substitutes for planks

Eponine7

I am doing Jillian Michaels' Body Revolution (love it so far) and in true Jillian fasion, she is heavy on the planks. Unfortunately, I struggle with rotator cuff impingement and all the planks seem to really set off the pain. The shoulder problem has been an issue off and on for years--but only when I aggravate it.

I know planks are awesome for core and was wondering if anyone here had any suggestions for plank substitutions when my shoulder acts up.

Thanks in advance for your help!