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Take a deep breath

In my experience, running athletes often assume that breathing through the mouth is superior given it allows greater amount of oxygen, and to the extend breathing training is useful at all it should focus on developing the correct breathing pattern. This attitude may come from an under appreciation of the complexity and multifaceted roles of the respiratory system, which is responsible for a much greater range of activities in the body than the simple transfer of oxygen from the external air to the blood and the removal of carbon dioxide.

Rediscovering the respiratory system

The respiratory system includes not only the lungs, airways, and blood vessels but also the respiratory muscles (e.g., diaphragm and obliques), which when fatigued may become a limiting factor in demanding endurance events such as marathon or ultra. The respiratory system communicates with the sympathetic and parasympathetic nervous systems, which impacts heart-rate, fatigue and recovery. Charles Levin and Steven Soap from the department of biology at Williams College, Massachusetts, USA showed the positive impact of deep breathing and alternate nostril breathing on heart rate variability. This finding is not entirely surprising given that the act of controlling one’s breath for the purpose of restoring or enhancing one’s health has been practised for thousands of years amongst Eastern cultures. The respiratory system also regulates the pH level in the blood, a critical homeostatic process. Respiratory alkalosis is the result of hyperventilation or an excessive removal of carbon dioxide (“CO2”), while respiratory acidosis is the result of hypoventilation or an impaired ability to eliminate CO2. The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2 via chemoreceptor cells in the carotide and brainstem. Finally the respiratory system is responsible for the production of nitric oxide (“NO”) in the paranasal sinuses (part of the nasal airways). Although the research is on-going, it appears that the role of NO in the sinuses is likely to enhance local host defense mechanisms, and that with every breath NO reaches the lungs to enhance pulmonary oxygen uptake via local vasodilation, supporting the ancient notion found in yoga, and more recently advocated by James Nestor in his book “Breath”, to the benefit of breathing through the nose.

The complexity of the respiratory system highlights the need to understand potential limiting factors, such as breathing muscle strength, chemoreceptor sensitivity, as well as weight the different effects before following specific recommendations, such as mouth breathing or forcing a given breathing pattern.

The role of CO2 & hypoxic/hypercapnic training

One potential misconception among runners is that less CO2 is better on the basis that CO2 is seen as a “waste” responsible for acidosis. This belief leads to forcing higher breathing frequency and using mouth versus nose breathing technique. Although accumulating excess CO2 is possible at high intensity, it’s often not the case for most of the sub-max running intensities. It's also important to take into account the Bohr effect, whereby an increase in CO2 concentration leads to an increase in oxygen delivery to the active muscles by lowering the bonding force between the molecules of O2 and its blood carrier, the hemoglobin (“Hb”).

The benefit of altitude hypoxic training on exercise economy and aerobic exercise performance is well documented. In addition to the well-known adaptations, which include increased in level of

  • haemoglobin,

  • oxidative enzyme activity,

  • mitochondrial volume,

  • free fatty acid utilization, and

  • capillary density

hypoxic training seems to decrease CO2 chemoreceptors’ sensitivity. This decrease sensitivity leads to a "vicious cyle": the increase in ventilation causes expiration of larger than normal quantities of CO2, which results in hypocapnia and respiratory alkalosis. The effect of the shift toward alkalosis inhibits the central respiratory center and to compensate for the alkalosis, the kidney increases excretion of bicarbonate and reduces clearance of hydrogen ions, lowering the pH and allowing ventilation to increase further.

Recent research conducted by Zoretić et al. at the University of Zagreb with 16 elite swimmers following an 8-week hypercapnic-hypoxic training program have shown an improvement in performance along with a 5.35% higher Hb concentration at the end of the program, which also caused a 10.79% increase in the VO2max. Keeping in mind the limitation of the study given the small sample size, the results are nevertheless consistent with larger studies conducted with elite free-divers and probably worth incorporating as part of a training plan for advanced athletes.

Effect of respiratory muscle training on performance

It is often believed that the lungs, airways, and respiratory muscles of healthy humans are ‘‘overbuilt’’ for the ventilatory and gas-exchange demands imposed by short-term exercise of any intensity and consequently that the specific respiratory muscles training (“RMT”) is not necessary. Although there is some truth in the above statement, it can’t be generalised. A meta-analysis conducted by Illi et al. (2012) incorporating 46 original studies revealed significant improvement in performance after RMT on constant load tests, time trials and intermittent incremental tests. As expected, less fit subjects benefit more from RMT than highly trained athletes. Dempsey et al. (2008) outlined how three respiratory system mechanisms, mainly arterial HbO2 desaturation, respiratory muscle work, and inspiratory and expiratory intrathoracic pressures, can become significant exercise limiting factors as the level of fitness required rises.

Consequently, it may make sense for runners to perform spirometer and metabolic tests measuring both static and dynamic breathing capacity (e.g., FVC, VC, etc.) and frequency (BF, VT, etc.) to determine whether respiratory capacity or utilization is a limiting factor.


We have briefly described the multifaceted functions of the respiratory system, which can act as a limiting factor in the progression of runners. We have highlighted two areas, namely hypoxic/hypercapnic training and RMT, potentially worth including in a well-rounded training program in the case there are found to be limiting factors. We also discussed the role of CO2 and NO, showing that outside high intensity effort, one may benefit from nasal breathing, which triggers NO production and deeper breathing, both promoting better gas exchange as well as avoiding potential respiratory alkalosis.

There is clearly no one-size fit all solution but mouth breathing following a 3-3 or 3-2 pattern is unlikely to be optimum for most runners.

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