Energy Systems

Understanding energy systems is important for personal trainers as this knowledge underpins our knowledge of how the human body works and relates to exercise principles. In this topic, we focus on energy systems. You will learn:

• what an energy system means
• the relationship between an energy system and fitness 
• the three key energy systems
• why it is important to burn fat.

Terminology and vocabulary reference guide

As an allied health professional, you need to be familiar with terms associated with basic exercise principles and use the terms correctly (and confidently) with clients, your colleagues, and other allied health professionals. You will be introduced to many terms and definitions. Add any unfamiliar terms to your own vocabulary reference guide.

Activities

There are several practical activities in the topic and an end of topic automated quiz. These are not part of your assessment but will provide practical experience that will help you in your work and help you prepare for your formal assessment. 

Before we get into learning about the energy systems, let’s define what is meant by fitness and fatigue as they help to explain why understanding each energy system is important to your work as a personal trainer.

Fitness vs fatigue

• Fitness is the ability to make enough energy to meet demands being placed on the body.

• Fatigue is the inability of the muscles to meet energy requirements being placed on it and a decrease in the ability to produce force because of, or after prolonged activity.

In simple terms, muscle fatigue often occurs after strenuous exercise (or other activities) and causes an individual to feel weak. It is a symptom that reduces the ability of the muscles to perform over time. Before we explore the body's energy systems, we need to know something interesting about the body and that is the type of fuel it uses.

What fuel does the body use?

Whilst adenosine triphosphate (ATP) is not the only fuel our bodies use, it is the key fuel and common to all movement production. We must transfer the energy from food into a more high-energy compound that our bodies can use to produce movement.

Let us look now at the key energy systems in more detail. 

What do energy systems do?

Energy systems provide the energy required for muscles to move. There are three energy systems that work together to ensure there is enough ATP for all our daily activities. Each system is different in the way they produce ATP from different sources and at different speeds. To extract the energy from the foods we eat and turn it into the chemical energy that our bodies can use, we have three separate energy production systems.

These energy systems also go by other common names, it is important when you review other literature and readings that you can use the terms interchangeably.

The ATP-PC system and the anaerobic glycolytic system (1 and 2 in the previous table) are both anaerobic systems, meaning that oxygen is not used by these systems to synthesise ATP. These systems are quick at producing energy, however they do not last very long (they fatigue quickly because they do not call on oxygen as fuel). From an exercise perspective, it is optimal to use aerobic energy production for as long as possible to avoid fatigue and enhance fat burn.

The following illustration shows the interconnection between the energy systems and fitness components.

The following video discusses energy systems in more detail from the perspective of a fitness trainer.

Why does a personal trainer need to know the energy systems?

The main reason it is essential for coaches and trainers to have a working knowledge of the energy systems is to ensure their training programs are set up in such a way that they provide correct intensities, durations, rest periods, training types that are specific to the sport or activity they are training. Your capacity to train depends on your ability to supply your muscles enough ATP and improving these systems is very specific to the training you do.

What is adenosine triphosphate (ATP)?

• ATP is the body's energy currency
• ATP is composed of an adenosine molecule and three phosphate groups (hence Triphosphate)
• The bonds which are holding the phosphates together (represented in the following illustration in red) are very high in energy.
• When they are split a large amount of energy is released which can be harnessed for bodily processes like muscle contraction.
• You only have about three seconds worth of ATP stored in the body.

Let us look at the breakdown of ATP during exercise Look at the following illustration then read the explanation.

Explanation

Imagine you are an ATP, stored in the bicep's muscle. Your body picks up a dumbbell and attempts to complete a biceps curl. Before any of the bicep's muscle fibres can contract, they must call on you to supply energy. The energy comes from ATP splitting as in the image above. The bond which holds the third phosphate (P) on the ATP is split by an enzyme. The breaking of this bond releases a large amount of energy, which is then used to power the contraction of the biceps muscle.

As you can see in the above diagram, once the third phosphate has been split off you are left with Adenosine Diphosphate (Di = two) or ADP. ADP has little remaining energy to fuel exercise so it is now the role of the three energy systems to rebuild the ADP back to ATP so exercise can be maintained.

Let us use a simple analogy to explain these. The ATP molecule is just like a rechargeable battery. When it is fully charged, it's ATP. When it is run down, it's ADP!

Commonly asked questions 

There is a lot of biology in this concept! To learn more, watch the following eight-minute video that explains the cycle in a clear and simple way. The presenter also explains what happens when the ATP is converted into ADP.

