There's a fancy name for everything. Technically, a green sea turtle's scientific name is Chelonia mydas. So, to similarly complicate things, I'll have you know the technical term for a side stitch is "exercise related transient abdominal pain" (ERTAP). ERTAP's exact causes are not known, but believed to be the simultaneous combination of running-induced jarring along with inhalation and exhalation. This repeated stretching of the ligaments around the diaphragm and internal organs can result in those sudden spasms we associate with a side-stitch. Interestingly, ERTAP is more common in runners whose exhalation and right-foot strike coincide, which is likely due to ligament stretching caused by the downward jarring of the liver as the diaphragm is moving up.
Just like injuries, prevention of ERTAP is key. A stronger core lowers the chances you'll be affected by a stitch. Breathe more deeply if your breathing tends to be shallow. If you're running after you eat, allow plenty of time for the food to digest (ideally 1-2 hours, but experimentation is always necessary to account for differences in each individual). And, along similar lines, stay hydrated not only for optimal performance and health, but to lessen the chances of a side stitch as well.
Okay, I get it. But how do I stop this ERTAP when I'm in the middle of a run?
Sometimes taking deep belly breaths and switching to a left-foot strike with exhalation will ease the tightness until the stitch disappears. The majority of people (70%) strike with their left foot while exhaling, but if you're part of the minority (30%) of right-foot-strike-exhalers, it may take a little conscious deviation from such tendencies. Deep breathing helps the diaphragm fully rise and lower, allowing the ligaments to relax. However, in the case that said things don't release you from your suffering...
Runners don't usually like to stop. Traffic lights, dead ends, obstacles on the trail - these can all make us (grudgingly) come to a stop. Sometimes, attempting to run through a painful side stitch will result in only more pain until you're forced to halt. So if the first suggestion fails, your Plan B is to: stop. (But don't drop and roll at the risk of making a spectacle of yourself. I know what elementary school teaches about fire safety, and it certainly doesn't apply here.)
Next, push up on your side just under the ribcage to lift the liver slightly. Stretch the affected side by raising your arm up and leaning to the opposite side, and massaging the area may help ease your discomfort, too. Of course, some stitches are more severe than others, in which case you should resume exercise with great discretion.
But I'm in great shape. This doesn't apply to me.
Oh, yes it does! Even the best runners can get side stitches, as a large number of factors can cause distress of the abdomen in even the most physically fit athletes. Speed demons can't run away from stitches any more than the average joe. Some estimates suggest as much as 70% of the runner population has suffered ERTAP in the last year. Never say never!
So, I end this post with a friendly reminder to be proactive and prevention-minded. Happy running!