Everybody seems to understand the importants of the abs. So we all do our ab-exercises, mostly plain crunches. Like all muscles abs should be trained in the way they are used. For running this means your hips are inline with your trunk and your abs hardly move they just trying to counter the forces from outside. For runners the illiopsoas should get extra attention because they are atteched to the abs.
Abs do not really respond well to instructions from the brain, they mainly contract in response to reflexes initiated by movement sensors in the abdominal muscles itself. These sensors respond to small disturbances with powerful reflexes. Like when you are being tickled.
If you choose your abdominals in a non-reactive area where no reflexes occur, and which is also the position of leg-pelvis-back is far away from those in the walking movement, then that is very good for the beach, but not for running .