Many authorities (including Dr. Stuart McGill) in the strength and conditioning genre believe that performing crunches (repeated flexion of the spine) can be harmful to our lumbar spine. The article below (which appeared in Strength and Conditioning Journal/August 2011) challenges this notion.
http://journals.lww.com/nsca-scj/Fulltext/2011/08000/To_Crunch_or_Not_to_Crunch__An_Evidence_Based.2.aspx#
