A side stitch (also called a side ache, a side cramp, a side crampie, a side sticker, a muscle stitch, or simply the stitch) is an intense stabbing pain under the lower edge of the ribcage that occurs while exercising. It is also referred to as exercise-related transient abdominal pain (ETAP). Some people think that this abdominal pain may be caused by the internal organs (like the liver and stomach) pulling downwards on the diaphragm, but that hypothesis is inconsistent with its frequent occurrence during swimming, which involves almost no downward force on these organs. If the pain is present only when exercising and is completely absent at rest, in an otherwise healthy person, it does not require investigation.
There is no single precise known reason for a stitch to occur. There are, however, a number of popular theories as to what may cause, increase the chances of, or otherwise exacerbate a stitch. A leading theory is that the pain may be caused by an increase in blood flow to the liver or spleen. Increases in the heart rate during exercise will force extra red blood cells into the liver which can cause temporary hepatomegaly and portal hypertension. Temporary hepatomegaly and portal hypertension can restrict blood flow through the portal vein of the liver thus slowing blood flow to the rest of the body; this is why most runner's cramps are felt on the right side near the liver. A plausible mechanism for the pain is that high internal pressure in the liver or spleen restricts blood flow, causing hypoxia.
There are other theories regarding side stitches than simple stretching of the visceral ligaments due to repeated vertical translation and jolting. Such theories include diaphragmatic ischemia, imbalances of the thoracic spine, irritation of the parietal peritoneum and strain on visceral ligaments by a fluid-engorged gut.
A further theory points to shallow breathing as a possible cause for a stitch and one possible preventative measure involves adjusting at what point in a runner's stride they inhale, or reducing the frequency of inhales (with an increase in inspiratory capacity).
The reasons for the variety of theories include, in particular, the prevalence of ETAP during swimming.
Most of the time, side stitches occur on the right side of the body. This may be because the largest organ in the abdominal cavity, the liver, is on that side. Certain athletes also report a pain in the tip of their shoulder blade. This is believed to be because this is a referred site of pain for the diaphragm via the phrenic nerve.[better source needed] When the side stitch is on the right side, published advice is to try to exhale when the left foot lands.
There is also a belief that an imbalance of electrolytes (such as calcium, potassium, and sodium) in the blood could also contribute to the side stitch.
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