What causes you to get a 'stitch in your side'?

We've all been there — you're just starting to find your running rhythm on the treadmill, or maybe you're hustling to catch a train, when you're suddenly struck by a sharp pain in your side, just under your rib cage.

A side stitch — medically known as "exercise-related transient abdominal pain" (ETAP) — is annoying and uncomfortable, albeit not medically serious. But what causes side stitches? 

Although side stitches are common, no one knows for sure what causes them, and there are competing theories as to what body part the pain comes from. 

For decades, after this theory was first proposed in 1941, scientists and sports doctors believed that a side stitch is caused by inadequate blood flow to the diaphragm, the large muscle that sits under the ribs and draws air in and out of the lungs. This lack of blood could cause a painful cramp or spasm. This idea was based on the theory that blood travels away from the diaphragm and concentrates in the muscles of the limbs when we exercise. 

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However, "this idea has lost popularity because the diaphragm also works harder during exercise which would redirect blood to it not away from it," Andrew Lavender, a senior lecturer in exercise and Sports Science at Federation University in Australia, told Live Science in an email.

Another idea is that the "constant jolting" that the body endures during activities like running puts strain on the liGaments that secure abdominal organs to the diaphragm, Lavender said. This theory possibly explains why runners often suffer from stitches, but it doesn't explain why a stitch can occur during swimming, rowing or cycling, he said.