Breath-Hold Walks for CO2 Tolerance Training

Audience and roadmap. This article is for endurance athletes, team‑sport players, tactical personnel, freedivers training on land, clinicians curious about off‑water protocols, and fitness enthusiasts who want a structured, evidence‑aware way to approach breath‑hold walking. We’ll cover what CO₂ “tolerance” actually means; how apnea walking differs from other breath‑hold work; the physiology you need (CO₂ chemoreception, the Bohr effect, spleen responses); what the research shows (and doesn’t) about performance; a complete, graded protocol; nasal recovery periods and why they help; SpO₂ monitoring with practical guardrails and limitations; integration with regular training; side effects and red flags; critical perspectives; and a simple action checklist. We’ll keep the tone conversational and clear while citing peer‑reviewed sources.

 

Let’s begin with plain language. CO₂ tolerance isn’t a superpower. It’s the learned ability to stay calm and functional as carbon dioxide rises and your brain’s “breathe now” alarm gets louder. That alarm is driven mainly by central and peripheral chemoreceptors—the brainstem and carotid bodies read changes in CO₂ and acidity and push ventilation accordingly.¹⁻³ When CO₂ goes up, pH drops, and hemoglobin loosens its grip on oxygen—the Bohr effect—which helps unload oxygen to working muscles.⁴,⁵ If you blow off too much CO₂ by over‑breathing, you temporarily shift hemoglobin in the other direction and can impair oxygen delivery, especially during intense work.⁴ That’s why CO₂ exposure work is about composure, not gasping contests.

 

Where does apnea walking fit? It’s a dry‑land drill in which you exhale to a comfortable resting lung volume, hold your breath, and walk at an easy pace until the first strong urge to breathe, then recover through the nose before repeating. Freediving schools popularized it because it’s simple, portable, and—done away from water with a partner—relatively controllable. Coaching materials describe it as a way to build hypercapnic comfort and to rehearse moving efficiently under a rising ventilatory drive.⁶ The peer‑reviewed literature doesn’t study “apnea walking” as a named protocol often, but it does study voluntary hypoventilation at low lung volume (VHL)—the same mechanics of end‑expiratory breath holds layered onto exercise—and static or dynamic apnea blocks on land. Those studies give us the best evidence we have for expected responses and training effects.⁷⁻¹⁵

 

What does the research say? On repeated‑sprint performance, several trials and a recent meta‑analysis report that adding brief end‑expiratory breath holds to intervals can improve repeated‑sprint ability within 3–6 weeks compared with the same training while breathing normally. A 4‑week study in basketball players (n=17; 8 control, 9 VHL) found that VHL reduced sprint‑to‑sprint performance decrements and improved muscle reoxygenation during recovery, without changing maximal speed.⁷ A broader synthesis in 2025 concluded that repeated‑sprint training in hypoxia induced by VHL confers likely gains in fatigue resistance versus normal breathing, though mechanisms remain under investigation.⁸ Complementary work shows altered glycolytic contribution and buffering after VHL blocks and hypoxemia levels comparable to simulated altitudes when end‑expiratory holds are used during effort.¹¹,¹³ These effects appear specific: VHL tends to help short, repeated efforts more than continuous endurance.

 

On hematology and the “built‑in oxygen bank,” serial apneas trigger spleen contraction, transiently raising circulating red cells and hemoglobin for minutes. In a classic experiment with 20 volunteers—10 with spleens, 10 splenectomized—five cold‑face apneas increased hemoglobin by ~3.3% and prolonged subsequent apneas only in the spleen‑intact group.¹⁶ Over weeks, static apnea training increased resting spleen volume by ~24% in healthy adults after eight weeks (daily sets), suggesting a slow structural adaptation even if the acute contraction amplitude didn’t change.¹⁷ Later studies confirm dose requirements and show exercise intensity and breath‑hold both evoke graded splenic responses.¹⁸⁻²⁰ That doesn’t mean apnea walking boosts hemoglobin long‑term like altitude camps, but it does mean short‑term oxygen‑carrying capacity can rise within a session via the spleen reflex.

 

What about nasal recovery periods? Nasal breathing routes air across paranasal sinuses that produce nitric oxide (NO), which reaches the lungs and can influence ventilation‑perfusion matching and pulmonary blood flow. Early and later work by Lundberg and colleagues measured high NO concentrations in sinus air and described its transport into the lower airways.²¹⁻²⁴ Trials in clinical populations (e.g., heart failure or coronary disease) indicate nasal breathing during submaximal exercise can improve ventilatory efficiency versus oral breathing, though performance benefits in healthy athletes remain uncertain.²⁵,²⁶ In practice, nasal recovery promotes slower, deeper breaths, tends to reduce hyperventilation, and offers a consistent cadence between holds, which is exactly what you need while training CO₂ composure.

 

SpO₂ monitoring: useful if you respect its limits. Clinical references treat SpO₂ below ~90% as hypoxemia, but there’s no single “magic” cut‑off for tissue hypoxia, and readings lag and vary with motion.²⁷⁻³¹ Medical‑grade finger oximeters can be accurate within about ±2–3 percentage points in lab testing, yet accuracy declines with low perfusion, motion, carboxyhemoglobin, methemoglobin, nail polish, and cold fingers.³²⁻³⁶ Consumer wearables often perform worse during activity.³³,³⁷ Racial bias is also documented: devices may overestimate oxygenation in people with darker skin, which prompted FDA advisory panels and draft guidance in 2024–2025 to mandate diverse skin‑tone validation cohorts and better labeling.³⁸,³⁹ With all that in mind, treat SpO₂ as a trend tool, not a scoreboard. Set conservative stop‑rules, corroborate with how you feel, and do not chase low saturations.

 

Now the protocol, built for safety and progression. First, environment and supervision: never train near water, pools, stairs, traffic, or heavy equipment. Wear flat shoes. Train with a partner who knows your plan and stays alert. Avoid sessions if you’re ill, sleep‑deprived, pregnant, or have cardiovascular, pulmonary, or neurological conditions without medical clearance. Avoid pre‑hold hyperventilation—it can delay urge‑to‑breathe while oxygen continues to fall and has been implicated in blackouts.⁴⁰,⁴¹ Start each session with 5–8 minutes of easy walking and nasal breathing. Add two relaxed breath‑holds at rest to feel the urge‑to‑breathe scale. When you begin the work, exhale normally to a resting lung volume (don’t force the air out), hold, and start walking.

 

A conservative structure that fits most people: three blocks of 6–8 breath‑hold walks. Each walk lasts until your first strong urge to breathe or a pre‑set cap, whichever comes first. Cap week 1 holds at ~20–30 seconds or ~15–30 steps; advanced athletes can extend but should increase gradually across weeks, not within a session. Between holds, recover with nasal breathing for at least twice the hold duration, or until your breathing is quiet and steady for 30–45 seconds. Keep walking easily between reps; the pace is conversational. Keep your mouth closed on recovery unless you must switch to mouth breathing to regain control. If you use a finger oximeter, spot‑check at the end of every second or third rep while standing still; expect readings to lag by 10–20 seconds.³⁴

 

Progress weekly by adding one rep per block (up to 10), or by adding 5–10 steps to the cap, not both. If dizziness, tunnel vision, tingling, chest pain, or confusion appear, stop the session. If your spot‑check SpO₂ trends into the low 90s and you also feel unwell, or if it reads <90% more than momentarily, stop and extend recovery; if values don’t rebound promptly or symptoms persist, end the day.²⁷,²⁸ Remember that darker skin can cause falsely high readings, so symptoms and partner observation trump the number.³⁸,³⁹ Two to three sessions per week is ample. Pair apnea walks on easier days or after your primary work, not before key speed or strength sessions. VHL‑style benefits relate to repeated, brief, end‑expiratory holds—not to maximal deprivation—so keep the effort submaximal and the technique tidy.⁷,⁸,¹¹

 

Technique cues that pay off: stay relaxed. Keep your shoulders low and stride quiet to minimize energy cost. Use a mental check at 10 steps: tension scan, jaw unclench, eyes steady. End the hold at the first strong urge, not the second or third. On recovery, think “soft inhale, soft exhale,” 4–6 nasal cycles before you even consider a deeper breath. Humming lightly on the first exhale can increase nasal NO flow and help you settle, an effect measured in lab settings, though it’s not a performance hack on its own.²³

 

How does this fit with real training? For team‑sport and court athletes targeting repeated‑sprint ability, replace one short‑interval accessory block with apnea walks during a 3–6‑week microcycle; the VHL literature suggests adaptations emerge on that timeline.⁷,⁸ Endurance athletes can place a short apnea‑walk block after aerobic maintenance runs to build CO₂ composure without compromising key workouts. Strength athletes may use it on off‑days for autonomic downshifting and breathing control practice. Across groups, log three variables: the number of steps per hold (or seconds), perceived urge‑to‑breathe on a 1–10 scale at stop, and the time to quiet breathing in recovery. Those tell you if your tolerance and control are improving.

