Russian twist

To target the deep muscles and sides of your stomach, hold a medicine ball, dumbbell, or weight plate and sit on the floor with your hips and knees bent at 90 degrees, feet off the floor. Hold the weight straight out in front of you and keep your back straight (your body should be at about 45 degrees to the floor). Twist your body as far as you can to the left and then to the right and repeat.