Standard Crunch - Pull your belly button towards your spine and flatten your lower back against the floor. Slowly contract your abdominals, bringing your shoulder blades about one or two inches off the floor. Exhale as you come up and keep your neck straight and your chin up. Slowly lower back down.
Side Plank Dips - Keep your body in a straight line (one plane) and keep the movement smooth and slow. From the straight side position slowly lower your hips down and back up. Your hips should just touch (but not rest on!) the ground. Do this exercise on both sides.
Lower Back Extensions - Lie down on a mat or carpet with your stomach to the floor. Slowly raise yourself up from your torso (using your lower back muslces) as far as you can go but do not over-extend yourself (basically at a 45 degree angle). Just to the point where you feel comfortable and you are receiving the maximum benefit. Hold this position for about 3 seconds.
Hover - Start in the plank position with your forearms and toes on the floor. Keep your torso straight and rigid and your body in a straight line from ears to toes with no sagging or bending. Your head is relaxed and you should be looking at the floor. Option: knees down.
At the end of your session, lie on your stomach for 2-3 minutes and gently come up into a cobra position to stretch your stomach muscles. Do this workout twice weekly.
This is such a common and uncomfortable condition for runners, and it is hard to pin point the cause. We are all so different physiologically, the triggers can be numerous!
If you do suffer from runners tummy this article may be of help: