if you wanna have abs of STEEL, then you might try the Bruce Lee's ab training that consists of:

Waist twists - four sets of 90 repetitions.
Sit-up twists - four sets of 20 repetitions.
Leg raises - four sets of 20 repetitions.
Leaning twists - four sets of 50 repetitions.
Frog kicks - four sets of 50 repetitions.



tried that only once couldn't make it of course