Everyone knows that running is a great cardiovascular exercise, but does running help you build abs? Fitness experts say, "yes." Running is a total body exercise. Running helps you build up abs while simultaneously reducing your body fat, which allows your abs to become visible.
Specifically, running helps you build your rectus abdominis, the long muscle that people are referring to when they say they want a "six pack." The rectus abdominis is utilized during running in order to stabilize your pelvis and back, keeping you moving forward. Your oblique muscles will also get worked as they rotate back and forth when you run.
Many people spend a lot of time doing crunches and they don't understand why they don't see results. Crunches and other calisthenics are great, but if you don't include cardiovascular exercise (like running) to help you build your abs, you wont see results. You might not think about your upper body when you are running, but the strength that propels you forward when running should come from your upper body first. Ironically, your upper body should be doing more work that your legs.
Next time you are out for a run, place one hand on your stomach and you will be able to feel the muscles working. To build your abs even more while you are running, make sure you are moving your arms (not holding them at a stiff 90 degree angle) and slightly sticking out your chest. This will force you to run taller and rely on your abs more.