 

Risks and side effects exist. Breath‑hold training can provoke syncope (fainting). On land that means falls; in water it can be fatal without rescue. Annual reports and reviews from Divers Alert Network (DAN) and AIDA emphasize never training alone and avoiding hyperventilation, because “shallow‑water blackout” is often preceded by aggressive breathing that suppresses the urge to breathe while oxygen quietly drops.⁴⁰⁻⁴³ Dry‑land protocols avoid immersion risk but not syncope; hence the conservative caps, partner supervision, and calm recovery. People with high blood pressure, arrhythmias, anemia, pulmonary disease, migraine with aura, seizure history, or recent concussion should seek medical input before attempting breath‑hold drills. If you’re on sedatives or stimulants, skip it. If you’re pregnant, skip it.

 

A critical perspective keeps us honest. Most VHL studies have small samples (often n≈12–20), short durations (3–6 weeks), and athlete‑specific contexts.⁷,¹¹,¹³ Meta‑analyses find likely benefits for repeated‑sprint ability, but mechanisms are inferred (muscle reoxygenation, altered K+ handling, glycolytic contribution) rather than proven across designs.⁷,⁸,¹¹ Spleen studies are physiologically compelling but do not demonstrate long‑term performance gains from spleen‑volume changes alone.¹⁶⁻¹⁹ Evidence for nasal breathing in healthy athletes is mixed; clinical groups show ventilatory‑efficiency improvements, while sport performance advantages remain uncertain.²⁵,²⁶ Finally, pulse‑ox data during movement are noisy and biased in ways that matter; they guide, but they don’t decide.³²⁻³⁹ All of this argues for cautious programming, steady progress, and a willingness to stop short rather than hunt for extremes.

 

If you like simple, here’s the action checklist. Pick a safe, flat route and a partner. Warm up with 5–8 minutes of easy nasal walking. Do three blocks of 6–8 end‑expiratory breath‑hold walks; cap each hold at the first strong urge or 15–30 steps in week 1. Recover nasally for at least twice the hold time while walking easily. Log steps, urge‑to‑breathe (1–10), and recovery time to quiet breathing. Progress week by week, not rep by rep. Spot‑check SpO₂ only when stationary, and stop if symptoms appear, if readings trend low alongside symptoms, or if numbers fail to rebound. Keep sessions 2–3 times weekly for 3–6 weeks, then reassess. When in doubt, do less and do it cleaner.

 

You might be wondering, “Will this help me run faster 10K times?” Not directly. The strongest data sit with short, repeated efforts, not continuous endurance.⁷,⁸ But you’ll likely gain steadier breathing under pressure, less panic at high CO₂, and better control between high‑intensity bouts. Combine that with regular training, sleep, and nutrition, and you have a stable platform for performance.

 

Summary for busy readers: apnea walking is a dry‑land, end‑expiratory breath‑hold drill to train composure under rising CO₂ while moving. Its closest research cousin—VHL layered onto intervals—improves repeated‑sprint ability in small, controlled studies over a few weeks. Spleen responses add a transient oxygen‑carrying bump within sessions; longer plans can enlarge resting spleen volume. Nasal recovery periods help regulate breathing and may support gas exchange; they’re practical even if not a performance panacea. SpO₂ monitoring can add insight but has lag, motion errors, and skin‑tone bias, so use conservative guardrails and focus on symptoms and technique. Keep sessions short, frequent, and tidy; never train alone or near water; and integrate the work thoughtfully.

 

Call to action. If this approach fits your goals, try the three‑block plan for the next month, log your metrics, and share your questions and results. If you coach, pilot it with one squad under clear safety rules and compare repeated‑sprint tests before and after. If you’re curious about the mechanisms, follow new VHL and apnea‑training trials as they publish.

 

Disclaimer. This educational content does not diagnose, treat, or prevent disease. Breath‑hold training carries risk of syncope and injury. Do not train alone or near water. Consult a qualified clinician before beginning if you have any cardiovascular, pulmonary, neurological, hematologic, or pregnancy‑related conditions. Pulse‑oximeter readings may be inaccurate in motion and across skin tones and should not replace clinical judgment. Follow local laws and organizational safety standards.

