When exercising, a side stitch (also called a side cramp, a side sticker or, more commonly in proper English, simply a stitch) is an intense stabbing pain under the lower edge of the ribcage. It is also referred to as exercise related transient abdominal pain (ETAP). This pain may be caused by the internal organs (like the liver and stomach) pulling downwards on the diaphragm. It is therefore more likely to occur in sports involving up and down actions - like running, jumping, swimming and equestrianism.
1. Conventional explanation: This very common pain is caused by contraction of the liver or spleen, which squeezes extra oxygen-carrying red blood cells into the circulation. Although there does not appear to be much muscle in the capsule of the spleen, there is direct & indirect evidence that its size does change with exercise. (see ref** below) This autotransfusion, (which is much larger in some animals,) increases excercise capacity but the associated pain may be severe, relieved only by rest. A plausible mechanism for the pain is that high internal pressure in the liver or spleen restricts blood flow, causing hypoxia. The pain may be referred to the shoulder. If the pain is present only when excercising and is completely absent at rest, in an otherwise healthy person, it is benign and does not require investigation. As this is a normal physiological response, there is probably not much you can do about it, although starting up more gradually may be helpful.
2. Diaphragmatic Ischemia
3. Imbalances of the thoracic spine
4. Irritation of the parietal peritoneum
The reasons for these 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 is due to the largest organ in the abdominal cavity, the liver, being 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.
These alternatives work by implementing the aforementioned function in combination with a coordinated task to occupy the sufferer's mind.
ref** 1) Sports Medicine 32(6): 2002. 261-269. The human spleen during physiological stress. Stewart & McKenzie
2) Clin Nucl Med. 1995 Oct;20(10):884-7. The effect of excercise on normal splenic volume measured with SPECT. Otto et al.
3) J Appl Physiol 74: 1024-1026,1993; Spleen emptying and venous hematocrit in humans during excercise. Laub et al.