Hank Green of SciShow explains the various theories about what causes the annoying “side stitch”, a pain that usually presents during exercise. Some scientists believe that it’s caused by contraction of the diaphragm, while others believe that it’s the jostling nature of exercise that causes temporary trauma to the body.
But the most likely explanation seems to point to the peritoneum, a two-layered membrane that lines your abdominal wall and helps support your organs. Now, normally, there’s fluid in between the layers to make sure they don’t scrape together too much. Because when they do, you end up with that sharp pain.
Luckily, a stitch is only temporary and can be relieved rather easily.
…wait a while after a big meal before jumping in the pool or going out for a run. Working to strengthen your core might help, too. That should reduce the movement in your abdomen while you’re exercising, meaning less strain on your ligaments and membranes.