most cruches and leg raisea are done improperly and work the leg muscles far more than the abs, this causes stress on the low back over time as the muscle imbalance builds. There are many exercises one can do, but here is a simple to see how well matched your abs are to yourhip flexors.
1. Lay on your back.
2. bend legs and bring kness into chest
3. Extend legs pressing your feet toward the ceiling.
4. concentrate on pressing your low back into the floor.
5. slowly lower your legs.
Here is the catch, most people can do this with ease, however once they get theri heels close to the floor, if you feel under their lower back, its heavily arched. What this indicates is that there ab strength has been surpased and could no longer support the weight and the hip flexors are tightened up pulling on the spine. which indicates a muscle imbalance and hip flexors that are much stronger than your abs. Do the above exercise, but once you feel your low back come away from the floor stop, bring knees into chest, rest a few seconds and repeat. Once you can get feet to the floor with low back flat against the floor youve made some good progress and can add weight to your feet(boots, ankle weights, rolerblades or whatever works) to keep reps low, after all If your looking to build strong abs, treat them like any other muscle and work on 3-10 reps sets, not 50 rep sets that most people do.