That annoying side stitch you get when you run may not be coming from mechanical factors.
It might be coming from your viscera or digestion system.
At least this is the current leading theory…
Morton & Callister ran surface EMG during stitch episodes specifically to test whether abdominal wall muscle was cramping, and titled the resulting paper "EMG activity is not elevated during exercise-related transient abdominal pain" (2008)
What to do:
1) Avoid large food/drink volumes 2 hours before exercise (possibly 3-4 hours for more susceptible people); small, regular fluid volumes during exercise may be better tolerated; avoid hypertonic beverages.
2) Use acute relief techniques (deep breathing 40%, pushing on the site 31%, stretching 22%, forward bending 18%) are all self-reported and actually somewhat contradictory as maneuvers — bending forward and stretching the site are opposite actions — which questions their real efficacy.
3) Torso support/restriction (wide supportive belt) may help, but improving functional core stability is preferable — two case studies specifically found trunk-stability exercises effective for preventing ETAP.
4) Get evaluated by a physical therapy that understands the complex mechanics involved