Stand tall atop an aerobic step (or equivalent). Brace your core and keep your chest up. This is your start position. Take a big step backward off the aerobic step and onto the floor with your right leg and bend at the knee, slowly lowering your body until your front knee is bent at least 90 degrees, whilst your rear knee should be hovering just off the floor. Inhale as you do so. Keep your torso upright the entire time and look forward. Pause at the bottom of the movement, then push off from your right foot to move forward, extending your knees and hips. As you come towards a standing position bring your right knee to your chest. Perform all the required reps on one leg before changing to the other.