The plank strengthens the dozens of muscles between your shoulders and hips.

But if you jack your feet in and out as you do it, the plank suddenly becomes so much more than just a core builder.

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The quick movement of your feet provides a great dynamic stability challenge that makes your abs scream, and ramps up your heart and metabolic rate.

And you can make it even more difficult by changing up your arm positioning. This creates an unbalanced foundation, so you have to work harder to perform the same exercise.

Put it all together using the plank jack complex below. Do each of the following four plank variations for 15 seconds each without resting. Once you’ve completed the fourth exercise, rest one minute. That’s one round. Perform up to five total rounds.