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[Text: QQs: What's causing that stitch in your side?]

Hank: You’re running. You’re in the last quarter of the marathon. You’re a beast! A champion! You’re incredible! But then, suddenly -- you feel a sharp pain in your side. It seems to get worse and worse with every breath. It’s an attack of the side stitch! And now you have to stop running for a while so it goes away. But don’t worry! Because you’re not alone.

A 2000 study found that 69 percent of runners had experienced a stitch, also known as exercise related transient abdominal pain, in the past year. And people who exercised in other ways, like swimming or bike riding, also reported having gotten a stitch. But even though they’re so common, researchers aren’t really sure where the pain comes from.

Remarkable that we don’t know these things, even about our own bodies. One possibility is that it’s all about your diaphragm, the dome-shaped muscle between your chest and abdomen. When your diaphragm contracts, your lungs expand and fill with air. When your diaphragm expands, your lungs get smaller and the air is forced back out. But when you exercise, you might be overextending that muscle. When you’re running, for example, your foot consistently hits the pavement at the same time as you exhale -- when your diaphragm is at it’s highest and tightest -- it might strain the diaphragm, causing it to spasm. It could also be that forceful movement, like the up-and-down motion of running, bounces your internal organs around, straining the ligaments trying to keep them from sliding around inside you. Which is a really gross thought.

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. When you eat a large meal, your stomach pushes out on the inner layer, and when you’re dehydrated -- like if you’ve been sweating a lot from exercising -- there’s less fluid between the layers. The way your body’s moving around might bump the two layers against each other, too.

So how do you make the pain go away? First, just stop exercising for a bit. The pain will eventually pass -- unless it doesn’t. In which case, go see your doctor. Because I am not a doctor. And if you want to make sure it doesn’t happen again, it’s probably worth listening to your mom’s advice: 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. This way, your internal organs and the stuff inside of your abdominal cavity can stay right where they’re supposed to be, without causing stitches that slow you down.

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