This question is one that many people ask when trying to implement a strategic approach to their new found fitness lifestyle.  Some may not ask this question and just assume they know what to do, while others have been misinformed by a peer or a heap of misinformation in some strength and fitness magazine.  Consider this the next time you plan to put your program together. 

If your goal is bodyfat reduction, then we recommend that you do your aerobic work directly after your resistance training at less than 60% of your VO2 max. If you are doing cardiovascular work on your off days, then we recommend that you train at 75% to 85% of your VO2 max for the first 30 minutes and drop the intensity the remaining 20 to 30 minutes to less than 65% of your VO2 max.  Warming up for five minutes prior to resistance training while beneficial is not considered cardiovascular work. Warming up for five minutes is necessary because, during exercise, blood flow patterns change. Through the action of the sympathetic nervous system, blood is redirected away from areas where it is not essential to those areas that are active during exercise. Only 15 to 20% of resting cardiac output go to muscle, but during exhaustive exercise, the muscles receive 80 to 85% of the cardiac output. This shift is accomplished by reducing blood flow to the kidneys, liver, stomach and intestines. The warm-up allows for this transition to occur.

Keep in mind that the major factor that determines the role of fat as a substrate during exercise is its availability to the muscle cell. In order to be metabolized, bodyfat (triglycerides) must be degraded into three molecules of free fatty acids and one molecule of glycerol. This splitting allows the free fatty acids to be converted to acetyl-CoA and enter the Krebs cycle. Hence, if fat is not available to the muscle cell then it can not be metabolized.

Fat can only be metabolized via Krebs cycle oxidation. It is essential to recognize that a reduction in Krebs cycle intermediates: whether it is the result of (1) low carbohydrate diets, (2) no carbohydrate diets, or (3) excessive prolonged aerobic sessions, resulting in a diminished rate of ATP production from fat metabolism. When carbohydrate stores are depleted in the body, the rate at which fat is metabolized is reduced. Therefore, carbohydrates are essential in the ability to metabolize fat.  It is only the free fatty acids that are metabolized via the Krebs cycle that are used in ATP production that go toward reducing bodyfat levels.

Therefore, when designing an exercise program to reduce bodyfat stores, it is vital to consider both (1) the total rate of energy expenditure and (2) the percentage of energy that is derived from fat metabolism. You must optimize the availability of fat to the muscle cells through selection of appropriate intensity and duration of exercise sessions. Since it takes approximately 20 minutes for lipolysis (fat burning) to occur, the session should exceed 20 minutes in duration for fat to be made available to muscle cells. At approximately 70% of VO2 max the availability of fat to the muscle cells diminish due to an increase in lactate production, which inhibits fat metabolism. Although engaging in activity at 20% of VO2 max will burn 60% of calories from fat as compared to working at 50% of VO2 max which would derive 40% of energy from fat, the total rate of energy expenditure is 2.5 times greater at 50% VO2 max. The absolute amount of fat metabolized is 33% higher during exercise at 50% of VO2 max. The ideal rate of work would be at 50% of VO2 max for duration of 60 minutes. Unfortunately, this physiological actuality has led many individuals to mistakenly believe that because the body utilizes a greater percentage of fat as fuel during aerobic exercise at a relatively low level of intensity, such exercise is more effective for fat loss than high intensity exercise. These individuals ignore two very important facts. First, the absolute amount of fat calories burned during high intensity exercise tends to be equal to or greater than the number burned during low intensity exercise, even though the percentage of calories burned from fat is higher during low intensity exercise. Second, when you eat, you replenish both carbohydrates and fats. As soon as an excess of calories (from either fats or carbohydrates) exists, your body will begin to store them as fat. Once you eat after exercising (including those activities which burn more fat than carbohydrates), you will rapidly replenish any of your carbohydrate stores you may have used up. Once they are replenished, your body will begin to store the rest of your caloric intake as fat. The net result is that your body’s fat stores will be virtually unchanged—if at all. The critical point is that low-intensity aerobic exercise does not (by itself) cause you to alter your body’s overall energy balance. Keep in mind that you lose weight and body fat when you expend more calories than you consume, not because you burn fat (or anything else) when you exercise. By the same token, all other factors considered, the most positive feature of low-intensity aerobic exercise is that it is relatively well-tolerated (orthopedically) by most individuals.

Larger mitochondria in greater numbers, increased levels of aerobic enzymes, coupled with increased blood flow all boosts the fat burning capabilities of the muscle fibers. Aerobics can lead to more routes for blood to reach working muscles and more oxygen, which is needed for oxidation of nutrients within the mitochondria. The more massive a bodybuilder becomes the more routes in the form of blood vessels are needed to supply these working muscles. From fat burning to improved cardiovascular health to improved recovery abilities, aerobic work should be an integral part of all training programs.

Once you deplete glycogen in your body you will no longer maintain an adequate level needed for resistance training.  You simply do not have enough energy to tackle the intense demands of strength training.  In short, aerobic should succeed resistance training.