My most recent week-long challenge was the three-legged plank, which is an excellent core workout. Based on that experience, I airily opined that following this with a week of tabletop crunches would be something of a breeze. I was half joking, of course, but it turns out I shouldn’t have been joking at all. There is no one core exercise to rule them all. You have to do a range of moves to get to every muscle.
