Why it's on the list: This cheap piece of equipment has gained a cult following over the years, and for good reason. EMG data suggests that using an ab wheel is equally if not more effective than hanging leg raises, sit-ups, and reverse crunches at activating the abs.

Why is the ab wheel so effective? It capitalizes on the concept of eccentric strength-building perfectly. As you roll out, your trunk must actively fire while those abs stretch to maintain a neutral spine without collapsing under your body weight and gravity.

Ab Roll-Out Variations for Ab Development:

• Ab wheel roll-out 
• Exercise ball roll-out  
• Suspended ab fall-out
• Reverse ab roll-out

In your workout: Build up to 3 sets of 8-12 reps, rolling all the way out and back on your knees. If you can't do all the reps prescribed or if your back is arching, reduce the distance and maintain a neutral spine, then slowly work to extend it over a few weeks.