Aerobic Metabolism and VO2 Max
Going back to my previous post, when I say “aerobic metabolism”, I’m really talking about the process of oxidative phosphorylation. This is the mechanism used to convert fuel (carbohydrates, fat, and to a lesser extent, protein) into ATP by using oxygen. One important point I forgot to make in the last post is that aerobic metabolism can utilize any of the available fuel sources. More specifically, fat can be “burned” through aerobic metabolism. Anaerobic metabolism only utilizes carbohydrates. Therefore, “fat burning” is not involved in anaerobic metabolism.
Aerobic energy production is a little slower than anaerobic, but since it can use fat, the fuel supply is almost limitless in most of us. We might only have 500 grams of carbohydrate stored away in our liver and muscles, but the average 70 kg (154 pound) person has about 9,000 grams of fat available to use aerobically. That is 80,000 calories that can be used for physical activity! This brings us to the first reason why aerobic capacity is important for endurance performance: our capacity to use fat for fuel during exercise is related to our aerobic capacity. In general, the more oxygen we can use, the better we are at using fat for fuel. The better we are at using fat for fuel, the more we can spare our much more limited carbohydrate reserves.
VO2max is simply a statement of one’s maximal aerobic capacity. It literally means “the amount of oxygen consumed during maximum work” and it reflects the size of your aerobic engine. In absolute terms, larger people often have a greater VO2max than smaller people. A larger person has more muscle to use oxygen therefore will have a larger absolute oxygen uptake. Absolute VO2 is usually expressed in liters per minute. A typical number for a 25 y/o male who weighs 154 pounds is about 3 liters / min. Women usually have smaller absolute aerobic capacities so an expected number for a 125 pound, 25 y/o woman is around 2.3 liters / minute. However, when assessing aerobic fitness, we generally look at aerobic capacity relative to body size (weight). Therefore, you’ll commonly see VO2max expressed in ml O2 / kg body weight / minute. That average 25 y/o male would have a relative VO2max of about 43 ml O2 / kg / minute and the value for the female would be about 40 ml 02 / kg / minute. So, relative to body size, the numbers start looking much closer to one another. If we take this one step further and look at maximum aerobic capacity relative to lean body mass, the numbers are even closer. This brings us to the second reason why relative aerobic capacity is important for endurance performance: relative VO2max reflects the size of your aerobic engine relative to your body weight. Think of it this way…a light-weight car with a powerful and efficient engine is going to out-perform a heavy-duty truck in a long distance race. The tables might be turned in a tug-of-war or even a short race. Visualize the body of a world-class sprinter as compared to a marathoner. The sprinter’s extra muscle is useful for generating the large amounts of force necessary to cover 100 meters in the smallest amount of time possible. However, when the distance is increased, that extra mass becomes an additional load that must be carried all of the way to the finish. This also demonstrates one of the most straightforward methods of increasing your relative VO2max. If you are carrying around a few extra pounds, shedding that weight and achieving a more ideal body weight will increase your relative aerobic capacity.
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