VO2max, lactate threshold, and exercise economy are all important determinants of endurance exercise performance. However, before we can discuss any of those, we have to think about the reason that each is important. The fundamental requirement for physical work is energy production. In order for our muscles to generate force, energy is required. Whether we’re running a 100-meter sprint, out for a century ride, or summiting Mount Ranier, we are going to be using energy. VO2max, lactate threshold, and exercise economy all describe how our body creates that energy and how efficiently we use it.
A simple way to think about energy use during exercise is by comparison to something that is very familiar to all of us—a car. Cars convert energy stored in fuel to mechanical energy through the process of combustion. The energy released is used to move the car. In much the same way, humans convert energy stored in food into muscle force production. Carbohydrates, fats, and to a lesser extent the proteins we eat in our diet are used as fuel. There are some important differences, however. While cars derive energy through combustion, humans convert fuel to energy through the biochemical processes of glycolysis and oxidative phosphorylation. Glycolysis doesn’t require oxygen so is often referred to as anaerobic metabolism. It is responsible for lactic acid production. Oxidative phosphorylation, on the other hand, requires the presence of oxygen. It is commonly thought of as aerobic metabolism and is not responsible for lactic acid production.
Another important difference lies in the way in which fuel is refined prior to use. Gasoline and diesel fuel start out as crude oil. The crude is taken to a refinery where it is processed into the fuel that drives our cars. That fuel is then delivered to a gas station for storage and distribution and eventually into the car’s fuel tank where it is ready for combustion. Just as our cars can’t run on crude oil, our muscles will not run on carbohydrates or fats. Those fuels have to be first converted (or refined) to a high-energy chemical called ATP. If carbohydrates and fats are the body’s crude oil, ATP is its gasoline. Our muscles run on ATP. This leads us to another very important difference. Our cars carry around refined gasoline in the fuel tank. The refining process is performed long before the gas is to be used. On the other hand, our bodies carry around a very limited supply of ATP. Instead, we carry our crude oil in the form of glycogen (stored carbohydrate) or fat (intramuscular fat and adipose tissue) and we refine that crude fuel just before we use it. We refine that crude fuel through aerobic and anaerobic metabolism to create ATP. How well we do that determines our endurance exercise performance.
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The next post will begin to look at aerobic metabolism and VO2max.