted breathing exercises.  Sit down in a quiet area, set a timer for 10 minutes, close your eyes, and use the following breath-count guidelines.  Breathe with the same pattern ad infinitum until the timer runs out.
    • Beginner: exhale slightly longer than inhale.  A good starting place would be to inhale for 8 seconds and exhale for 10 seconds.
    • Intermediate: exhale longer than inhale, and utilize a breath hold at the top of each inhale.  A good starting place would be to inhale for 8 seconds, hold your breath for 4 seconds at the top of the inhale, and exhale for 10 seconds.
    • Advanced: exhale much longer than inhale, and utilize a breath hold at the bottom (and top as well if you want the extra challenge) of the exhale.  A good starting place would be to inhale for 8 seconds, exhale for 10 seconds, and hold your breath for 4 seconds at the bottom of the exhale.


So there you have it!  Use the CO2 Tolerance Test to assess your breathing function, interpret your results, use the exercises listed above, and see how much your overall stress levels drop while your gym performance skyrockets!  As always, if you’d like to know more about, and start taking responsibility for your own health, fitness, and wellness, please reach out to Crux Fitness Richmond for any of your personal training needs.

Patrick Koo – Certified Strength and Conditioning Specialist and Personal Trainer at Crux Fitness Richmond