Let me start by saying this. There is no good or bad movement as such. All movement is good, generally. We should and can be doing a variety of movements in life. However there are some movements that are more appropriate and better for us at certain stages.
Abdominal crunches, or sit-ups, can place huge loads on our spine and put our discs at risk of herniation if we are not managing deep core muscles and breathing very well.
It also creates a ton of pressure in our abdominal cavity and many of us struggle to contain it well, putting much load onto the pelvic floor, spinal discs and potentially weaker areas in the abdomen. This puts us at risk of exacerbating pelvic floor issues, disc herniations or abdominal wall herniation, unless you do these crunches in very good form.
If you think about it, we are doing these exercises in flexion (rounded spine), against gravity, just from the strength of our abs, and that is a big challenge. It is also not a very functional movement - meaning, this sort of movement does not need to get done much in day-to-day life. Many of us hold our breath through it, bearing down onto the pelvic floor and bracing in the abdominals, and in the process we lose the connection into the deep belly muscles that act like a corset for our middle and help stabilise the spine.
If we lose connection to the deep belly muscles, we also overwork the superficial abdominal muscles and create conditions for them separating - this is called diastasis recti, separation of the rectus muscles.
So it is not terribly good for the core, that's the bottom line. It is not functional, tricky to get right, deals with huge loads, causes potential shearing of superficial abdominal muscles, and can put your back at risk.
If you have been given ab crunches or sit ups as part of your daily recovery and strengthening routine, and you have back pain, I would not recommend doing this exercise now. You definitely should not be doing these if you have abdominal muscles separation or existing pelvic floor issues. And limit it, generally, even when you feel strong, unless someone works with you to closely review your form and you know you are not bracing or bearing down whilst doing this.
After all, we do not want our exercise regime to get us into a risk of hernias, or pelvic floor issues, or diastasis recti.
What are good alternatives? Working on breathing drills as part of your core strengthening, for example breathing into a balloon, rolling on the floor left to right, crawling, bird-dog, Pilates progressions of abdominal exercises are all good safe alternatives.
Above all, it all needs to start with correct breathing, freeing up the diaphragm and strengthening the low belly muscles by learning to exhale without bracing.
Enjoy your movement, and choose the right ones that gives you the most benefit.
Have a lovely weekend all!