If you're like the majority of runners, you've experienced this at one point or another: You're out for a run, and then in creeps a sharp, localised pain, right under one of your ribs. You think, "What is that, and how to I banish it to hell forever?" This may help. Welcome to Fitmodo , Gizmodo's gym for your brain and backbone. Don't suffer through life as a sniveling, sickly weakling -- brace up, man, get the blood pumping! Check back on Wednesdays for the latest in fitness science, workout gear, exercise techniques, and enough vim and vigor to whip you into shape. What Is a "Runner's Stitch"? It goes by many names: side stitch, runner's cramp, side ache, Beelzebub, but the medical name for it is " exercise-related transient abdominal pain ," or ETAP. Despite it being one of the most common phenomenas in the exercise world (particularly common in runners and swimmers) there is little consensus as to what precisely it is and what causes it. There are, however, commonalities in most of the prevailing theories. Virtually everyone agrees that it has to do with the diaphragm (the muscle, not the contraceptive device). The thoracic diaphragm is a sheet of muscle that extends along the bottom of the rib cage, and it's largely responsible for your breathing. As the diaphragm contracts it pulls air down into your lungs. As it relaxes, it pushes the air out. It is believed that the sensation of ETAP occurs when that muscle