What most people do not realize is that the function of the abdominal muscles is to flex the lumbar (lower back) spine. So if your lower back and spine is tight, if you cannot comfortably roll down and up in a ball, your abdominal muscles cannot do their job. Not only in the gym but in the rest of your day, so you end up utilizing your hip flexors and lower back each time you bend and move.
Remember, you are constantly exercising against the force of gravity, but if a tight spine is preventing your abdominal muscles from doing their job, you are missing out on abdominal contractions all day long!!! So instead of thinking about sit ups, think about getting more flexible, it will make your workouts much more efficient.