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CHAPTER 3: Aerobic and anaerobic pathways – an introduction to the energy systems 45ISBN 9780170186261

3Aerobic and anaerobic pathways – an introduction to the energy systems 3333Aerobic and anaerobic Aerobic and anaerobic Aerobic and anaerobic Aerobic and anaerobic Aerobic and anaerobic Aerobic and anaerobic Aerobic and anaerobic

This knowledge includes:

• introduction to the characteristics of aerobic and anaerobic pathways (with or without oxygen) and their contribution to movement and dominant ÿ bre type associated with each pathway.

These skills include the ability to:

• identify the dominant energy pathway utilised in a variety of aerobic or anaerobic activities determined by the intensity and duration of the activity. Collect, analyse and report on primary data related to responses to exercise and anaerobic and aerobic pathways.

knowledgekey

key skills

Energysystems

Muscle fibre typesFast twitchSlow twitch

Anaerobic glycolysisenergy system

Lactic acid systemLactacid system

Interplay

ATP-PC energy systemAlactacid system

Phosphocreatine system

Aerobic energy systemAerobic glycolysis system

Oxygen system

Movement

Food fuels

03 PE VCE 1&2 5ED TXT 4pp.indd 45 15/09/10 10:14 AM

Nelson Physical Education VCE Units 1 & 246 ISBN 9780170186261

All body movements require energy for muscle contraction, whether it be walking, riding a bike, dancing, swimming, playing a team sport or running. Where does this energy come from, how is it produced and used by the body at rest and during exercise, and what does this energy enable us to do? A clear understanding of energy systems will help you to understand your body better. This chapter will provide you with a comprehensive introduction to the characteristics of energy systems and their contribution to movement and the dominant muscle fi bre type associated with each pathway.

Foods and their conversion to energyFood is the primary source of energy, but it cannot be used directly. Digestion is the body’s way of breaking down food into nutrients that are then absorbed. Nutrients are carried away in the bloodstream to the cells of the body. Some of this fuel is used immediately for energy production. The remainder is stored in various forms and at different sites (see Table 3.1). The foods we eat contain nutrients, which are essentially the chemical substances carbohydrates, fats, proteins, vitamins, minerals and water.

Carbohydrates (CHO), fats and proteins are the only sources of fuel energy. To utilise these nutrients for muscle action, the body converts the nutrients to a common ‘energy compound’ called adenosine triphosphate (ATP). The energy in food is chemical energy, which is converted into mechanical energy to allow for muscular contractions. Movement, therefore, is a result of the chemical breakdown of food. The chemical energy is supplied through the breakdown of ATP, which is resynthesised almost as quickly as it is broken down by the breakdown of the stored nutrients releasing energy.

CarbohydratesWe need carbohydrates to fuel physical activity – they are the body’s preferred source of fuel, particularly during exercise. A carbohydrate-rich diet is essential for a physically active person. Carbohydrates play a key role in the performance of exercise lasting an hour or more. Therefore, carbohydrate intake before, during and after exercise to meet the fuel requirements of the activity is vital.

Carbohydrates are the sugars and starches found in fruit, cereal, bread, pasta and vegetables. Carbohydrates serve as major food fuels for the production of ATP. There are two forms of carbohydrates used for this purpose – blood glucose and

glucosea sugar

How food is stored in the body

Food fuel Stored as Site

Carbohydrate GlucoseGlycogenExcess as adipose tissue

BloodMuscle and liverAround the body

Fat Fatty acidsTriglyceridesAdipose tissue

BloodMuscleAround the body

Protein MuscleAmino acids

Around the body

3.1



muscle and liver glycogen. Muscle glycogen and blood glucose play key roles in intense exercise bouts, with the body’s glycogen stores playing an important part in sustained aerobic activity such as distance running.

We don’t eat glucose and glycogen directly – carbohydrates in foods are converted to glucose for immediate energy and to glycogen to be stored in the muscle. Glucose is a specifi c form of sugar and is found in the blood. Glucose is absorbed from the intestines into the bloodstream after you eat. The blood carries glucose to the muscle, where it enters the muscle through the aid of insulin.

The pancreas secretes insulin in response to the increase in blood glucose. Insulin is an important hormone that regulates the level of glucose in the blood by allowing glucose to be transferred from the bloodstream into the muscle. Without insulin, your body would not be able to process glucose and would therefore have no energy for movement or other functions.

Once inside the muscle, glucose is used as fuel for everyday activity, such as breathing, walking and running. However, not all the glucose is used immediately; excess glucose is stored in the liver as glycogen. When extra fuel is required, the glycogen is converted back to glucose to be used within the muscle. The liver stores about one-third of the body’s total glycogen. About two-thirds is stored within skeletal muscles, where it is very important as a primary source of energy for muscle contractions, especially during high-intensity exercise.

FatsFats are essential in our diet and perform many vital roles. For example, fats are involved in:

• protecting body organs

• maintaining body temperature

• hormone production

• energy stores for the body.

It is the role of fats as a fuel source in physical activity that is of greatest interest to students of physical education.

