Breathe In Through Your Nose and Out Through Your Mouth
There’s no evidence that you must breathe air in through your nose when running and breathe air out through your mouth. Breathe however makes you feel most comfortable.
Breathing Through Your Nose Increases the Amount of Oxygen You Take In
You don’t inhale more oxygen by breathing through your nose compared to through your mouth. That doesn’t matter anyway, since your blood is nearly 100% saturated with oxygen, even when running as fast as you can (unless you’re running at very high altitudes).
Warming Up Before a Run Helps Your Lungs Work Better
Your lungs don’t “work” like your muscles do. They are passive structures, beautifully designed with an enormous area (the size of a tennis court) and a very thin wall for the passive diffusion of oxygen and carbon dioxide. You can’t get your lungs to “work better” when you run.
Take Deeper Breaths to Get in More Oxygen
Unless you’re running at very high altitudes, oxygen has no problem diffusing from the air into your lungs. Your blood is already nearly 100% saturated with oxygen anyway. Taking deeper breaths directs more of the cardiac output to the breathing muscles, potentially “stealing” oxygen from your leg muscles.
Your Lungs Limit Your Running Performance
False (unless you have asthma or some other pulmonary pathology). At sea-level, you breathe more when you run primarily to exhale carbon dioxide, not to inhale more oxygen. To get more oxygen to your muscles, that’s your heart’s and blood’s responsibility. Don’t blame your lungs!
Running is All About Breathing
Breathing is natural; you do it without thinking. So don’t think about it. Just run. As you become fitter, you’ll notice you don’t breathe as much at the same pace. That happens because your cardiovascular system improves its ability to send blood with oxygen to your muscles.