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The Three Energy Systems

Comprehending the energy systems of the body underpins the study of the human body.   We all know that we derive energy from the food we eat, but we should also know that the energy derived from food requires various chemical pathways to breakdown the food for repair, maintenance, and growth. Before we delve into elaborate the energy systems let us ponder and query what energy is.

Simply said, energy is the ability to do work, and any activity requires energy. Food is the major source of energy for the human body.  But food doesn’t provide energy undeviating. As stated above, it has to go through various chemical pathways to provide energy.   All energy requiring processes in the cells rely ultimately on the basic energy molecule of the body, ATP (adenosine triposhphate).  An ATP molecule consists of adenosine linked with three phosphate bonds, precisely the two outermost phosphate bonds.   When the two phosphate bonds are broken, a burst of energy is released and the ATP is broken down to ADP (Adenosine diphosphate).  This energy is produced in the mitochondria (power house) of the cell. Mitochondria are organelles that take in nutrients, break them down and create energy for the cell.  The production of energy from nutrients is known as metabolism.

Coming back to our discussion on three energy systems, they are divided into three groups:

The short term energy system

The intermediate energy system

The long term energy system

The short term energy system

The short term energy system is also called the ATP/CP (creatine phosphate) system.  This is the immediate source of energy that is stored in muscle cells.  But the ATP is depleted within a few seconds of strenuous all out physical activity, as there is small quantity of ATP reserves in the cell.  But CP stores have three to five times as much as ATP.  This allows the body to refuel intense effort for a maximum of 20-30 seconds.    Sprinters, wrestlers, and weightlifters will have a well developed ATP system.  In addition, as the duration of the exercise increases the intensity has to decrease to enable the ATP/CP stores to be replenished by the other energy systems.

The intermediate system

As soon as the CP reserves are depleted the intermediate system takes over. Also termed as the glycolytic system, it is distinguished into oxygen-independent glycolysis and oxygen-dependent glycolysis.  Oxygen-independent glycolytic system lasts between 30 seconds to 3 minutes.  Energy is produced by breaking down muscle and liver glycogen without the use of oxygen. The glycogen converts to pyurvic acid (an end product of the metabolism of sugar or starch) and ATP.  Lactic acid is also produced and serves as an additional energy source, as pyuruvate is reduced to lactic acid when oxygen is deficient.  Sports, such as gymnastics where the routines lasts 3 to 4 minutes have well developed anaerobic glycolytic system.  Oxygen-dependent glycolytic system takes over after 3 minutes and can last an hour.  This system produces energy by breaking down muscle and liver glycogen in the presence of oxygen.  This system does not produce energy as fast as the ATP/PC or the oxygen-independent glycolytic system. An example would be a typical bodybuilding routine which lasts an hour or so.

The long term energy system

The long term energy system uses carbohydrates stored in muscles, liver and blood stream or fats from stored body fats.  Protein sometimes may be also used as an energy source, as   protein converts to pyurvic acid to enter the citric acid cycle (Krebs cycle).  Krebs cycle occur inside the mitochondria of the cell and provides energy required for the activity.     Long distance running, cycling and swimming are the best examples of the long term energy system. What interests us/fitness enthusiasts is that fats are the preferred energy fuel of the resting muscles and the liver.  Fat furnishes about two-thirds of the energy we require at rest.

It is known that once the ATP/CP reserves are depleted, the oxygen-independent glycolytic system takes over and then the oxygen-dependent glycolytic system followed by the long term energy system. It is assumed that the energy systems come into play one after the other, for it seems that any one of the energy system is dominant during any physical activity. But is should be known that all these energy systems occur concurrently, as Dr Mel Siff categorically states that all movements involves muscle actions which are simultaneously stabilize and move the limbs, so that the body may rely on the long term energy system for its overall movement in a particular event, while postural muscles are fuelled by the short term or the intermediate system.  Thus, the long term energy system might be dominant systematically, where as locally; short and intermediate system also might be highly active.  It is incorrect to state that only one energy system is active during a specific activity.

What do we have two perceive from the above explanation? As fitness enthusiast we should strive to obtain advantage and derive the benefit of fitness which encompasses the three energy systems.  An integrated approach aimed towards fitness in its totality would be a wiser approach for better health.

7 thoughts on “The Three Energy Systems

    1. Prithwin, the ATP/CP reserves deplete very soon that is 20-30 seconds, but we continue to do the drills, so the intermediate energy system comes into play to sustain the continued demand. I hope I am being clear.

  1. I see, but what i want to know is ,are we resting between the sprints so that we replenish the atp and then use it up immediately with another sprint? that is to say the purpose of the sprint was to burn ATP and not to be really start consuming the intermediate energy source?

    1. Prithwin, unfortunately, the ATP/CP reserves deplete right after an all-out effort, so the intermediate system has to come into play to generate ATP. For us to sustain repeated bouts of sprints, the ATP stores cannot provide energy for a longer time. I hope I am clear.

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