This was a question that someone asked me on another board and I thought I would share my insight on this question. So what is the optimal order for cardio? Should it be done before or after your lifting session? Well, it really depends on what your goals are and how you have your training schedule is setup.
For most of us we want to get the most out of our lifting as possible and that means the most energy to lift the most weight that we can. By doing your cardio beforehand you are depleting your energy stores and more importantly your glycogen stores which help your muscles during your weight training session. This means that you won't have as much stored energy to get the big weights up. Now, does that mean that doing a few minutes of cardio is going to ruin your lifting session if you mainly use it as a warmup? Well no, but if your doing 30 minutes or some intense cardio like HIT (high intensity training) before your lifting it is just common sense that you won't have as much energy.
The best thing you can do is to do your cardio on your non lifting days. This way when your not lifting you are still getting some exercise and your giving your muscles a chance to rest. If you have a training schedule where you have to do both cardio and lifting on the same days then I would recommend doing your cardio after your lifting session or come back to the gym later in the day and do you cardio. By this point your glycogen stores should be low or depleted anyways. With all this being said there has been no scientific evidence to show which the optimal order of cardio and lifting, but the way that your body uses its glycemic science led me to this conclusion.
Any feedback on the topic is welcome.