It would be nice if getting the facts about anything related to diet and exercise were black and white, but if that was the case there would not be any gray matter. I'm telling you from first hand experience all the good shit happens in the gray matter, before it returns to black and white.
When I first began my bodybuilding career I was so certain I knew everything about all kinds of things, like best way to train to pack on muscle, or how much protein one should consume on a daily basis. Name it, and I knew. It was a black and white answer.
Fast forward with a few more years of experience under my belt, learning more about those very same things, I realized that my confident certainty about black and white conclusions had changed. Now it seemed everything I knew about diet and exercise was all gray matter. Nothing was black and white any longer. All of a sudden I was in a time frame of my career where everything was a work in progress, and I didn't seem to conclusively know shit anymore. How could it be I knew less even though I had more experience.
I continued to learn at the school of hard knocks, simply needing more years of first hand, in the trenches of real time experiences. I had to fail, and fail some more to start getting the answers to all the gray matter that had once been black and white. Albert Einstein nailed it when he said real knowledge is experience.
So, to address the question of when you should do cardio. The black and white answer is if you're an athlete training for sport, do it after your weight training. If you're not an athlete training for sport, rather for overall fitness, it doesn't matter, PERIOD...
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Best, The Truth Talker (Leo Costa Jr)