As you may recall, each system provides ATP at different levels of exercise intensity and duration. The systems are ordered from the highest intensity to the lowest intensity. There are two anaerobic systems and one aerobic energy system. So, what do anaerobic and aerobic mean?

Let us look more closely at each system.

Think of this system as an eight-cylinder car engine, it is powerful but not very efficient!

In other words, high power and short duration. This system is the simplest and therefore the quickest at producing energy for exercise. It is also the fastest at converting spent ADP back into usable ATP again. It provides energy for up to 5 to 12 seconds of maximal intensity physical activity.

The ATP-PC system is used for activities such as jumping, short sprints, heavy lifting or other maximal activities. However, the system is active at the beginning of all exercises, regardless of their intensity. The system relies upon the breakdown (hydrolysis) of ATP via an enzyme or creatine phosphate (CP) as shown in the following equation.

What is creatine phosphate?

Creatine phosphate is the fuel that powers the ATP-PC system. Creatine is generally made by the liver after which around 95% is absorbed by lean muscle and quickly converted to Creatine Phosphate (or Phosphocreatine (PCr)). During intense exercise, ATP is used up in seconds and needs to be replenished using CP so that exercise can continue for up to 12 seconds.

Creatine phosphate is a common supplement for those looking to increase speed, power or strength. Consider the following question. How would supplementing creatine improve physical performance? So the body is either in an energy release phase (i.e. during exercise or sport) or in an energy regeneration phase (i.e. during recovery from exercise)

Study the following illustration and then read the explanation.

Explanation

Let’s walk through the diagram above step by step.

1. Find the ATP molecule (blue) on the above diagram; this is our starting point. This represents the small amount of ATP already stored in the muscles ready to go instantaneously.
2. Follow the cycle to the next station and you will see a Phosphate (P) split off from the ATP releasing energy for muscle contraction leaving us with ADP (two P with one split off).
3. Next, you will see the ADP meet up with Creatine Phosphate (CP), the CP donates its P to the ADP and we are now back to where we started with a regenerated ATP molecule ready to make the muscle fibre contract again.

What is time to fatigue?

The US National Library of Medicine (2017) define muscle fatigue in humans as 'exercise-induced decrease in the ability to produce force'. Take a look at the graph below and then read the explanation.

Explanation

During the first few seconds of intense muscular activity (sprinting) ATP is maintained relatively constant, but PC declines as it used to replenish ATP. The main limitation of very intense exercise duration is PC depletion. At exhaustion, both ATP and PC levels are low.

Your ATP and PC stores can sustain the muscle’s energy needs for 5-12 seconds in an all-out sprint. Beyond this point, ATP-PC contribution decreases, and the muscles rely more on other systems of ATP formation. That is the glycolytic and oxidative energy systems.

A one-off maximal contraction like a shot put toss, Olympic weight lifting lift, a single vertical jump does not require the use of the ATP-PC system. There is stored ATP in the muscle which can provide for these instantaneous explosive movements. The ATP-PC system must be activated once the stored ATP has been depleted after two to three seconds.

What is the Wattbike test?

The Wattbike was developed by British cycling as a specialised ergometer for elite cyclists. It was designed to mimic real cycling and is equipped with a wide range of tests and training programmes well suited for anyone looking to improve their conditioning in a low impact manner. Due to their programmability and functionality, they are almost universally used in elite sport and other training facilities.

The Wattbike has a six-second sprint test that measures the power of your ATP-PC system. It requires you to pedal as fast as you can for six seconds and measures the power you are able to produce over this timeframe. The example here shows the subject was able to average 1850 watts and 200 rpm over six seconds. This is an Olympic sprint cyclist level! Test results may look like the following.

A cadence peak in cycling, or pedal speed, is measured in pedal stroke revolutions per minute (RPM). For example, a cadence of 50 RPM means that one pedal makes a complete revolution 50 times in one minute.

A practical example of ATP-PC system training

You are a trainer and have prescribed some maximal strength training for a client wishing to improve her strength. You have prescribed five sets of four reps of some compound exercises with 90 seconds rest between sets.

This would be a good session for maximal strength and ATP-PC system development except the rest periods are too short. The fourth heavy rep will completely deplete your client's phosphocreatine fuel. It will now require approximately three minutes of rest to replenish the PC stores fully.

If the client started the second set after 90 seconds rest, she would start with the PC stores only about 70% replenished which could result in running out of PC at rep three. As soon as the PC stores run low the body will need to switch to the next available energy system, the glycolytic. However, this system is not as powerful as the ATP-PC so the client would need to lower intensity, stop early, or lower weight. Over a few weeks, this would result in greatly diminished strength training adaptation.

