Carbon Dioxide Tolerance Test (CO2TT)

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There is a rapidly growing recognition of the value of improving CO2 tolerance for health, physical wellbeing, and athletic performance [1-9]. The CO2TT has been used by Free divers to help determine daily training tables. The CO2TT can be used as a test and an activity. The benefits of the activity are derived from first engaging (stimulating) the nervous and biochemical systems, then allowing them to quiet to a blissful place.

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The Procedure – 4 Breaths

  • Take three easy, calm breaths in and out through the nose
  • Take one full nasal inhale, filling the abdomen, upper chest, and lungs
  • Start a timer as you exhale through the nose, as slow as possible
  • Stop the timer when you run out of air, swallow, or feel that you must take a breath in

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How we use the CO2TT

As a test, it can be used to help determine the appropriate BYT phase in which to play.

  • < 30 seconds = BYT Phase 1 activities
  • 30-60 seconds = BYT Phase 2 activities
  • >60 seconds = BYT Phase 3 activities

 Click here to access BYT Phase Activities

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More Interpretations

SH//FT, an athlete training system report:

>80 seconds –> Elite. Reflects an advanced pulmonary adaptation, excellent motor control, and low arousal. You get to play big.
60-80 seconds –> Advanced. Reflects a healthy pulmonary system, good motor control, and relatively low arousal.
40-60 seconds –> Intermediate. This range generally improves quickly with a focus on CO2 tolerance training.
20-40 seconds –> Average. Moderate to high arousal state. Breathing mechanics may need improvement.
<20 seconds –> Awesome candidate for breathwork. Very high arousal and stress sensitivity.

Option: See the SH//FT calculator or recommended apnea and cadence breathing schedules

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  1. McKeown, P., The oxygen advantage : the simple, scientifically proven breathing techniques for a healthier, slimmer, faster, and fitter you. First edition. ed. 2015, New York, NY: William Morrow, an imprint of HarperCollinsPublishers. xiv, 352 pages.
  2. Camacho-Cardenosa, A., et al., Repeated sprint in hypoxia as a time-metabolic efficient strategy to improve physical fitness of obese women. Eur J Appl Physiol, 2020. 120(5): p. 1051-1061.
  3. Galvin, H.M., et al., Repeated sprint training in normobaric hypoxia. Br J Sports Med, 2013. 47 Suppl 1: p. i74-9.
  4. Haider, T., et al., Interval hypoxic training improves autonomic cardiovascular and respiratory control in patients with mild chronic obstructive pulmonary disease. J Hypertens, 2009. 27(8): p. 1648-54.
  5. Hamlin, M.J., et al., Hypoxic Repeat Sprint Training Improves Rugby Player’s Repeated Sprint but Not Endurance Performance. Front Physiol, 2017. 8: p. 24.
  6. Hinghofer, H., Intermittent hypoxic training: Risks versus benefits. European journal of applied physiology, 2009. 108: p. 417.
  7. Serebrovska, T.V., Z.O. Serebrovska, and E. Egorov, Fitness and therapeutic potential of intermittent hypoxia training: a matter of dose. Fiziol Zh, 2016. 62(3): p. 78-91.
  8. Nestor, J., Breath : the new science of a lost art. 2020, New York: Riverhead Books. pages cm.
  9. Nestor, J., Deep : freediving, renegade science, and what the ocean tells us about ourselves. 2014, Boston: Houghton Mifflin Harcourt. 266 pages, 8 unnumbered pages of plates.
  10. Bentley, T. and B. Mackenzie CO2 Tolerance and Anxiety Study. 2020.

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The Science

The science on this test is scarce. A recent study, yet to be published by researchers at USC and California State University – Fullerton, showed an inverse relationship between the CO2TT score and anxiety levels[10]. The authors of this study caution against this measure being used for anything more than an “in-this-moment” assessment.

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