The exact cause for stitches is still up for debate. The reason I have been hearing most support for lately is that the pain is due to stretching of the ligaments which extend downward from your diagphragm to hold your liver in place. When you run, your liver can get jostled about, pulling on the ligaments and causing pain.
The liver is located on the right side of your body, just below the rib cage. As you exhale, the diaphragm moves upward. As you step down on your right foot, your liver (and other organs on the right side of your body) move down. Most walkers or runners tend to have a rhythmic breathing pattern as they exercise — that is, they tend to breathe out on one foot (e.g., “exhale - step - step - step - exhale…”, or “exhale - step - exhale - step” for sprinting.) So, the theory goes, if you’re a right-footed exhaler, you are much more apt to end up with stitches.
So, the first thing to do when you get a stitch is to slow down, and press your fist into your liver, pressing it upward against your diaphragm to release the pressure. (Bending over to tie your shoe like enolancooper could also have the same effect — but generally, it’s not a great idea to drop your head below heart level when your heart is pumping away like that.) Pursing your lips and blowing out forcefully (like you are going to blow out a birthday candle) like enolancooper suggested could also be helpful, because it seems to push the diaphragm downward. Be sure you breathe in deeply, because this lets the diaphragm fully lower to reduce stress on the ligaments.
Making a conscious effort to exhale on your left foot instead of your right foot may help you to prevent stitches. Also, running downhill tends to exacerbate stitches, since you strike with a lot more force when going downhill — so if the forced breathing isn’t doing it for you, either avoid the downhills, or slow it down to a walk then. Your knees will thank you, too. (It’s probably the force of impact rather than vibration that causes the problem, Rufusleaking — that’s why, as you noted, it happens when you run, but not when you bike.)
Why does this happen to you on the treadmill, floating cheese? Probably because you run with a shorter, choppier stride on the treadmill than you would on the open road — thus you have a lot more bouncing up and down. If you’re running on a treadmill with too short a belt, you may be out of luck. If it’s long enough, though, think about lengthening your stride. You should look like you are gliding, not bouncing up and down (Think “Chariots of Fire.”) If you’re in a gym with mirrors, watch your head. Does it bob up and down? If so, your stride isn’t smooth enough, and needs to be lengthened.
What if you have a stitch on the left side? I dunno — maybe it was something you et.
Hope this helps----