The problem is after weight training your fast twitch muscle is depleted so this is the time when you should be filling up with qigh quality protein and carbs to get the growing started. If you head out and do cardion, what little glucose you have in your blood stream will go to fueling slow twitch fibers as you jog(or whatever) around. I for one feel it would be detrimental to overall growth since you are not taking advantage of the window to fuel the muscles after a workout.