Usually when I work my abs, I use a non-slip mat to cushion my lumbar region. But two days ago, I used a rug the size of a small bath towel to do the job (I was too lazy to unfold the huge mat, so I grabbed whatever was handy at the time). I have marble floors in my house, so the rug slid all over the place due to the smooth surface. I did the abs section in ME. Basically, I had to engage my core to avoid myself from slipping everywhere. At the end of the workout, I thought nothing of it but my abs were sore the very next day! I guess this method has the same concept as stability balls, but I'm loving this more because I can do reverse crunches, double crunches etc ..