Think of this system as a six-cylinder car engine, it is moderately powerful with better efficiency compared to a V8.

The glycolysis system equates to moderate power and moderate duration. ​​​Glycolysis refers to the splitting of glucose. This process involves the anaerobic breakdown of glucose to produce ATP and provides energy for up to two minutes. Blood glucose and muscle glycogen compose the primary fuel for glycolysis.

Muscle glycogen is a stored form of glucose located in the muscle and liver. If you are not immediately active after a meal most of the glucose from CHO breakdown will be stored ready for future exercise. When you exercise next, this glycogen will break down to glucose enter glycolysis

Glycolysis is far more complex than the ATP- PC system and involves twelve reactions which ultimately produce pyruvic acid.  

• Anaerobic glycolysis is used during high-intensity activity (predominantly between 15-60 seconds) where glucose is broken down to pyruvic acid when then converts to lactic acid.
• Aerobic glycolysis is used during low intensity activities (greater than 60 seconds). Due to the lower intensity, pyruvic acid has time to be transported into the aerobic system where it is fully processed.

For every glucose used, two ATPs are formed from glycolysis. Look at the following diagram and then read the explanation. 

Explanation

• The fuel for glycolysis is glucose which you can see at the top of the previous diagram. Glucose has six carbon atoms represented by the orange circles.
• The addition of two phosphate molecules arises from the breakdown of two ATP molecules.
• This six-carbon, two-phosphate compound is then split into two parts, each with three carbons and one phosphate molecule.
• After several complex chemical reactions, the net products are pyruvic acid (or pyruvate) and ATP. 
• You will note on this diagram NAD → NADH +H being produced. This NADH will be transported to the aerobic system where it will produce large amounts of ATP when oxygen is present. 
• Pyruvate, the end product of glycolysis faces one of two fates; being utilised as an energy source by its conversion into lactate (a process known as anaerobic glycolysis) or being taken up by the mitochondria of the cells (a process known as aerobic glycolysis).

Lactic acid  

Anaerobic glycolysis causes lactic acid to accumulate in the muscle, which impairs muscle contraction. This is why anaerobic glycolysis is also called the lactic acid energy system. The human body does not tolerate well any changes in pH (or acidity) well and lactic acid that cannot be immediately removed will rapidly drop a body's pH severely affecting a person's physical performance. Lactic acid is removed rapidly from the body post-exercise but is removed even faster with an active cool-down following exercise.

A practical example of the glycolytic system training

You are a trainer and have a fit athletic client who has just started doing some hard glycolytic training to prepare for an upcoming sports season. The client lets you know that he has tried doing a series of maximal 30-second efforts with a full four minutes recovery between reps. He explains that after the first rep he cannot recover, and he gets slower and slower with each rep. He is worried that he is not fit enough to finish the workouts.

This is a common issue with glycolytic training. The client is correct in choosing 30-second efforts to maximally test this system. However, what you will find is the glycolytic system is a ‘one-hit-wonder’; it is good for maintaining one maximal prolonged effort but is not able to recover well for additional reps.

The following graphs demonstrate what has occurred. In this example, the client did two maximal sprints of 30 seconds with four minutes recovery between; and you can see the ATP contribution each system is making to these efforts. As you would expect, the first rep is predominately supported by the glycolytic system. However, in the second rep, the glycolytic system is still fatigued and has not recovered, therefore the aerobic system steps in to take the load.

The aerobic system is not as powerful as the glycolytic system, so the client's speed drops considerably. The aerobic contribution increases and power drops with each successive rep. This is a natural response to this style of training, so you could let the client know it may not be low fitness levels causing this. 

Glycolysis summarised

• The glycolytic energy system is the second anaerobic system the body uses to regenerate ATP during exercise.
• It operates at a moderate intensity for a moderate period of time (compared to the ATP-PC system) to provide a longer sprint or high-intensity burst for a relatively short time.
• It uses glucose as a fuel because glucose doesn’t require oxygen to be metabolised during glycolysis.
• Glycolysis can be aerobic or anaerobic but only anaerobic glycolysis produces lactic acid.

Think of this as a four-cylinder car engine, not much power but very efficient!

Without the oxidative system, exercise might be limited to only a few minutes. This system is the long-term provider of ATP. The oxidative system is aerobic and involves the breakdown of glucose, fat, and protein in the presence of oxygen. The more oxygen you can supply the more fuel (CHO, FAT, PRO) you can burn!

