There are many ab exercises for core strength without requiring a sit-up. Planks, side planks, obliques, crunches, and leg raises are recommended. These exercises work your entire core just like a full sit-up. Apart from the fact that sit-ups are boring, the rounding of the spine when doing sit-ups puts stress on the lower back, which may cause injury over time. Moreover, planks, leg raises, side crunches, and squats engage your core abdominals in a wider range of motions without any limitations. You can do these workouts any time of the day, and they cannot limit the true amount of core strength you can build.