Stand with an aerobic step (or equivalent) roughly one foot in front of you, with a pair of dumbbells in each hand. Stand tall with your core braced and a flat back. This is your start position. Take a big step forward onto the aerobic step with your left 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 extend your knee as you push off from your left foot to returning to the start position. Repeat on the other leg. Continue to alternate legs until all the required reps (or time) are completed.