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Energy Pathways – Aerobic and Anaerobic Metabolism


For our body to move it requires a form of energy. In a car that energy is produced from the combustion of petrol, and the petrol is produced from refined oil. In our body the energy is produced from the breakdown of a molecule called Adenosine Triphosphate or ATP. This is our petrol.  Carbohydrates, protein and fats are similar to the oil, after being consumed they are broken down into smaller particles eventually creating ATP. ATP binds to proteins in the muscle causing a contraction. How this ATP is produced varies based on the speed at which we require it.


The way this ATP molecule is produced is important for health and to understand the way the body works when performing training.


There are 3 systems when referring to the production of energy (ATP) in our body.


The first 2 systems are immediate systems and are used to produce energy quickly. These systems are referred to as Anaerobic Systems. Anaerobic means without oxygen. The Third system is what is termed our aerobic system. Our Aerobic system requires oxygen for the production of energy.


Below is a more in depth view of each system



Speed – Rapid - During the first seconds of movement we use the stored ATP in our muscle our next step is breaking down Phosphocreatine (PC) to restore the used ATP. This occurs in the first 10 seconds of movement. An activity such as a jump or a sprint uses the ATP-PC system. See diagram A.

The legal substance Creatine Phosphate is a protein that increases intramuscular stores of Phosphocreatine. Increasing these store creates an improved ATP-PC system.

When this system becomes depleted ATP need to be restored from other external sources. The second system inwhich this can be achieved is the Anaerobic Glycolysis pathway.



Anaerobic Glycolysis

      Speed – Moderate - The name of this system means the breaking down of glucose (glycolysis) without oxygen (anaerobic). This system is still referred to as a high intensity system. Activities such as a 400m run, running up a flight of stairs or sprinting to the finish of a running race utilize this system. Anaerobic glycolysis still occurs during longer events but it contributes a lower percentage of the total energy produced. See diagram A. During Anaerobic Glycolysis glucose is partly broken down to lactic acid which is then taken out into the blood stream. The liver later converts this lactic acid back to glucose for later use. This is why lactic acid is referred to when talking about high intensity exercise. A persons lactate threshold is the point at which anaerobic glycolysis produces lactic acid faster then it is taken away and used. This anaerobic threshold is an indicator of endurance running performance as continued anaerobic glycolysis causes muscles to become acidic and prevents further contraction occurring. Aerobic metabolism doesn’t produce this acidity level and there a fewer beproduct which can cause fatigue. Anaerobic Glycolisis produces 2 ATP molecules for each Glucose molecule broken down


Aerobic metabolism

      Speed – Slow to moderate - Aerobic metabolism occurs for prolonged activities that require sustained energy. This is the system that is developed in endurance runners, swimmers and cyclist. This system uses the complete breakdown of carbohydrates (unlike anaerobic glycolysis) and fats to form energy (ATP). This system is slower, and as a result has a limited rate of production. This rate is referred to as your VO2 max or or velocity of oxygen max. This is the peak velocity that oxygen can produce energy within the cells. Tests such as the 2.4km run test or the multi stage beep test are used to measure VO2max. Fats unlike carbohydrates cannot produce energy anaerobically. For this reason the fat burning zones are mentioned when referring to aerobic training. High intensity exercise still results in fat loss, so don’t view aerobic exercise as the only way to burn fat. If you look at diagram B you can see fats enter the Krebs Cycle to produce ATP, carbohydrates that have undergone anaerobic metabolism also enter this cycle. The Krebs Cycle requires oxygen for it to continue to produce energy. In short term exercise like a 400 meters race the energy demand of the exercise is above the ability of the body to deliver oxygen to the Krebs Cycle. This means you need to use system 2 – Anaerobic Glycolysis. As shown in the diagram this can produce 2 units of ATP. Aerobic metabolism of fats or carbohydrates can produce 28 ATP units. So for continued contractions the aerobic system and Krebs Cycle is required.


Figure A - Energy Systems and time to fatigue
Pathways of ATP production

This leads us to the importance of understanding these systems for our health and wellbeing. The heart is a muscle similar to skeletal muscles. The heart requires oxygen to produce energy via aerobic metabolism. Without oxygen the heart muscle cannot survive. When oxygen is cut off due to a heart blockage we experience chest pain as the muscle function is prevented due to the use of anaerobic metabolism. See article on exercise and heart disease. Keeping a well developed aerobic system ensures better oxygen delivery to the heart, and reduces the chances of blockages.


Aerobic metabolism is also important for weight loss. Any form of exercise that lasts for longer then 1 minute requires aerobic metabolism. To burn large amounts of energy continuos movement is required. Aerobic metabolism is the only system that enables continued movement. This is why its important for maintaining body weight and preventing diseases related to weight gain such as Type 2 Diabetes. See article of Exercise and Type 2 Diabetes


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