Let's talk about the anatomy of the plank. It is a great exercise for your core when done right. What does 'right' mean? Try to keep a straight line from your heel to your head (as straight as possible), tighten your core so that your lower back does not sag in, and tighten your glutes so your bum does not lift up. Look down. Looking forward or upward causes your back to curve in as well.
Want better abs? Work on your planks.
Cheers Eights & Weights!