Where does the oxidative system occur in the muscle? Look at the following two images and read the explanations.

Explanation

All oxidative metabolism occurs in the part of the cell called the mitochondria (as shown in the preceding image. In the following image of the muscle fibres, you will see that the mitochondria (where all the aerobic processes take place), the supply of fuel in glycogen and fat are very close. ATP will leave the mitochondria and go straight to the muscle next door. One of the adaptations of aerobic training is an increased number and size of the mitochondria.

Explanation

The burning or metabolism of CHO to produce ATP involves two processes which both take place in the mitochondria:   

• Krebs cycle
• Electron transport chain

We will now look at each process in more detail.

What is the Krebs cycle?

The Krebs Cycle (also known as the citric acid cycle) accepts the pyruvic acid generated previously in glycolysis and processes it further in the presence of oxygen. Pyruvic acid is converted to acetyl CoA which is a universal entry point into the oxidative system.

Acetyl CoA enters the Krebs cycle, which is a complex series of chemical reactions that spin like a wheel producing ATP and C0². NADH is a high energy compound that will be transported to the electron transport chain (ETC) latter to produce ATP. Acetyl CoA can be considered the universal entry point of all macronutrients (CHO, FAT, PRO) into the aerobic (oxidative) system.

Remember, pyruvic acid is simply the glucose we started glycolysis with, that split in two halves. Universal simply means that the oxidative/aerobic system can only use one fuel type, much like your car (diesel in a petrol car won't work). Your oxidative system can only use Acetyl CoA as fuel.

Look at the diagram of the Krebs cycle and read the accompanying explanation.

Explanation

• Pyruvate (or pyruvic acid) is at the top of the diagram. It simply follows on from glycolysis which you learnt earlier. The pyruvic acid is made of three carbons, it loses one carbon (C) to CO² and turns into Acetyl CoA which is made of the remaining two carbons.
• The Acetyl CoA then enters the Krebs cycle where it spins around and gets further metabolised, producing more CO², ATP, NADH/FADH. The CO² is breathed out, the ATP is used to do work, and the NADH/FADH₂ is transported to the ETC (electron transport chain) where it makes more ATP later.
• The harder you exercise, the faster this cycle spins and the more pyruvic acid you need from glycolysis.

What is the electron transport chain (ETC)?

The ETC is a complex series of reactions that take place in the mitochondria. This third stage is responsible for most of the ATP production during exercise. It works by taking the FADH₂ and NADH formed during the Krebs cycle and glycolysis and uses them to produce more ATPs. Exercise, as well as good nutrition, allows your ETC to work more effectively providing more energy (ATP) during exercise.

A practical example of the oxidative system training

You are a trainer and have prescribed some aerobic exercise to a client wanting to lose weight. You give the client 30 minutes of cardiovascular exercise. Unfortunately, you miscalculate their current fitness level and start the client off at a treadmill speed of 10 kph (6 mins/km). However, their aerobic threshold is 8 kph. This means their aerobic system can provide enough ATP up to 8 kph after which the glycolytic system is going to need to ‘top-up’ any deficit or shortfall in ATP production.

As soon as you need to top-up the shortfall with the glycolytic system you will produce lactic acid which causes rapid fatigue as well as physical discomfort. This will diminish the quality and enjoyment of this aerobic exercise session by the client and potentially hinder their progression.

Fitness testing will help you find appropriate training intensities for your clients.

Recap on the oxidative system

Test yourself and make sure you can explain the following points about the oxidative system.

• Your oxidative system is also commonly referred to as the aerobic or cardiovascular system
• It is the energy system you use for 99.9% of your life so it is functioning continuously from sleep up to when you are doing maximal exercise. The anaerobic energy systems are only there to top-up the aerobic system when it can’t meet a person's energy demands.
• The oxidative system is the only system that can use all the macronutrients (CHO, PRO, FAT) as a fuel source. All these fuels come out the other end as H₂0, CO₂ and heat. Remember that PRO only contributes about 5% of energy needs under one hour and after that, it can jump to 15%.
• It produces large amounts of ATP energy but is relatively slow when compared to anaerobic systems.
• The oxidative system is composed of two processes: the Krebs cycle and the electron transport chain (ETC).
• It takes at least two minutes to get up and running at full capacity but can go indefinitely or until it runs out of fuel.

Why is burning fat so important? Fat is an almost limitless source of energy (for example, it can last weeks), whereas CHO is a very limited source of energy (which may last 60-90 minutes) to fuel ATP production. Someone who can supply the O₂ (strong heart and lungs) required to burn fat will generally have better endurance ability due to the sparing of valuable CHO supplies.

