Made up of 50 one-minute intervals, Core Force by popular Anni Mairs is designed to maximize recruitment of your core and ab muscles. You'll use light hand weights (1 to 3 lbs) and an optional step. The intervals range from cardio to standing strength to lying-down core work. The full length of the workout is 50 minutes, but Anni gives you the option of stopping after 20, 30, or 40 minutes if you prefer. The fast-changing moves leave no time for boredom, and it's a great way to get that ab work in without resorting to tired old crunches.