Sit-ups suck! The function of our core (anterior and posterior) is to stabilize the spine and rotation of the trunk to propel us forward in motion, not to cause flexion of the spine especially the lumbar spine (which is not designed for repetitive flexion Study 1 Study 2). Sit-ups use a lot more of the hip flexors (not a bad thing if we do not already have overactive hip flexors and these are the targeted muscles) to help us sit up rather than the abdominal that most of us are targeting.
How should we train our core? Check out my how to plank blog. Start by mastering the basic such as activation of the Transverse Abdominis and Plank training. All core training should focus on strengthening your posture and stabilizing the spine. Once you can master the plank, as I lay out in the blog, you can then progress by adding movements such as TRX reach outs(a plank with movement added) and balancing exercises.
Train the Core the Right Way Dr. Stu McGill