I am not doing anything at all other than swimming and running. I have been running for like 4 years and didn't notice any significant development on my core. Three months ago I went back to swimming and just this recent I saw some development on my cores. They are on their way in getting into shapes. My cousin who is a swimmer told me the when she was training, every after it her core really aches and stiffens. so I am guessing that it's the swimming responsible for the shapes on my core.