I noticed in pictures recently that while standing up straight, my back is consistently arched. Despite my best conscious efforts, it seems like I may have an imbalance where my back is stronger than my core. Honestly, I don’t do much ab work. Seems like that’s catching up to me. I lift weights 3 days per week, broken down by 4 full body (compound) exercises; 2 isolated exercises. Is there a recommended ab exercise I could throw in outside of knocking out sit-ups on a swiss ball?
There’s always going to be some natural arch to your back because there’s supposed to be. If it’s drastic (presuming you don’t want to post a side pic to show what’s what), it may be an anterior pelvic tilt and not specifically about your back itself or lack of ab work - though, of course, fixing that glaring oversight should be a priority.
This talks a bunch about APT, along with some drills to work on everything involved: Tip: Fix Your Anterior Pelvic Tilt, Feel Awesome | T NATION
As for specific ab work, it’s pretty hard to screw up. Any kind of crunch variation and any kind of static core hold (Pallof press, plank, 1-arm plank, etc.) would be a simple start. Hit abs directly with one of your isolation exercises twice a week and you should get back on track.