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Cardio Before OR After Weight Lifting?
    

Brian Bambl, B.S. Exercise Science 2010-01-29

This question often arises from the idea that if you perform weight training/resistance training prior to Cardio that you will utilize your glycogen (carbohydrate storage) during your weight training session and in turn burn more fat during your cardio session.

The reason behind this thinking is that weight training or any type of anaerobic exercise (exercise in which oxygen is not directly needed for the production of ATP which is the body’s universal currency for energy) causes our bodies to utilize glycogen as their primary fuel source.  On the other hand aerobic exercise (exercise in which oxygen is directly used for the production of ATP) type activities causes our bodies to utilize fat as their primary fuel source.  However, no exercise is purely anaerobic or purely aerobic.  Weight lifting is approximately 90% anaerobic and 10% aerobic.  The amount of energy coming from carbohydrates and fat depend on the storage levels of glycogen (carbohydrate stores) and the exercise intensity.

Crossover effect

(adapted from Powers S.K., Howley E.T.: Exercise Physiology: Theory and Application to Fitness and Performance.  5th Ed. McGraw Hill, 2004, pg. 60)

At low levels of exercise intensity the body’s primary fuel source is fat.  For example at rest approximately 75% of our energy comes from fat.  As our exercise intensity increases our bodies become more reliant on carbohydrates for energy.  At around 30 to 35% of our VO2 max (highest level of oxygen that an individual can utilize oxygen) our bodies begin to transition from fats to carbohydrates as their primary fuel source.

Exercising at 30% VO2 max is quite low and most people will exercise well beyond this level.  For example a 150 lb. individual, with a VO2 max of 62.5 ml/kg/min, exercising at 30% VO2 max, on a 0% grade would be walking at only 2.85 miles per hour.  Now if the same individual were running at a more practical speed, say a 7:24 minute per mile pace (8.10 MPH), then they would be exercising at 75% of their VO2 max.  And at this exercise intensity approximately 75% of calories would be coming from carbohydrates and 25% from fat.  We don’t exclusively use carbohydrates as our energy source until we’re at 100% VO2 max and this intensity cannot be maintained for very long anyways.  For example that same 150 lb individual with a VO2 max of 62.5 ml/kg/min, running at 100% VO2 max, on a 0% grade, would be running at a 5:27 mile pace.  They could probably only maintain this pace for about 5 or 6 minutes.

The fact is that our exercise intensity is a major predictor of what our bodies use as their preferred energy source.  On the other hand the availability of energy sources can limit our exercise intensity level.  As stated earlier as exercise intensity increases, carbohydrates become the preferred source of energy and if carbohydrates availability is limited then exercise intensity levels will become limited. 

Even if we were to suppose that substrate (energy source our bodies enzymes are using to produce ATP for exercise) availability were the only factor in determining what our energy source is during cardio we would still run into another problem with the idea that weight training should be done before cardio.  The problem is that we have such large storage levels of carbohydrates.  The average individual stores about 100 grams of carbohydrates in their liver and another 400 grams of carbohydrates in their skeletal muscle.  This large storage equates to 1,600 Calories (4 Calories per gram of carbohydrate).  Now, if our 150 lb. individual performed resistance training for 1 hour which is a fairly long weight training session then they would burn only 430 Calories.  If we assumed that 100% of Calories came from carbohydrates then that the individual would still have 1,170 Calories of carbohydrate stores remaining.

Conclusion   

Exercise intensity is a major predictor of our bodies preferred source of energy during exercise.  We have such large storages of carbohydrates that it is not practical to think that we would utilize the majority of carbohydrates during weight training and thus be that significantly reliant on fats during a cardio session at “normal” exercise intensities.   

Even if we were capable of exclusively selecting fat as our energy source by the type and intensity of exercise that we perform it still we would still be hindered by the fact that any excess Calories that our bodies take in will be stored as fat.  So, if you choose to perform cardio or weight training first it really is not that significant in terms of lowering you body’s level of fat.

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