March 2, 2018 Randy Brown Mantis Boxing, Training Tips Improve Cardio in Boxing - The Double Under March 2, 2018 Randy Brown Mantis Boxing, Training Tips Looking to improve your cardio on and off the mats? This is one of the best exercises for increasing your cardio. The Jump Rope is a phenomenal tool for this, but getting to the Double Under ramps it up to a whole new level.