So your personal trainer gets you to do sit-ups to "strengthen" your abs and get that six-pack. And you think it's gonna happen...
The majority of the population are walking around with dysfunctional abdominal muscles which means that they either have a sequencing problem with the muscles that function to create flexion at the spine and hips or that they have an imbalance in strength/function of these muscles. Or both, which is often the case.
This only leads to one outcome - increased pressure on the lumbar discs
Yip, not a six-pack, not strong abs but more than likely a disc herniation somewhere down the track. It could take weeks/months/years to develop.
I refuse to let any of my clients do sit-ups and refuse to do them myself for the above reasons. There are multiple exercises out there that are heaps more effective than the sit-up in improving abdominal function/strength and they are 100x safer!
The curl up is an acceptable exercise if only your shoulders leave the ground and you exhale as you crunch, think about drawing your lower ribs towards your pelvis. Don't pull on the back of your neck but just support it loosely with your hands.
Even more ideally, I have my favourite Top 4 abdominal exercises I like to use in order to safely and effectively develop a functional central stability system.