Start: Get into a modified pushup position with your weight on your forearms and toes. Your body should form a straight line from head to heels .
Finish: Pull your abs in as far as you can, and hold this position for 20 to 60 seconds, breathing steadily. Relax, If you can do the entire 60 seconds, 1 rep is enough, If not, try for any combination of reps that gets you up to 60 seconds.