“How should I breathe when I run?”
I get asked this question all the time. We often take breathing for granted.
Many runners are told to “belly breathe,” to breathe from their diaphragm and take deeper breaths to take in more oxygen. However, the main stimulus to breathe, especially at sea-level, is to exhale carbon dioxide, as the partial pressure of CO2 rises in the blood from metabolism. At sea-level, your blood is nearly 100% saturated with oxygen, even while running a race, so it’s fruitless to take deeper breaths since what matters is how much oxygen is transported in your blood, not how much oxygen is in your lungs. The best distance runners in the world are small people, and so they have small lungs. When I measured total lung capacity of runners in the lab, I found that the smaller runners had smaller lung volumes, even though they were better runners. There is no relationship between your lung volume and how fast you run a 5K or marathon.
Since your diaphragm and other breathing muscles also use oxygen, the muscle contractions associated with deeper breaths can potentially “steal” some of the oxygen your leg muscles need to run. When you travel (or live) at higher altitude, you breathe more to compensate for the decreased partial pressure of oxygen in the environmental air. (Oxygen molecules travel from areas of higher pressure to areas of lower pressure, so the greater the difference in oxygen pressure between the environmental air and the air in our lungs, the greater the driving force for oxygen to travel from the environment into our lungs. The higher up in altitude you go, the less the pressure difference between the environmental air and the air in our lungs.)
While taking deeper breaths won’t help your running, coordinating your breathing to your stride rate can. Most animals, like the cheetah, coordinate their breathing patterns to their stride rates because of how the movement of their forelimbs assists the movement of their chest cavity. And so four-legged animals take a 1-to-1 step-to-breath ratio: For every step, they take one breath.
This coordination of breathing to stride rate, which is called “entrainment,” also happens in humans, although it is not as tightly coupled since humans have a greater anatomical separation between the movement of their legs and chest cavity. In humans, the entrainment of breathing to stride rate is linked to running experience, with more fit and experienced runners exhibiting greater entrainment. For my doctoral dissertation, I measured this entrainment of breathing and stride rate, and found that human runners take 3 or 4 steps per breath (inhalation + exhalation) when running at easy to moderate paces, and two steps per breath when running fast. With many miles of training, runners may learn how to most effectively ventilate their lungs and minimize the metabolic cost of breathing, which can improve running economy (the amount of oxygen you use to maintain a given submaximal pace).
Although the entrainment of breathing to stride rate seems to happen naturally with training, you can voluntarily coordinate the two rhythms. When you run, begin to exhale when either your right foot or left foot lands on the ground, and go through a complete breath (exhalation + inhalation) every 3 to 4 steps, always exhaling when your foot lands on the ground. When you run fast, try to take two steps per breath.