• Sunshine Coast Rehabilitation & Exercise Physiology

Energy Metabolism: the What and the How.

For us to move around and participate in life, the body has 3 complex pathways that are responsible for the creation of energy.


First of all, what is energy? Energy is defined as the ability or capacity to do work. Sometimes when there is a large demand placed upon our body, we require higher levels of energy, alternatively, during low workload requirements, our need for energy is less. Because of the variability and unpredictability of life, our body needs to be able to adjust the energy availability to the energy need.


The body uses a molecule called ATP (Adenosine Triphosphate) as the building block for energy. Energy is generated when one of the phosphate molecules is cleaved off of the ATP molecule, creating ADP + P (adenosine diphosphate and phosphate) molecules. Because the body needs a constant supply of energy to maintain bodily functions (e.g. breathing, thinking, moving, etc), there is constant resynthesis and cleaving off of the phosphate molecules to meet the demand for energy.


The body has 3 energy pathways, with all three always contributing to the supply of energy, but with one being more dominant depending on the demand placed on the body. The first two energy pathways do not require oxygen to generate energy and are call the anaerobic (an = no, aerobic = oxygen) pathways.


The first energy pathway is called the phosphagen energy pathway and involves a molecule called creatine phosphate (PCr). When the phosphate molecule is cleaved off the ATP molecule, ADP is left over. For ADP to become ATP again, it needs to regain a phosphate molecule. This is where the PCr comes in. PCr will donate its phosphate group to ADP to recreate ATP. This re-synthesisation of ATP occurs rapidly and is responsible for rapid and immediate energy – think of things like jumping, sprinting, etc. We generally see the phosphagen energy system being dominant in short energy bursts lasting up to 30 seconds.


The second energy system is called the anaerobic glycolytic energy system. This energy system still provides a rapid generation of energy but over a longer period than when compared to the creatine phosphate system, with anaerobic glycolysis being the main contributor in 30-90 second efforts. Instead of breaking down creatine phosphate, the glycolytic energy system involves burning glucose and glucose-variants to produce energy without the involvement of oxygen. There are many complex steps involved in this system, however, the main result is glucose being broken down into pyruvate and hydrogen ions. Pyruvate then binds to hydrogen resulting in the creation of lactate (not lactic acid) which acts to fuel energy production. The accumulation of lactate and hydrogen within the cell can create a cellular environment that is not ideal for efficient energy production if the rate of lactate production is greater than lactate clearance. This results in cellular state called metabolic acidosis, or as some people like to describe it ‘lactate legs’, where their limbs feel heavy and fatigued. We can train the body to become more efficient at utilising and clearing lactate from our system, allowing us to perform at a higher level for longer periods of time.


The third and last energy system is known as the aerobic energy system, which utilises oxygen to break down metabolic substrates to produce a continuous supply of energy. The aerobic energy system produces a slower rate of energy and generally takes 2-3 minutes to become the main contributor to energy production. The aerobic system is dependant upon the individual’s capacity to utilise oxygen, so if the oxygen demand is greater than the oxygen availability, the body will switch back to the anaerobic pathways to meet the energy demand. This system primarily utilises carbohydrates and fatty acids breakdown through a process called oxidative phosphorylation.


Exercise physiologists are trained to manipulate the body’s energy pathways to help individuals become more efficient at creating and utilising their energy stores. If you want to become more ‘energy efficient’ and live life to the fullest, please give our friendly clinic staff a call and book in with one of our exercise physiologists.


Metabolism and Exercise, exercise physiologists Sunshine Coast


















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