Breathing Mechanics and Performance: What Nasal Breathing, CO2 Tolerance, and Respiratory Muscle Training Do

Breathing seems to be having a moment. Some claims are well supported by physiology and clinical research. Others are overstated or outright wrong. The challenge is telling the difference.

This post breaks down the science of breathing mechanics as they relate to exercise performance: what happens in your respiratory system during effort, why carbon dioxide tolerance matters more than most people realize, what the evidence says about training your respiratory muscles, and when nasal breathing is genuinely beneficial versus when it is just limiting your ventilation.

How breathing works during exercise: the basics

At rest, you breathe approximately 12 to 20 times per minute, moving roughly 6 to 8 liters of air per minute (your minute ventilation). During maximal exercise, minute ventilation can increase to 100 to 200 liters per minute in trained individuals, achieved through increases in both breathing rate and tidal volume (the amount of air per breath).

This massive increase in ventilation is not driven primarily by a need for more oxygen. At moderate intensities, hemoglobin oxygen saturation remains close to 98 percent, your blood is almost fully saturated with O2. What drives the ventilatory response is primarily the need to remove carbon dioxide and manage blood pH.

During exercise, your working muscles produce CO2 as a metabolic byproduct. CO2 dissolves in blood to form carbonic acid, which dissociates into bicarbonate and hydrogen ions. The rising hydrogen ion concentration (falling pH) is detected by central and peripheral chemoreceptors, which stimulate the respiratory centers in the brainstem to increase ventilation. In simple terms: you breathe harder mainly to blow off CO2, not to get more oxygen in.

This distinction matters because it reframes how we should think about breathing limitations. For most people, the ceiling on performance is not oxygen delivery to the lungs: it is one or more of the following: the capacity of the respiratory muscles to sustain high ventilation rates, the efficiency of gas exchange at the alveolar level, and the tolerance of the nervous system for elevated CO2 levels.

Carbon dioxide tolerance: the underappreciated variable

CO2 tolerance refers to your body's ability to maintain composure (so to speak), both physiologically and psychologically, in the presence of elevated blood CO2 levels. When CO2 rises, most people experience an increasing sense of air hunger, anxiety, and the urge to breathe faster. This response is mediated by chemoreceptors and has a significant learned component: you can, within limits, train your nervous system to tolerate higher CO2 levels before the urge to breathe becomes overwhelming.

Why does this matter for performance? Because many recreational athletes hyperventilate, they breathe faster than metabolically necessary, driven by discomfort with the sensation of rising CO2 rather than an actual physiological need for more oxygen. This hyperventilation blows off too much CO2, causing respiratory alkalosis (rising blood pH), which paradoxically impairs oxygen delivery to tissues via the Bohr effect. The Bohr effect describes how hemoglobin's affinity for oxygen increases when blood pH rises, meaning hemoglobin holds onto oxygen more tightly and releases less of it to working muscles.

In other words, breathing too much during moderate-intensity exercise can actually reduce oxygen delivery to the tissues that need it most. Learning to tolerate a slightly higher CO2 level allows you to breathe more efficiently, maintain a more appropriate blood pH, and improve oxygen offloading to muscles.

Dempsey et al. (2008) suggests that ventilatory inefficiency, including excessive ventilatory drive relative to metabolic demand, is a significant contributor to perceived exertion and exercise intolerance, particularly in less-trained individuals.

CO2 tolerance exercises typically involve controlled breath holds, extended exhales, or reduced-frequency breathing during low-intensity exercise. These are low-risk practices for healthy individuals and can meaningfully improve breathing efficiency. However, they should be progressed gradually, and individuals with asthma, panic disorder, or cardiovascular conditions should consult with a professional before attempting breath-hold training.

Nasal breathing: when it helps and when it doesn’t

Nasal breathing has become heavily promoted in fitness circles, with proponents claiming benefits ranging from better oxygen utilization to reduced cortisol.