In today’s society, the ability to burn fat is also seen as an aesthetic advantage to being lean.

Fats are made up of free fatty acids (FFA) that can produce ATP in a process called beta-oxidation. Beta oxidation occurs within the mitochondria and involves converting FFA to acetyl CoA. Acetyl CoA enters the Krebs cycle just as it does with glucose metabolism, but more acetyl CoA is produced from fat than carbohydrate, so the energy yield is far greater.

_{* This is how much ATP is produced if glucose is used as a fuel in the oxidative system. The body prefers to use glucose as a fuel because it requires less oxygen to burn.}

_{** This is how much ATP is produced if fat is used as fuel in the oxidative system. You can see that fat is an excellent source of stored fuel as opposed to CHO. However, large amounts of oxygen are required to burn fat.}

The chemistry behind burning fat as a fuel

We have already mentioned that we have an almost unlimited supply of fat stored in our bodies and a very limited store of glucose. However, the body preferentially prefers to use glucose as fuel!  Why is this? Take a closer look at the chemical equations that represent the burning of glucose and fat.

Glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Fat: C₅₅H₁₀₄O₆ + 78O₂ → 55CO₂ + 52H₂O + Energy

Explanation

This might all look quite complicated but really, it's quite simple! You can see both fat and glucose are made of the same things: carbon, hydrogen, and oxygen, but in different quantities. The key difference for our focus here is the amount of oxygen needed to burn fat as compared to glucose; 78 for fat compared to just six for glucose.

Where does this oxygen come from? It has to be supplied by your cardio-respiratory system (your heart, lungs, blood and blood vessels). As so much oxygen is quite hard to supply, the body likes the ‘easy option’ of just having to supply six O₂s.

Not only does your cardio-respiratory system have to transport the 78 O₂ to the muscle, but it also needs to transport the 55 CO₂ back to the lungs to be exhaled. This is why unfit people fatigue quickly. They are not able to burn fat effectively because they cannot transport enough oxygen and tap into glucose which runs out fast but also produces lactic acid.

If you have a powerful aerobic system you will obviously be very well placed to supply this oxygen during times of need, like hard exercise!

Aerobic power (V0₂max)

Aerobic power describes how effective your oxidative energy system is at producing ATP during exercise.

One of the leading predictors of good health is your aerobic power or VO₂max. This is the ability of your muscles to use oxygen it receives from the heart and lungs to produce energy (ATP). For this reason, the largest VO₂max is typically seen in endurance sport athletes.

VO₂max is typically measured in millilitres per kg of body weight per min (ml/kg/min) at maximal exercise. The higher this number the more aerobically fit you are. It is a good fitness test for clients as it is an accurate predictor of aerobic fitness. The VO₂max test typically involves laboratory equipment and performed with qualified professionals.

Did you know? The highest VO₂max recorded in a human was 96 ml/kg/min for a cross country skier. VO₂max has been calculated for other animals. For example:

• a thoroughbred racehorse has a VO₂max around 190 ml/kg/min at 515 kg
• a Husky dog around 230 ml/kg/min 
• the highest VO₂max in a mammal is the Pronghorn Antelope with an estimated 300 ml/kg/min allowing it to run 10 kilometres in about 10 minutes!

The following table compares female and male values in millimetres (ml), kilograms (kg), and minutes (min). 

Female (value in ml/kg/min)

Male (value in ml/kg/min)

You have been exposed to a lot of information and chemistry. The following three tables will summarise the three energy systems.

Table 1: Three energy systems, activity, and ATP produced

The following table summarises the three energy systems, the type of activity powered, and the number of ATP produced by the activity.

Fat >100

Table 2: Speed of ATP produced by each system

Table 3: The three systems (intensity, duration, fuel, by-products, and recovery)

This has been a long topic and we have covered a lot of information about energy systems. It is important you are confident if you need to explain any of these concepts to a client. Focus on reviewing the energy systems and becoming comfortable with describing these to your clients, practice telling your friends and family about these, and sharpen your vocabulary in preparation for wowing your clients. the more you understand and can relay to your clients, the more they will be able to understand and feel both confident and comfortable working with you. Creating your own reference documents or mindmaps can be a useful way to reinforce this information. 

In this topic, we focused on energy systems. You learnt about:

• what an energy system is
• energy systems and fitness components
• the three key energy systems: the ATP-PC system, the glycolysis system, and the oxidative system (aerobic)
• why it is important to burn fat.