Carbon - I was just about to PM you this link, as I've noticed in the past that you're interested in sports-medicine and very "evidence based." Read this article by McGill:

http://www.backfitpro.com/pdf/select..._exercises.pdf

Re. kettlebells and core fitness, "The Kettlebell Bible" has made my jaw drop with its thoroughness. One of the authors was senior PTI for the Royal Marines and seems to have spent all his time reading biochemistry and physiology papers - there's lots of good stuff on metabolic threshold and training that should apply very nicely to cycling even if you never touch a k-bell. You might need to order this book from the author's website if you want it:

http://www.intensefitness.co.uk/index.html

A really nice bonus is that k-bells have the potential to replace plyo work at a much lower injury risk - I really hesitate to do plyo anymore now that I'm out of my 30s.

Obviously, if McGill and the above disagree at any point, I'd go with McGill (I just got round to ordering my own copy - in the past I have only surfed a friend's copy, notorious cheapskate that I am.)

Anyway, I stopped doing high-rep crunches several years ago - and my abs and core are probably better than they have ever been.