To raise the intensity of this workout, do these moves after 2 flat-tummy circuits. Stand with your feet shoulder-width apart. Push your butt back and down to a squat, then lower to a crouch. Place your hands on the floor in front of you, shoulder-width apart. Hop your feet back to a plank, body straight, then hop them back to your hands. Return to standing. Repeat for 1 minute. Assume a push-up position, with your hands under your shoulders, back flat, legs straight, feet together and belly button drawn in. Keeping your core tight, bring one knee forwards, toward your chest. Return that leg to the starting position and repeat with the opposite knee. Repeat as fast as you can for 1 minute.