From Alwyn's blog yesterday -

By Mike Robertson and Eric Cressey:
"Believe it or not, crunches can actually lead to shoulder, upper back, neck, and elbow/forearm/hand problems over time. Your rectus abdominus runs from the rib cage to the pelvis, so when it contracts during a brunch, it depresses the rib cage. If the rectus abdominus chronically shortens from hundreds of crunches every day, you'll develop a kyphotic posture that, in turn, allows the scapulae to wing out and up, compromising the muscles and nerves of the upper extremities. Therefore, the best rectus abdominus exercises emphasize pure stabilization rather than actual muscle shortening; these exercises can be complemented with reverse crunch variations (external oblique emphasis), as these exercises will posteriorly tilt out pelvis without pulling the rib cage downward."

I want to shout this from the roof tops and scream this a people I see "crunching" for their "core work"!

There is a HUGE difference between Core activation, Core training and abdominal training - Crunches are an example of an abdominal exercise - they are NOT a Core exercise!