Running causes your body to move up and down with each stride. Breathing also involves an up and down motion of your diaphragm. But what happens when your breathing and running stride are not in sync? You guessed it -- you get a side stitch.
When you inhale, your lungs fill with air and your diaphragm is pushed downward. When you exhale, your diaphragm moves up. But if your footstrike occurs during exhalation, the diaphragm gets pulled in two different directions. If this happens repeatedly, the muscle responds by going into a spasm [source: Bodyresults.com].
Side stitches are particularly common among beginners. This is due in part to the fact that beginners tend to breathe more rapidly, which never allows the diaphragm to fully relax on the downward cycle. With the continuous upward tension and downward tug brought on by the pounding of your feet on the pavement, a side stitch is more likely to occur [source: Bodyresults.com].
It's more common for side stitches to occur on the left side of the body than the right. That's because the biggest internal organ in your abdomen -- the liver -- is located on the right and is connected to the diaphragm by two ligatures [source: Johnson]. Pockets of air and gas or a belly full of food can also create an imbalance and additional stress on the diaphragm.
While runners come in all shapes, sizes and abilities, they tend to have one thing in common: They breathe out when their left foot hits the ground and breathe in when the right foot lands. This is true -- by some estimates -- in 70 percent of all runners [source: The Stretching Institute]. This is how it should be; however, if your breathing is out of whack during a particular run, you are more likely to create a tugging of the diaphragm and internal organs, which can result in the dreaded side stitch.
So, how do you treat the problem? Breathe deeply and turn to the next page.