M.D. Until now, every explanation for the cause of a side stitch has been wrong. You're running at a fast clip and suddenly you feel a dull ache just underneath your ribs on the right side and as you keep running, the pain worsens until it hurts so much you double over. When you stop, the pain usually disappears. A side stitch is caused by stretching the ligaments that attach your liver to your diaphragm.
Humans breathe out once for every two steps.
More than 70 percent of humans breath out when their left foot hits the ground, while 30 percent breathe out when their right foot hits the ground. Those who breathe out when their right foot hits the ground are the ones most likely to suffer side stitches because the force of the right foot strike causes the liver to go down when their diaphragm goes up during breathing out. So the ligaments are stretched and hurt. When you get a side stitch, stop running immediately, reach your fingers into the right side of your belly and push your liver up. And breathe out with you lips pursed at the same time.