Compared with crunches and bent-knee sit-ups, non-traditional ab moves like these (and the Arm Roll-Outs, which are on page 5) do a significantly better job of activating the upper and lower abdominals as well as the obliques, found a study published in the Journal of Orthopaedic & Sports Physical Therapy. Montenegro suggests doing this move on a smooth surface, like a wooden or tile floor (not a rug, where the towels won't slide).
1. Start in a high plank position with your palms on the ground underneath your shoulders, arms straight, core engaged, with a small towel under the toes of each foot. (If you have an exercise ball, you can use that instead of the towels.)
2. Use your abdominals to bring both knees in toward your chest. You should be in a crouch position with your knees between your elbows.
3. Hold for 1 count. Push from your abdominals to slide the towels under your feet back to starting plank position.
4. Do 12 reps.