I've done the cardio portions of HSTA on the Bosu, and this is a BLAST. This morning, I also did the genie arms move on Legs & Glutes using the Bosu, and it added a NICE extra challenge here. You could also certainly do any ab work, especially any crunches on a stability ball, on the Bosu. I actually find that crunches on the Bosu are more challenging for me than on the stability ball, for some reason.
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