What Causes a Side Stitch?

The pain is caused by a spasm of the diaphragm muscle. The diaphragm is a dome shaped muscle that separates the thoracic (lung) cavity from the abdominal cavity and moves up and down when you inhale and exhale. To understand why the diaphragm muscle spasms, we need to understand what is happening during exercise: Let’s use the sport of running as an example.

It is interesting to note that more than 70% of humans exhale when their left foot strikes the ground, while less than 30% exhale when their right foot hits the ground.

When you inhale, your lungs fill with air and force your diaphragm downward. Conversely, when you exhale your lungs contract and your diaphragm rises. This pattern of rising and falling occurs quite rapidly when you’re running and as most side stitches occur on the right hand side, consider what happens to your diaphragm when your right foot strikes the ground.

As your right foot strikes the ground, gravity forces your internal organs downward. Some of these organs are attached to the diaphragm, which in turn pulls the diaphragm downward. Now if you’re also exhaling at the same time as your right foot hits the ground, your diaphragm is being pulled upward as your lungs contract. This creates a stretching of the diaphragm muscle and the ligaments that are attached to your internal organs, which in turn causes the pain.