Full-sit ups tend to recruit the hip flexors to get the person into an upright position. This is where the crunch came from- They were trying to take the hip flexors out of the equation.
Having said that, full sit-ups aren't all bad for everyone. If you are someone with weak hip flexors they can actually help to strengthen them. And, hip flexors aside, they DO bring the abs through a fuller range of motion, thereby recruiting more ab muscle.
What I see they make the kids in school do is more of a "sit-up," & based on targeting/isolating the abs, the "crunch"is probably more effective. I felt compelled to ask a PE teacher recently, what exactly were the kids working, while doing his...."sit ups?" :)
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