side ache while running

The most effective way to prevent a side stitch is to avoid "shallow" breathing. Shallow breathing can be defined as taking in a small volume of air with each breath, using only a small portion of the total lung capacity. When this occurs while running, the diaphragm remains in a consistently high position and never lowers enough to allow the connective ligaments of the liver to relax. The diaphragm becomes stressed and a spasm or "stitch" results.

Instead, one should breathe "deeply", also known as "belly breathing" while running. This allows the diaphragm to fully lower and reduce the stress on it.

Here's an exercise to try. Lie down on the floor, place a hand on your belly and breathe deeply. You are belly breathing correctly if you feel your hand raise slightly. If only your chest moves up, you are not breathing deep enough.

A technique that is very successful in preventing side stitches while running, is to periodically "purse" the lips while exhaling, as if blowing out the candles on a birthday cake. Again, deep breathing is required to be effective. (This works best for me as well as most of the runner's I work with.)

Another technique that helps, is to exhale as the left foot strikes the ground, instead of the right foot. The organs attached to the diaphragm on the left side of the body aren't as big as those on the right side, so there is less strain on the diaphragm.