Stuart McGill talks about spinal flexion and the consequences.
Here's some quotes from http://www.exercisebiology.
"There are only so many bends or a 'fatigue life'," in your spinal disks," says Stuart M. McGill, a professor of spine biomechanics at the University of Waterloo. Inside each disk is a mucus-like nucleus, he says, and "if you keep flexing your spine and bending the disk over and over again, that nucleus slowly breaches the layers and causes a disk bulge, or a disk herniation." A herniated disk won't show through your swimsuit, but it's no fun, and can cause persistent back and leg pain, weakness, and tingling.
Think of the oft-repeated advice for movers: bend at the hips and lift with your legs, not your back. And what is a sit-up but a back bend done in a lying position? "When people are doing curl up over gym balls and sit-ups, and this kind of thing, they are replicating a very potent injury mechanism on their back," says McGill. "Every time they bend it they are one repetition closer to damaging the disk."