Those poor, pathetic bastards who think they're blasting their cores by doing planks and little bitty plastic dumbbell curls while perched on a squishy Bosu Ball!
They're fools, we tell you, fools!
Do you really want to develop a core that practically rivals the Earth's in strength, only without all the hot, molten magna, absurdly strong gravitational pull, and soulless mole people? Don't say yes unless you mean it!
Okay, but only if you mean it!
Get thee two dumbbells, one that weighs about 15 pounds and one that weighs 30. (Sure, sure, those are kind of light, but you need to crawl before you walk.)
Grab one dumbbell in each hand. Raise the lighter dumbbell straight overhead, lock the elbow, and keep it there for the duration of the set.
Let the other dumbbell hang straight down, also for the duration of the set.
Now simply begin squatting as low as you can go (without the weight hitting the floor, for you Orangutan arms out there).
Shoot for about 6 to 10 reps.
Reverse the dumbbells and do another 6 to 10 reps.
Quite simply, just about any movement that involves holding a weight overhead works the core muscles exceedingly well, but Single DB Overhead squats take core exercise to a different level as the second weight works as a counterbalance and tries to tip you over.
The very act of keeping yourself erect is extremely tough on the core musculature. Oh yeah, in case you didn't notice, they also work the quads, glutes, and spinal erectors quite nicely.
As you progress, remember to adhere to the 2 to 1 weight ratio. In other words, the weight of the hanging dumbbell should be twice as much as the weight of the overhead dumbbell.
Now gird those loins and blast that core!