Physiology of Endurance Training Part 1: Energy Systems

Physiology of Endurance Training Part 1: Energy Systems

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Understanding how our bodies work will be quite helpful in our pursuit to optimise Endurance performance. A person training for competing in endurance-based events requires a significant amount of energy.

In this part, we will learn about energy and the various pathways through which our body derives it from.

What is Energy?

To begin, Energy is simply the ability to do work. In human physiology, work would mean all physiological functions like a muscular contraction, digestion, blood circulation, nerve transmission, respiration etc.

The basic unit of Energy within our bodies is Adenosine Triphosphate (ATP) molecule. For convenience, think of it as a “1 rupee coin”.

Each of us has millions of ATP molecules in our body which provide us with energy. We constantly use and replenish ATP, even when we are at rest. Based on the above Rupee analogy, ATP replenishment and utilization can be viewed as similar to our daily lives wherein we earn and spend cash to maintain our lifestyle.

ATP molecule is made up of (1) Adenine, (2) Ribose, and (3) three phosphate groups. The phosphate groups are bonded to each other via a high-energy bond.

The elaborate chemical composition and reactions are beyond the scope of this discussion and hence we will not delve deeper into it.

When an ATP molecule comes in contact with the enzyme ATPase and water, one of its high-energy bonds is broken, releasing a burst of energy, which the body can use for its physiological functions. When this bond is broken, it results in the formation of adenosine diphosphate(ADP) and a phosphate(Pi) molecule.

Having understood, what energy is, let’s take a look at various ways in which ATP is produced.

The body has three energy pathways (producing systems) namely, the immediate (ATP-CP), short-term (glycolysis), and long-term (oxidative phosphorylation) systems.

All the three pathways ultimately produce ATP, but they differ in how quickly ATP is produced and the amount of ATP produced. The immediate and short-term energy systems are anaerobic in nature, they do not require oxygen to produce ATP. On the other hand, the long-term energy system is aerobic in nature, depends on oxygen to produce ATP.

ATP – CP system (Immediate energy system):

The immediate energy system is also known as the ATP-CP system, where CP stands for Creatine Phosphate. As discussed earlier, when a high-energy bond is broken from an ATP molecule, ADP is produced (having two phosphate groups). This ADP is not simply thrown away by the body after the initial reaction. In fact, it undergoes a recycling process with CP (which has one phosphate group). CP donates its phosphate group to ADP to produce another ATP molecule, leaving behind creatine (CR) molecule, which later bonds with another phosphate molecule.

If we use our Rupee analogy, the ATP-CP system is similar to the cash we carry in our wallet:

  • We can quickly access it.
  • The amount of cash we carry in our wallet is often limited.

Similarly, the ATP – CP system can produce ATP very quickly, but on the downside, the amount of ATP it can produce is quite limited. It is the dominant energy system during very high-intensity, short duration activities lasting approximately 10 seconds or less. E.g 100-meter sprint.

Anaerobic Glycolysis (Short-term energy system):

This energy system is known as Anaerobic Glycolysis because the initial biochemical reactions involve the conversion of glycogen (stored glucose) to free glucose. In this system, lactic acid is also produced as a byproduct along with ATP.

Applying the Rupee analogy again, this energy system is similar to the money a person has in a savings account:

  • Compared to the money in a person’s wallet, the savings account would have more money.
  • However, to access money from a savings account in the form of cash is a slightly lengthier process.

The Anaerobic Glycolysis system too has the advantage of producing more ATP than the ATP-CP system, but the disadvantage is that it takes longer to do so. Also, this energy system produces lactic acid, which is further converted to lactate and positively charged hydrogen ions (H+). High concentration of H+ ions creates the burning sensation in the exercising muscle, causing premature fatigue, leading to the cessation of the activity. In terms of performance, this energy system is dominant during high-intensity, moderate-duration activity lasting approx. 30 to 120 secs. E.g. 400-metre sprint.

Oxidative Phosphorylation (Long term energy system):

This system is dependent on oxygen for ATP production. Through this system, 32 molecules of ATP can be produced from a single molecule of glucose.

Using our cash analogy, this energy system is similar to the money a person has in long-term investments such as fixed-deposits, mutual funds or stocks :

  • Significantly larger amount of money will be available in long-term investments compared to the money in savings account or cash in the wallet.
  • However, the person has to go through several steps and has to wait for a longer duration to access the funds and convert them into cash.

The advantage of this system is that a very large amount of ATP can be produced compared to other energy systems. However, the disadvantage of this system is that it takes a longer time than the other energy systems to produce ATP. It takes longer to produce ATP because it depends on oxygen and the only place where oxygen can be utilised is in the mitochondrion in the cell, which is similar to a very large ATP factory with multiple “halts on the assembly line”.

In terms of performance, this energy system is dominant in low-to moderate-intensity, long duration activities lasting longer than 5 minutes. E.g. A marathon race.

Although the long-term energy system is the dominant energy system used in endurance activities, athletes need to understand that it is not the only energy system used in endurance sports, we shall cover this aspect in the following section.

The interplay of Energy Systems:

We have seen previously that ATP (Energy) can be produced via three distinct energy systems. Although we looked at each system individually, this, however, does not imply that only one energy system is in use at any given time. For our understanding, we can use the analogy of an orchestra performance. The orchestra will include multiple instrument groups, and each group plays loudly, moderately or softly depending on the musical score.

It is possible, that at the beginning of the musical, the violin group may be loud, the pianist group maybe moderate and the flute group maybe soft. By the end of the musical, this interplay could be reversed to reflect soft music by the flute group and loud music by the violin group. The same is true for energy production during exercise. Each of the energy systems is in dynamic flux.

Like the instrument groups, each energy system is operating constantly, but the systems operate at different levels of ATP production depending on the intensity of the exercise.

For example in a cycle race, during pack riding the intensity is moderate and the duration is long, the oxidative phosphorylation (Long-term system) will be the dominant system here but not the only energy system. The other two systems too are active, but they are “playing softly”.

During an uphill climb, the intensity rises for a shorter duration of time, here the anaerobic glycolysis (short-term) energy system will be playing “loudest” (dominant), the ATP-CP system will be moderate and the oxidative phosphorylation system will be playing “softly”.

In the final sprint phase of the race, the energy dynamics will be completely reversed. In this phase, the ATP-CP will clearly be the loudest followed by short term energy system and the long-term energy system.

All three energy systems contribute towards performance in an endurance event and thus it is crucial to train each one of them correctly. The following blogs will shed more light on scientific program designing principles.


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