Very gradually, I've developed some core strength. One exercise, called "crank plank", is a good example. When I first began to attempt that move, over two years ago, I couldn't even get into the beginning position! I'd have to try to hold it for a few seconds, with my free arm extended outwards. Crank plank still isn't easy, but I've gotten better at it.
One of the muscle sets target by core work is the obliques. I have to be careful with that word. If I repeat it more than a few times, I end up thinking of Bartholomew Cubbins and the Oobleck!