Abdominals are made to be strong, but you can’t start with the toughest abs exercise and expect to ace it right away. Like any type of exercise, there is a progression needed, from the easiest to the toughest versions.
Take the regular floor crunches as an example.
Level 1 Arms lying on your sides
Level 2 Arms crossed on your chest
Level 3 Arms behind your head
Level 4 Arms straight over your head
Level 5 Weight on your chest
Level 6 Weight behind the head
Level 7 Holding weight with arms straight over the head
Level 8 low pulley with rope
For each level, there are 3 sub levels. Sub 1: where your feet are anchored under a bench or something holds your feet down with your legs bent at 45°. Sub 2: where your legs are bent at 45° without your feet being held. Sub 3: where your legs are straight and both on the floor.
As you can see, there are so many variations. You can basically do the same with a swiss ball, adding another 24 variations to your arsenal.