This is one reason why I'd recommend people doing chin ups/pull ups (if he/she can manage) and front squats instead of planks. For pull ups, the abs are worked on when one stabilizes the body from moving/jerking about too much during the movement - legs flying and kicking about, hips swaying left to right etc. etc.

For front squat, the abs work in a pretty similar way - as a stabilizer.

This post has been edited by Vbs: Dec 20 2010, 02:12 PM