Isometric Crunch and Chair Crunch
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These two core-tighteners are the perfect antidote to breakroom donuts: For the Isometric Chair Crunch, pull in your abs as if someone is about to punch you in the stomach, says Holland. Continuing to breathe, squeeze your abs for 30 to 60 seconds.

Then, perform regular Chair Crunches: Keeping knees bent to 90 degrees, lean back in your chair while lifting both feet off the ground. Pull both knees towards your chest, then return to starting position. Do 3 sets of 15 to 20 reps. For added difficulty, perform these while keeping your legs straight, Holland suggests.