Get in push-up position with your feet together and your hands placed wider than shoulder-width apart. Your shoulders, hips and knees should all be in a straight line. Draw-in your navel and contract your glutes. Quickly pull your knees to your chest and jump straight into the air, driving your arms over your head. Land softly, placing hands back in the starting position and extend your hips and knees into a straight line. Repeat quickly.