Crunches are a great way to target your rectus abdominis -- also known as your six-pack muscles. But throw a BOSU ball beneath you and suddenly the classic crunch becomes a core-crushing challenge that hammers every muscle in your midsection (in a good way, of course!). "The BOSU ball adds an element of instability, forcing your core to work even harder with every rep," says Todd Durkin, author of The IMPACT! Body Plan and creator of this weekend's fitness test. Ready to try it? Watch the video to see Durkin perform the challenge with perfect form. He completed 61 reps in 60 seconds. What was your score? Let us know in the comments below.

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Want more ways to carve your core? Try Durkin's 6 moves to sculpt beach abs.

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