Using an empty bar allows you to work through slightly more range of motion than an ab wheel. The bar sits a little lower to the ground which means your body can fall flatter.
This will also place the points of contact (the bar and your knees, or feet if you're doing them standing) further apart. The further the distance between these contact points, the harder your abs must work to keep your lumbo-pelvic region positioned correctly.
related posts
The Success Quiz
There's nothing more frustrating than committing to a training program that doesn't pan out. This is equally true for an athlete who might work hard yet still falls short of his expectations and the strength coach whose biggest satisfaction comes from the great strides his athletes make.
The Superior Circuit - Part 3
Instead of using one dimensional thinking and throwing rocks by doing cyclical modes of cardio, why not try to cover as many needs as possible in a short period of time with a circuit of exercises?
The Delt Triad
The perfect way to torch your delts in the shortest time possible.
Does Lifting Boost Testosterone?
Does manipulating your hormone levels through exercise actually lead to more muscle? Or is that Broscience 101?