When should I do my cardio: Before or After my weight lifting routine?

Once you understand the energy systems in the body, this question is really easy to answer.  You have 3 energy systems:  a quick response system, a long-term regenerating system and an in-between system.  We really only need to look at the first two.  In the quick-response system, you are utilizing the energy stored in your muscles.  That energy is finite.  Once it’s gone, the muscles have to rest to re-create and store more (and they must have the building blocks for that energy readily available).  In the long-term regenerating system, the muscles actually have the capacity to create “on-the-go” energy.  So you can keep moving and the muscles don’t have to rest to regenerate energy. Some of the fuel that is used in that “regeneration” process is fat, so it always has the building blocks available.  Consider your goals with weight lifting and with cardio.  When you’re lifting, you are trying to build/tone muscle.  When you’re doing your cardio, you are trying to increase your endurance/improve your cardiovascular systems/burn body fat.  Here’s the key:  the quick-response energy will be used first – no matter which kind of exercise you are doing.  If you use it up on your cardio, your body will seamlessly switch to the long-term system.  But then, if you try to lift weights afterwards, your lifting session will be less than ideal, because lifting primarily uses the quick-response system and there is no longer any fuel available for that system.   So, to get the most out of both exercise components, you should lift first using the quick-response system, and then get on the cardio equipment and utilize your stored fuel (fat) as you dip into the long-term system.  It’s all about efficiency and effectiveness for your body!