Blast Belly Fat 15 minute workout- THE MATRIX- Grab a five- to 10-pound medicine ball (or dumbbell) and kneel on the floor with your knees hip-width apart. Keep your torso upright and hold the ball against your abs (a). Slowly lean back as far as possible, keeping your knees planted (b). Hold the reclined position for three seconds, then use your core to slowly come up to the starting position. Do 12 to 15 reps.