What nasal breathing does: it warms and humidifies inhaled air (reducing airway irritation and exercise-induced bronchoconstriction), filters particulate matter, and stimulates the production of nitric oxide in the nasal sinuses. Nitric oxide is a potent vasodilator, Lundberg et al. (1995) showed that nasal breathing delivers significantly more NO to the lungs than mouth breathing, which may improve local pulmonary blood flow and gas exchange efficiency.

Recinto et al. (2017) published a study comparing nasal-only versus oral-only breathing during submaximal exercise. Nasal breathing resulted similar oxygen consumption, and a lower ventilatory equivalent for oxygen (VE/VO2), meaning subjects achieved the same metabolic work with less ventilatory effort, but also results in higher heart rates…meaning potentially greater cardiovascular stress.

The nasal passages create significantly more resistance to airflow than the mouth. At rest and during low-to-moderate intensity exercise, this is fine. But once exercise intensity increases to roughly 75-85% of VO2 max (depending on the individual), nasal airflow cannot meet ventilatory demands (see here). Forcing nasal-only breathing above this threshold creates excessive work for the respiratory muscles, increases the sensation of breathlessness, and limits performance.

The practical takeaway: nasal breathing is a useful training tool for warm-ups, cooldowns, Zone 2 aerobic work, and CO2 tolerance development. During high-intensity intervals, threshold work, or competition, breathe through both your nose and mouth. Dogmatic nasal-only breathing at all intensities is not supported by the evidence and will limit your performance when it matters most.

Respiratory muscle training: what the research shows

Your diaphragm, intercostals (muscles between your ribs), and accessory respiratory muscles are skeletal muscles. Like any skeletal muscle, they can be trained to become stronger and more fatigue-resistant. Respiratory muscle training (RMT) devices, typically inspiratory muscle trainers (IMT) that add resistance to inhalation, have been studied extensively in both athletic and clinical populations.

HajGhanbari et al. (2013) published a meta-analysis examining the effects of RMT on exercise performance. Across 21 studies, the review found that IMT improved maximal inspiratory pressure produced small but statistically significant improvements in time trial performance, time to exhaustion, and perceived exertion during exercise. The effects were more pronounced in less-trained individuals and in endurance events lasting longer than several minutes.

The proposed mechanism is the "respiratory muscle metaboreflex." When respiratory muscles fatigue during heavy exercise, they trigger a sympathetic vasoconstrictor response that redirects blood flow away from locomotor muscles, essentially stealing blood from your legs to keep your diaphragm working. By training the respiratory muscles to resist fatigue, IMT may delay the onset of this metaboreflex, preserving blood flow to working muscles for longer.

Important caveats: the performance improvements from RMT are small. For competitive athletes where marginal gains matter, this is meaningful. For recreational exercisers, the benefit may not justify the daily training commitment. Additionally, RMT does not improve VO2 max or change lung volume, its benefits are specific to respiratory muscle endurance and the downstream effects on blood flow redistribution.

Diaphragmatic breathing: the foundation

Before considering any advanced breathing technique, it is worth assessing whether you are using your primary breathing muscle effectively. The diaphragm is a dome-shaped muscle that sits at the base of the rib cage. When it contracts, it descends, creating negative pressure in the thoracic cavity that draws air into the lungs. This is an incredibly efficient pattern of breathing, it maximizes tidal volume with minimal energy cost.

Many adults, particularly those with desk jobs, chronic stress, or a history of low back pain, default to an upper-chest breathing pattern that relies heavily on accessory muscles (scalenes, sternocleidomastoid, upper trapezius).

Reassessing your breathing pattern does not require fancy equipment. Place one hand on your chest and one on your belly. During relaxed breathing, the hand on your belly should move first and more prominently. If the hand on your chest rises first or your shoulders elevate with each breath, you are likely over-relying on accessory muscles.

P.S. Your nasal strips don’t do anything, I’m sorry (read here). Don’t believe me? Read even more. Still think I’m mistaken, check this out! There’s an abundance of evidence. Maybe, just maybe, if you have some sort of nasal obstruction, a nasal strip could be helpful in improving airflow; however, for most of us, this isn’t the case.