Nothing beats planks when it comes to strengthening your deepest abdominal muscles, which flatten out the stomach. Plus, the row builds strength and definition in upper back, shoulders, biceps, and triceps.
How to do it: Grab a set of dumbbells and come into plank position with your feet slightly wider than hip-width apart. Brace your core as you lift one dumbbell off the ground in a rowing movement. Bring the weight back to the ground and switch sides. Continue alternating at a quick pace. Make sure to keep hips as still as possible and legs engaged the entire time. Do 8 reps per side.