From the New York Times: Sit Ups Devastate Spines

A professor of spine biomechanics, Stuart McGill, says that sit ups place devastating loads on the discs. Instead, there are other important core strengthening exercises. Not sure 365 Bird Dogs, Planks, and Approved Crunches have quite the same appeal, but this is worth pondering. 

Yes, he really says, If they did hundreds of sit ups their spines would break before they trained to a high level. 

Ouch.