The action of exercising abdominal muscles is generally not only for the middle or upper and lower parts of abdominal muscles, but for a whole rectus abdominis.
The key is the intensity of the action, not the more times the better. For example, if you can do 200 sit-ups at a time, you might as well do 10 from both ends. Because being able to do more than 200 exercises is endurance, the thickening of muscle fibers is not obvious.
Actions to exercise abdominal muscles: sit-ups, sit-ups and leg lifts, leg lifts at both ends, leg suspension and pull-ups (the intensity increases in turn).
You can also hold dumbbells with suitable personal exercise weight at the same time, and then sit on the fitness chair and turn around to a great extent.
Practice four times a week, and practice three groups with one movement at a time, each group doing about 15.