What causes a side ache when running or swimming?

Q:  What causes a side ache when running or swimming?

A:  Side ache refers to the nagging abdominal pain that tends to result from running or swimming.  The scientific term, "exercise-related transient abdominal pain" (ETAP), has plagued exercise enthusiasts for centuries and yet there is still no definitive explanation to the cause of the pain.  Below are the three leading theories:

Drinking before exercise - One recent study found people that consumed reconstituted fruit juices and beverages that were high in carbohydrates before exercise were prone to get ETAP.  ETAP didn’t seem to be related to the volume of the beverage consumed.

Diaphragmatic ischemia - During exercise, blood flow is increased to the working muscles and reduced to other parts of the body.  The theory suggests that the decreased blood flow (ischemia) to the diaphragm (muscle which expands your lungs for breathing) causes localized pain.  However, recent studies have found that runners prone to ETAP had no difference in inhalation strength compared to non-prone runners.  The findings imply that poor blood flow to the diaphragm was not a major cause of ETAP.

Ligament irritation - This theory suggests impact during exercise pulls the organs in your abdomen down, putting stress on the ligaments in the upper abdomen and results in pain.  However, side aches affect up to 75% of swimmers. Swimming does not involve the up-and-down strain found in running. Therefore, the up-and-down strain does not fully explain the origin of all ETAP.

Several theories exist, but they all have some short comings.

The following websites were used as references in answering the question:

http://www.levelrenner.com/tag/diaphragmatic-ischemia/

http://sportsmedicine.about.com/cs/injuries/a/aa053100a.htm