Dumbbell Crossover Crunch This combination exercise for your core, obliques and upper body will help flatten your stomach and strengthen your arms at the same time. Be careful to use good form and resist swinging your body as you come up to sitting for maximal results. Start down on the mat with your knees bent and your feet flat, one dumbbell in each hand. Come up to sitting and twist to the right as you punch across your body with your left hand. Return to the mat and repeat on your left side. Continue slow and controlled for 10 repetitions on each side. Image Source: POPSUGAR Photography