It turns out, if you want rock-hard abs, you're going to have to get them the hard way.
At the request of the American Council on Exercise (ACE) for exclusive research, study authors at the University of Wisconsin, La Crosse looked at the effectiveness of popular abdominal workout equipment and exercises compared to crunches. They looked at products like the Ab Circle Pro, Perfect Sit-Up and Ab Straps, among many others. They also looked at other ab exercises like the yoga boat pose, the stability ball crunch, the decline bench curl-up, the bicycle crunch, and various planks poses.
They had study subjects complete a series of the workouts, and the researchers attached electrodes to their abdominal muscles, external obliques, and the rectus femoris (which is on the thigh) and measured their contraction on each piece of equipment and exercise. They found that traditional crunches always had the greatest muscle activation.
“It's important, however, to keep in mind that there is no single abdominal exercise that challenges all the abdominal muscles in the most effective way,” Dr. Cedric X. Bryant, ACE's Chief Science Officer said in a statement, “and the best way to work the abs is with different types of exercises. For example, the plank exercise effectively engages the transverse abdominis, which plays an important role in spinal stabilization.”
A copy of the study is available, here.