That's the first time I've heard that one and I don't think it has anything to do with the ab routine. It could be muscle exhaustion/lactic acid build up I guess if you're doing the cardio right after the ab routine. I guess it could be your obliques because depending on what type of cardio you are doing your internal obliques are responsible for twisting your upper body. I dunno... never experience it and I don't ever do cardio.