What's your hardest plank?

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I've been doing just the regular ones on the floor. Even the two minute ones are getting too easy now. I want to mix it up. What kinds are you guys doing, what's the hardest for you? I did Jillian Michaels six pack ab video a few times over my rest week between stage 4-5 and there was this crazy *kitten* side plank thing that was a killer. What variations are you loving (or hating, rather lol)?

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