from the article at Quick and Effective Ab Workouts, some of which I like:
“train as much as necessary (to improve, and to win), not as much as possible!”
The Plank and Side Plank
One of the best isometric exercise is as follows.
For myself, I end a workout with a simple Ab-core torso exercise, called the plank and side plank (see image on left) I do a single 90 second hold for the plank, and 60 seconds each for the side plank. Total time: 3.5 minutes.
The average person can’t do the plank for 60 seconds! So don’t settle for average!
and some of which I am not too keen on.