This static core move engages just about every muscle in your body.
How to do it: Get into full plank position with your feet wider than hip-width apart. Brace your abs in tight (don't let your hips sag) as you raise your right arm, lifting it up by your ear, palm facing in. Hold for 1 count, return right arm to ground, and repeat with the left arm. Alternate arms each time, keeping your body as steady and as still as possible, for 1 minute.