Instead of using incremental weight, gymnasts gradually reduce their leverage from taking an easy calisthenic to a ridiculously hard one - poor leverage instead of heavy weight. Resistance is resistance - your muscles don't know the difference. However, you get the added benefit of developing exceptional balance and coordination.
If you think developing a strong core is simply a matter of doing situps and hyper extensions, you'd be quickly humbled and perhaps humiliated in attempting to duplicate what some of these athletes are capable of doing. They are truly pound for pound the strongest -at least upper body wise.