 

References

1. Benner A, Brewer K. Physiology, Carbon Dioxide Response Curve. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023. (https://www.ncbi.nlm.nih.gov/books/NBK538146/).

2. Brinkman JE, Sharma S. Physiology, Respiratory Drive. StatPearls [Internet]. 2023. (https://www.ncbi.nlm.nih.gov/books/NBK482414/).

3. Dempsey JA, et al. Rethinking O₂, CO₂ and breathing during wakefulness and sleep. J Physiol. 2024. (https://doi.org/10.1113/JP284551).

4. Benner A, Al‑Ani M. Physiology, Bohr Effect. StatPearls [Internet]. 2023. (https://www.ncbi.nlm.nih.gov/books/NBK526028/).

5. Malte H, et al. The Bohr/Haldane effect: a model‑based uncovering of the full... J Appl Physiol. 2018. (https://doi.org/10.1152/japplphysiol.00140.2018).

6. Performance Freediving International (PFI). What is an apnea walk? 2020. (https://www.tdisdi.com/pfi-diver-news/what-is-an-apnea-walk/).

7. Lapointe J, et al. Impact of hypoventilation training on muscle oxygenation and RSA in basketball players. Front Sports Act Living. 2020;2:29. (https://www.frontiersin.org/articles/10.3389/fspor.2020.00029/full).

8. Précart C, et al. Repeated‑sprint training in hypoxia induced by voluntary hypoventilation at low lung volume: Meta‑analysis. Sports Med Open. 2025;11(1):55. (PMC12084195).

9. Woorons X, et al. Effects of a 4‑week training with voluntary hypoventilation at low pulmonary volumes. Respir Physiol Neurobiol. 2008. (https://doi.org/10.1016/S1569-9048%2807%2900232-7).

10. Woorons X, et al. Physiological adaptations to repeated‑sprint training in hypoxia induced by voluntary hypoventilation at low lung volume. Eur J Appl Physiol. 2019;119:1959–1970. (https://pubmed.ncbi.nlm.nih.gov/31286240/).

11. Woorons X, Millet GP, Mucci P. Hypoventilation training at supramaximal intensity improves anaerobic performance. Med Sci Sports Exerc. 2016.

12. Elia A, et al. Six weeks of dynamic apnoeic training stimulates cardiorespiratory adaptations. Eur J Appl Physiol. 2020;120(2):441–453. (https://pmc.ncbi.nlm.nih.gov/articles/PMC7892731/).

13. Cardiorespiratory and muscle oxygenation responses to VHL. Eur J Appl Physiol. 2024. (https://link.springer.com/article/10.1007/s00421-024-05569-1).

14. de Asís‑Fernández F, et al. Effects of apnoea training on aerobic and anaerobic metabolism. Biology (Basel). 2022. (https://pmc.ncbi.nlm.nih.gov/articles/PMC9551563/).

15. Patrician A, et al. Breath‑Hold Diving—The Physiology of Diving Deep and Returning. Front Physiol. 2021;12:639377. (https://pmc.ncbi.nlm.nih.gov/articles/PMC8176094/).

16. Schagatay E, et al. Role of spleen emptying in prolonging apneas in humans. J Appl Physiol. 2001;90(4):1623–1629. (https://pubmed.ncbi.nlm.nih.gov/11247970/).

17. Bouten J, et al. Eight weeks of static apnea training increases spleen volume but not acute spleen contraction. Respir Physiol Neurobiol. 2019;266:144–149. (https://pubmed.ncbi.nlm.nih.gov/31009754/).

18. Lindblom H, et al. Effect of exercise intensity and apnea on splenic contraction and hemoglobin. Eur J Appl Physiol. 2024. (https://pmc.ncbi.nlm.nih.gov/articles/PMC11199288/).

19. Keeler JM, et al. Increased spleen volume with head‑out water immersion. Am J Physiol Regul Integr Comp Physiol. 2022. (https://pmc.ncbi.nlm.nih.gov/articles/PMC9639762/).

20. Persson G, et al. Splenic contraction and cardiovascular responses are individualized adaptations. Front Physiol. 2023. (https://pmc.ncbi.nlm.nih.gov/articles/PMC10028099/).

21. Lundberg JON, et al. High nitric oxide production in human paranasal sinuses. Nat Med. 1995;1:370–373. (https://pubmed.ncbi.nlm.nih.gov/7585069/).

