Belly breathing -- also called diaphragm breathing -- is widely preferred by runners over regular chest breathing because it allows them to completely fill their lungs. It also means they're exhaling more carbon dioxide and breathing in more oxygen.
When you breathe normally, the lungs expand on the inhale and compress on the exhale. With belly breathing, you use your diaphragm to help push as much air out of the lungs as possible, allowing more fresh air to come in.
If done right, you should see and feel your stomach moving in and out. You can practice belly breathing by lying on your back with a book on your stomach. As you exhale, pull your belly in toward your spine, helping to empty your lungs of as much air as possible. Imagine yawning as you inhale -- it will help you completely fill the lungs with oxygen. The book on your stomach should rise and fall with each inhalation and exhalation, while your chest will stay relatively still.
Another deep breathing exercise can be done in a standing position with your arms at your sides. As you inhale, slowly raise your arms until they're at shoulder height. This helps your rib cage move out and up, making space for the lungs to expand. As you exhale, slowly lower your arms back down.
Once you understand how to belly breathe you can begin incorporating it into your running. Keep in mind that it may not feel natural at first, but it will eventually become routine.
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