Why do you recommend against cardio with the X3?
As the X3 is a form of High Intensity Interval Training, workouts are segmented into brief but highly intense sets. It appears that HIIT provides many or all of the health benefits of cardio. It is also the case that many people engage in cardio workouts for weight loss. We think that X3 will provide superior benefit to cardio for that purpose, due to the growth hormone response one would expect from stabilization firing and heavy lifting.
Over 100 studies analyzed (meta-analysis) show brief, high-intensity interval training to increase endurance as much or more than traditional endurance training. Importantly, duration appears to be less important than intensity. These data suggest that the shorter duration and higher intensity of resistance training may give it an advantage over intervals in building cardiovascular fitness.
Steele, J., Fisher, J., & Bruce-Low, S. (2012). Resistance training to momentary muscular failure improves cardiovascular fitness in humans: a review of acute physiological responses and chronic physiological adaptations. Journal of Exercise Physiology Online, 15(3), 53-80.