The muscles in our bodies contain a chemical called glycogen. It is also stored in the liver. Glycogen is a natural carbohydrate reserve and available to be readily converted by the body to glucose as needed to satisfy its energy needs like during a weight training session.â When all glycogen is exhausted from the muscles, the body will look for other alternative sources for energy and will begin to burn any excess fat that it finds.
Simply, if cardio is performed “before” a weight training session, all glycogen in the muscles will be used thus diminishing the energy that can be tapped for lifting heavy weights. Since lifting weights does not raise the heart rate as cardio exercise does, there would be minimal fat burning taking place.
Performing cardio “after” a weight training session would allow a person to lift more weight in the beginning to power through a grueling workout and take advantage of the “heavy weights” to build muscle mass, the goal of most bodybuilders. A person needs to be more focused and alert at the beginning of a weight training session and proper form would be easier to keep. Form plays a greater role in weight lifting than it does in cardio. Proper form is needed while lifting weights to achieve a good muscle growth and avoid injury. Focus and form are key when lifting heavy weights.
In this scenario, once glycogen is used by the weight training, the body will again look for other ways to continue to generate the fuel it still needs. At this point, performing an intense cardio session will burn more calories at a faster rate thus resulting in greater fat loss. In bodybuilding, this is known as “shredding.”
Simply, if the goal of the training session is fat loss, performing high intensity cardio on the days other than weight training days is probably going to be the best choice. If the goal for doing steady state cardio is to improve your endurance then leave it for after a weight training session.