If there is pain on the left side of the abdomen when running, it is usually due to the following factors:1. The most common factor is gastrointestinal flatulence, when running with the mouth open to breathe there will be gas inhaled into the respiratory tract, digestive tract, which will lead to gas accumulation and flatulence in the gastrointestinal tract, and this will cause intestinal colic on the left side of the abdomen for a long period of time.2. If you have too much exercise when running and do not do the warm-up exercise when you have a sudden and strenuous exercise, this will lead to the abdominal tract being stretched, causing peritoneal pain or fascial stretching or strain and leading to abdominal pain. 3. cause peritoneal pain, or fascia stretching, strain and lead to abdominal pain. 3, if the amount of exercise is too large when running and the exercise is too strenuous, it will lead to mesenteric adhesion of the gastrointestinal tract, mesenteric hemorrhage, or lead to severe mechanical, strangulation intestinal obstruction, all of which will cause pain on the left side of the abdomen.