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If you work out regularly at a health club or fitness center, you might¡¯ve noticed an odd-looking piece of equipment tucked in among the free weights, yoga mats and kettlebells.?

Called a ¡°BOSU¡± ball or a ¡°BOSU balance trainer,¡± this device has a soft, inflatable dome on one side and a hard, flat platform on the other.

But what are you supposed to do with a BOSU ball? Can it really make exercise more challenging? Should...