Energy Pathways

There are two primary systems by which your body transports energy to your muscles, one of which can be subdivided into two separate systems. During less intense exercises, the muscles rely on the Aerobic Energy System (with oxygen), which can produce energy over hours. On the other hand, during intense exercise, the muscles rely on the Anaerobic Energy System (without oxygen), which can only produce a certain amount of energy at a time. This latter system can be subdivided into the ATP-CP Anaerobic Energy System and the Glycolysis Anaeroboic Energy System.

Fueling the Energy Pathways
Before I dive into each of the energy systems, I want to look at how they are fueled. Muscles use something called ATP (adenosine triphosphate) as an energy source. Nutrients such as carbohydrate, fat and protein get converted to energy in the form of ATP. It is from the energy released by the breakdown of ATP that allows muscle cells to contract. However, each nutrient has unique properties that determine how it gets converted to ATP. Carbohydrate is the main nutrient that fuels exercise of a moderate to high intensity, while fat can fuel low intensity exercise for long periods of time. Proteins are generally used to maintain and repair body tissues and are not normally used to power muscle activity.

Aerobic & Anaerobic Energy Pathways
Because the body cannot easily store ATP (and what is stored gets used up within a few seconds), it is necessary to continually create ATP during exercise. ATP is created in essentially two ways.

Aerobic Energy System
One way that ATP is created is by aerobic metabolism using oxygen carried in the bloodstream. The aerobic energy system utilizes proteins, fats and carbohydrates (glycogen) to resynthesize ATP. This is a sure and steady way to create virtually unlimited sources of energy. However, it has a limit on how fast it can work. The cardiovascular system is limited in its ability to quickly deliver blood and oxygen to the working muscles.

ATP-CP Anaerobic Energy System
Because of the cardiovascular system's limitations, during high intensity exercise, ATP is also produced by anaerobic metabolism that does not require an ongoing supply of oxygen. Anaerobic pathways can provide ATP immediately, but they also have limited stores that need to be refilled after they are used up. The ATP-CP anerobic energy system, also known as the Phosphate System, supplies about 10 seconds worth of energy and is used for short bursts of exercises such as a 100-meter sprint. This pathway doesn't require any oxygen to create ATP. It first uses up any ATP stored in the muscle (about 2-3 seconds worth) and then it uses creatine phosphate (CP) to resynthesize ATP until the CP runs out (another 6-8 seconds). After the ATP and CP are used the body will move on to either aerobic or anaerobic metabolism (glycolysis) to continue to create ATP to fuel exercise.

Glycolysis Anaerobic Energy System
This system creates ATP exclusively from carbohydrates, with lactic acid being a by-product. Anaerobic glycolysis provides energy by the (partial) breakdown of glucose without the need for oxygen. Anaerobic metabolism produces energy for short, high-intensity bursts of activity known as the lactate threshold and muscle pain, burning and fatigue make it difficult to maintain such intensity. Click on the image below to see how the various energy pathways respond during different types of exercise.
Energy Pathway Cycle
During exercise, an athlete will move through each of these metabolic pathways. As exercise begins, ATP is produced via anaerobic metabolism. With an increase in breathing and heart rate, there is more oxygen available and aerobic metabolism begins and continues until the lactate threshold is reached. If this level is surpassed, the body cannot deliver oxygen quickly enough to generate ATP and anaerobic metabolism kicks in again. Since this system is short-lived and lactic acid levels rise, the intensity cannot be sustained and the athlete will need to decrease intensity to remove lactic acid build-up.

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