This movement requires attention to form like all lunges, with the added challenge of keeping your balance. For these reasons, it's a great way to strengthen all of the legs stabilizer muscles. To perform this movement, do one lunge forward, put weight in your front foot to propel yourself back up, swinging that same leg directly into a back lunge. Remember to keep the front knee in line with the ankle and avoid letting knees jut in front of the ankle or too much to either side. It is not necessary to touch back knee to the ground.