Hi Rhonda, I use an 8 pound medicine ball. My core is very, very weak. I neglected this area (not sure why) for so long. The muscles in my abs have separated due to a pregnancy and my lower abs look like a medicine ball. (I think I hear ya laughing..) but it's true.
I can do planks as long as Cathe does in her workouts (but I shake like crazy to get through it).
Susan C.M.