We've all had this one, a sudden sharp pain in the side of the upper abdomen at the base of the ribs. The side stitch typically strikes when you're really pushing yourself and fades quickly when you slow down or stop. The stitch is particularly common for new runners still adjusting to the rigors of running.
The pain is caused by a spasm of the diaphragm, the muscle that controls your breathing. There are a number of possible reasons for this. If your breathing isn't controlled and disciplined, the diaphragm may be complaining. If you are running too soon after eating, your heavy stomach may literally be tugging at the ligaments connected to the diaphragm. Or you may simply be running too fast for your body's breathing machinery to keep up.
A stitch will usually go away quickly after just slowing down or stopping. If you're in a race or you just don't want to stop, however, you can often make it go away by bringing your breathing into careful control. Concentrate on belly breathing, pushing your belly out when you breathe in and relaxing it as you breathe out. Take deep breaths on the intake, and exhale suddenly, even noisily. To get the diaphragm to contract in rhythm with your steps, try to inhale and exhale as you land on your left foot. Strange but true, this can help prevent spasms by encouraging the diaphragm to bounce along in sync with your stride.
If the pain is just too much and you have to stop, try bending over and raising your knee on the stitch side while pressing your fingers deep into the painful area and tightening your stomach muscles. Or just walk while belly breathing.