A BOSU ball looks like an exercise ball cut in half that's inflated on one side with a flat platform on the other.

You can use BOSU balls to increase the difficulty in stability exercises such as planks or one leg stand. It can even make some stable exercises require more stability work like push ups.

The resistance is it's unstable environment where you need to balance on top of one side or the other.

(Least Difficult Stability To Most Difficult Stability) Floor > Balance Beam > Half Foam Roll > Foam Pad > Balance Disc > Wobble Board > BOSU Ball

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