I always do cardio after legs. Normally I do cardio with zero incline at about 2.8mph. After legs, I always bumb it up to 3 degree incline and speed it up to about 3.5mph. Or I may even do the stepmill ala Ron Coleman. It makes my legs much more flexible and easier to move. When I dont do cardio regularly my legs get real stiff & inflexible, they feel heavy and make me feel very awkward when im walking. The cardio really helps with keeping my legs functional; and it doesnt seem to slow down growth either.