The Stitch in Your Side

It shows up mid-run, mid-stride, like a sharp hook just under one side of your ribs — sudden enough to make you slow down or bend over. Almost everyone who runs has felt it. The mechanism behind it took scientists a surprisingly long time to pin down.

What it actually is

The sharp, cramping pain runners get, usually just below the ribs, has a real clinical name: exercise-related transient abdominal pain, or ETAP. The exact mechanism isn't fully settled, but the leading explanation involves the ligaments that suspend organs like the liver and stomach from the underside of the diaphragm.

The ligament-tension theory

With every running stride, the torso jostles up and down, and that motion tugs on the visceral ligaments connecting organs to the diaphragm. Combine that repetitive tugging with the diaphragm's own constant up-and-down motion from breathing, and the ligaments come under repeated strain — strain that, past a certain point, registers as pain.

Why it's more common after eating or drinking

A fuller stomach adds extra mass pulling on those same ligaments with every stride, which is a big part of why a side stitch is more likely to show up after a heavy meal or a large amount of fluid close to a run. It's also why the standard advice is to leave more time between eating and higher-intensity exercise.

Why breathing pattern matters

Most runners unconsciously exhale on the same foot-strike every time, and that habit concentrates the ligament strain on one particular side rather than distributing it evenly. Deliberately varying which foot you exhale on, or focusing on deeper belly breathing, is a commonly recommended way to reduce how often stitches occur. When one does hit, slowing down, pressing into the area, and taking a slow, deep exhale is usually enough to ease it within a few minutes.