Side aches, also known as exercise-related transient abdominal pain (ETAP), are a common complaint among runners. They are characterized by a sharp, stabbing pain in the side that can be debilitating. Side aches can occur on either side of the abdomen and can range in severity from mild to severe.
The exact cause of side aches is unknown, but several theories exist. One theory is that side aches are caused by a lack of blood flow to the diaphragm, the muscle that separates the chest cavity from the abdominal cavity. Another theory is that side aches are caused by irritation of the nerves that supply the diaphragm. Whatever the cause, side aches can be a nuisance and can make running difficult or even impossible.