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To do the Inchworm Plank, set up in a forearm plank from your toes with your elbows under your shoulders and your body in a nice straight line down to your heels.
Begin to walk your feet in toward your head, keeping your legs straight as you take very small steps forward. Your butt should go up in the air as you walk your feet in to pike up. Walk in as far as you mobility allows. Then jump both feet back out (beginners may walk back out instead).
Do not let your hips sag toward the ground as you return to the forearm plank. Then repeat walking back in before jumping back out.