Regardless of whether you are weight lifting heavy for low reps, middling in the 8-10 range, or performing higher reps with lighter weight, any cardio activity you add to the day must be performed after your weight training, not before.
The most obvious reason for this rule becomes painfully clear if you have ever gone through a bout of cardio exercise and then stepped into the weight room to perform heavy weight lifting. You can’t. So don’t. In order to build as much lean fat-burning muscle as possible, you have to lift weights and do so with the most intensity you can muster in order to lift as much as possible with proper form. Cardiovascular activities will drain your body of the energy required to perform your weight lifting to the best of your ability, sacrificing weight, form, and reps – which means no new muscle growth.
When you perform exercises with weights your body will use glycogen as its preferred energy source. Glycogen is stored in the muscles and readily available for this strenuous activity, it is the energy your body needs to move heavy weights. Performing cardio before a weight training workout can drain your body of the fuel it requires to lift with intensity, meaning you are not able to stress your body sufficiently enough to initiate muscle growth.
So the first point is that in order to maximize your results to build new fat burning muscle, you must lift weights first while your energy reserves are highest.
Secondly, fat burning cardio is best performed when your body is forced to tap into your stored fat as its source of energy to perform cardio. Your body will look to glycogen, but you already burned through much of that when training with weights, so the next logical step is to eat away at your stored fat and convert that into the energy source required by your cardiovascular activity. If you perform cardio before weight lifting you will be using stored glycogen first, and will not be burning away your stored fat.
The benefits to cardio exercise after weight training are two-fold: You will have more energy to lift more weight for more reps, which translates to new muscle growth, which then leads to further fat loss as muscle burns calories. And, when you do perform your cardio after weights, you will burn stored stubborn fat rather than utilizing your glycogen reserves.
Train hard and smart – weights before cardio!