22. Dubois AB, et al. Production and absorption of nitric oxide gas in the nose. J Appl Physiol. 1998;84(4):1217–1224. (https://journals.physiology.org/doi/abs/10.1152/jappl.1998.84.4.1217).

23. Maniscalco M, et al. Humming increases nasal nitric oxide. Eur Respir J. 2003;22(2):323–327. (https://publications.ersnet.org/content/erj/22/2/323).

24. Lundberg JO, Weitzberg E. Nitric oxide and the paranasal sinuses. Rhinology. 2008;46(1):1–7. (https://pubmed.ncbi.nlm.nih.gov/18951492/).

25. Eser P, et al. Improved ventilatory efficiency with nasal breathing during submaximal exercise in HF/CCS. Front Physiol. 2024;15:1380562. (https://www.frontiersin.org/articles/10.3389/fphys.2024.1380562/full).

26. Lörinczi F, et al. Acute effect of different breathing regimens on muscular endurance. BMC Sports Sci Med Rehabil. 2024;16:18. (https://bmcsportsscimedrehabil.biomedcentral.com/articles/10.1186/s13102-024-00840-6).

27. Torp KD, et al. Pulse Oximetry. StatPearls [Internet]. 2023. (https://www.ncbi.nlm.nih.gov/books/NBK470348/).

28. Open Critical Care. Oxygenation terminology. 2023. (https://opencriticalcare.org/encyclopedia/physiologic-oxygen-terminology/).

29. Norton LH, et al. Accuracy of pulse oximetry during exhaustive exercise. Int J Sports Med. 1992. (https://pubmed.ncbi.nlm.nih.gov/1459747/).

30. Barker SJ, Shah NK. The effects of motion on pulse oximetry performance. Anesth Analg. 1997. (https://pubmed.ncbi.nlm.nih.gov/9009945/).

31. DeMeulenaere S. Pulse oximetry: uses and limitations. Nurse Pract. 2007;32(4):13–18. (https://www.npjournal.org/article/S1555-4155%2807%2900210-3/fulltext).

32. Marinari S, et al. Accuracy of a new pulse oximeter vs reference. Sensors (Basel). 2022;22(13):5031. (https://www.mdpi.com/1424-8220/22/13/5031).

33. Kovesi T, et al. Accuracy of readily available consumer‑grade oximeters. Can Respir J. 2024. (https://pmc.ncbi.nlm.nih.gov/articles/PMC11108112/).

34. Bent B, et al. Investigating sources of inaccuracy in wearable optical sensors. npj Digit Med. 2020;3:18. (https://www.nature.com/articles/s41746-020-0226-6).

35. FDA. Pulse oximeters: Technology, accuracy limitations, and considerations. 2024. (https://www.fda.gov/media/176236/download).

36. FDA. Executive summary—Performance evaluation of pulse oximeters. 2024. (https://www.fda.gov/media/175828/download).

37. Rajakariar K, et al. Smartwatch pulse oximetry in hospitalized patients. MCP Digital Health. 2024. (https://www.mcpdigitalhealth.org/article/S2949-7612%2824%2900010-5/fulltext).

38. Sjoding MW, et al. Racial bias in pulse oximetry measurement. N Engl J Med. 2020;383:2477–2478. (https://www.nejm.org/doi/full/10.1056/NEJMc2029240).

39. Reuters. FDA seeks to boost blood‑oxygen monitor accuracy across skin tones. Jan 6, 2025. (https://www.reuters.com/business/healthcare-pharmaceuticals/fda-seeks-boost-blood-oxygen-device-accuracy-across-skin-tones-2025-01-06/) and AP News. Jan 2025. (https://apnews.com/article/d5fde9b81251ac9d4e39c11264638909).

40. Divers Alert Network (DAN). Freediving safety awareness. 2023. (https://dan.org/alert-diver/article/freediving-safety-awareness/).

41. DAN Southern Africa. Freediving risk assessment. 2024. (https://www.dansa.org/blog/2024/05/21/freediving-risk-assessment).

42. DAN. Breath‑hold diving—Annual diving report 2020 edition. 2021. (https://www.ncbi.nlm.nih.gov/books/NBK582509/).

43. AIDA International. Competition rules—Blackout guidance. 2019. (https://www.aidainternational.org/PostDetails/78).

 

Strong finish: Master the calm first, then add the steps—because in breath‑hold work, control beats bravado every time.