I said risk, not guarantee of injury. This isn't just my opinion though, I already asked my professors at the university (I study in Kinesiology) about situps and the advice they gave me was basically not to do any situps but to do different types of crunches (ex crunches on the ground, on a stability ball, doing them by touching the knee to the opposite elbow etc...). The reason for this is that while doing situps the movement is mostly accomplished by the muscles around the hips to femur articulation (mostly the iliopsoas) and during crunches it is mostly the abdominal muscles that do the work. However, it is easy to cheat while doing the crunches movement, for them to be effective, you have to do them with the proper technique.
Riiight. And how many of your professors have ever done one situp?
How many situps have you ever done? More than one?
The problem with your theorems and kinesiological research is this: The topic here is not crunches. It is how to be able to do more situps.
Not crunches. Situp. Situps are not crunches. This is not designed to give you sexy abs. This is designed to make you better at doing situps.
As it is the case that we are discussing situps, I want to remind you that though your professors, not one of whom has offered any proof whatsoever experimentally, that using a dumbell for situps is anything but beneficial, state that in theory it is a possibility that maybe someone could perhaps injure themself (I have no idea how) by doing a situp with a dumbell behind their neck, that I do them, have done them, never got within a Mac50 range of injuring myself therby.
Please, don't cling to theories when faced with real world realities. If you are under fire somewhere and your mates tell you to duck, will you instead stand tall, quoting your professors that said that enemies don't exist in that region?
In theory, they couldn't go to the moon, fly in a heavier-than-air craft, and I can't do deadlifts without a weight belt. For 25 years. All these theories are false.
If you were talking to the Wright brothers or someone who said that in theory, their plane should not have flown, who would you believe?
Consider your sources. Facts trump theories every single time. Your obvious great regard for your professors, who, from what you have said, offer no proof from their own personal experience that:
A) they have any knowledge of how to even DO a situp
B) they know how to increase situp capability
C) they have ever done ONE THING to increase any physical capability whatsoever
D) have ever even been in the same province as someone who was, at that time, to their knowledge, done a situp with a dumbell behind their head
should not prevent you from putting more stock in real-world experience.
Theories look great. Until reality makes them obviously ridiculous.