Check out their ranking of abs exercises (from best to worst) below:
4. Crunches with Vertical Legs: This exercise is just like a traditional crunch (see #11 below), but with your legs extended up into the air, in line with the hips.
5. Torso Track Machine
6. Crunches with Arms Extended: This exercise is just like a traditional crunch (see #11 below), but you extend your arms overhead, squeezing your upper arms by your ears as your crunch up and lower down.
7. Reverse Crunches: Click here for a demo.
8. Crunches with Heel Push: This exercise is just like the Crunches with Vertical Legs (see #4 above), except that as you crunch up you also slightly lift your hips off the ground (feet towards the ceiling).
9. Ab Roller Machine
10. Plank: Click here for a demo.
11. Traditional Crunches: Click here for a demo.
12. Exercise Tubing Pull
13. Ab Rocker Machine
To view the entire study report from ACE, click here. (You will need Adobe Acrobat Reader to download this PDF.)
Action Sparked: It's important to remember that every individual performs exercises differently, and even in this study, an exercise that proved effective for one person was sometimes ineffective or uncomfortable for another person. Especially for people with back, spine or disc issues, it can be tricky to find exercises that don't aggravate your problems. So listen to your body and work at a level that is comfortable for you, and never perform an exercise that causes pain.
The big thing you can take from this study is that you really don't need to buy anything special to train your abs—traditional exercises prove to be better. But if you are going to invest in one piece of exercise equipment, a simple and inexpensive stability ball is the way to go. Not only are crunches on the ball ranked as #3 above, but the ball is extremely versatile for all types of exercises.
Remember that your abs are just like any other muscles in your body, so train them accordingly. That means 1-3 sessions per week, and 1-2 days of rest in between abs workouts. Also, be sure to avoid the top 10 abs training mistakes.