Fats are not all the same – some fats are better for you than others. Essential fats (EFAs), such as omega-3, are vital to overall health and wellbeing. Tuna and salmon are sources of omega-3. The overconsumption of fat, however, can increase the risk of diseases.

There are many sources of fats including butter, margarine, eggs, oil and nuts. Fats, or lipids, are found in the body in the form of triglycerides, stored in the fat cells (adipose tissue) located throughout the body and in skeletal muscle. Triglycerides are broken down into free fatty acids, which are broken down aerobically to provide energy for movement.

did you know?

The body can store enough glycogen

to provide energy for a few hours at

most during exercise and for less than

a day during rest. In order to meet the

body’s daily needs, the body requires

a continual supply of glucose, which is

achieved through the daily consumption

of carbohydrates. The brain maintains

a small amount of glycogen as an

emergency reserve during times of

severe glucose deprivation.

glycogenthe form in which carbohydrates are stored in the muscle and liver

insulina hormone that regulates the level of glucose in the blood

pancreasa gland that is both an endocrine and exocrine gland



Fats are the body’s preferred source of fuel at rest; as you sit here reading this book, your body is using fat. Fats are also used during prolonged submaximal exercise. A greater amount of oxygen is required by the body to utilise fats as a fuel than to use carbohydrates to produce ATP. Fats are capable of producing more ATP than carbohydrates. However, the conversion rate is less effi cient in terms of the amount of oxygen required to produce the same amount of ATP. In terms of the total amount of energy produced, fat has a higher energy content than carbohydrate, so is a more powerful fuel.

ProteinFoods that are rich in protein include meat, fi sh, poultry, eggs and cereal. Proteins are more complex and have larger molecules than either carbohydrates or fats. Their main role in the body is for growth and repair of tissue. Proteins are sometimes referred to as the building blocks

of the body. All enzymes (which speed up chemical reactions) are proteins. The basic structural unit of proteins are amino acids. Like carbohydrates and fats, proteins have a vital role in energy production – protein is used as a fuel source in long-duration endurance exercise. Protein is also often associated with strength-building exercise.

enzymeschemical substances that facilitate or speed up the rate of reactions occurring within the body

THINKING THINGS THROUGH

1 Identify which foods carbohydrates are typically found in.2 Identify which foods fats are typically found in.3 Which foods are proteins typically found in?4 Outline why carbohydrates are the body’s preferred source of fuel.

Foods for rest and exerciseThe type of food used for energy production depends on the duration and intensity of the exercise. Similarly, the usage of food fuels is determined by whether or not a person is exercising or resting. At rest, the aerobic energy system is the dominant system in operation, with fats contributing about two-thirds of the food fuel and carbohydrates contributing about one-third. This is because the body is working submaximally, with the cardiorespiratory system working to supply the cells with enough oxygen for the production of ATP. There are no fatiguing by-products. The contribution of proteins is minor.

figure 3.1 A healthy

diet should contain

balanced amounts of

carbohydrates, fats and

proteins.

>>answers



Once a person starts to exercise, both the anaerobic and aerobic energy systems contribute to ATP resynthesis. The duration and the intensity of the exercise being undertaken largely determines which energy system is predominant.

A marathon runner who runs for around 2 hours 30 minutes will be using the aerobic energy system, with carbohydrates being the preferred fuel. This is because the marathon is predominantly a low-intensity submaximal activity. The longer the marathoner runs, the more their glycogen stores will gradually drain away, and fats will become the main food fuel. The exact time when fats become the more dominant fuel will vary for different athletes and will depend on, among other things, the athlete’s training, their diet and their initial glycogen stores. For a runner in the 400-metre event, which is predominantly a high-intensity activity, the anaerobic system will be dominant, carbohydrates being the preferred fuel. In the case of the pole-vaulter, who may take only 9 seconds from the approach to the push away, the fuel is chemical, not a food fuel because of the time and duration and the need for immediate fuel.

Energy systemsThe breakdown of ATP releases energy for muscular contraction. Stored within the muscle is a limited quantity of ATP, responsible for movement. ATP is continually being used and resynthesised, which enables the body to keep moving. There are three energy systems responsible for the manufacturing of ATP and there are essentially two mechanisms for producing ATP – the aerobic and anaerobic pathways. ‘Aerobic’ literally means with oxygen, while ‘anaerobic’ means without oxygen.

There are two types of anaerobic system:

• The ATP-PC system – also known as the alactacid or phosphocreatine (or creatine phosphate) system

• The anaerobic glycolysis system – also known as the lactic acid or lactacid system

There is only one aerobic system, which is also known as the aerobic glycolysis or oxygen system.

did you know?

When you start to feel tired or fatigued, eating a

snack such as an apple, orange or a snack bar made

from grains (low GI) will help maintain your blood

glucose levels at a consistent level.

Eating a bar of chocolate (high GI) or other sugar-

rich foods will result in a rapid rise in blood sugar

levels and the production of too much insulin, raising

insulin levels above the optimal level. In turn this will

cause your blood glucose levels to rise dramatically.

Exclusively eating high GI food can leave you

feeling lethargic and doesn’t allow you to perform

at your best. Eating a balanced diet will allow you to

maintain consistent blood sugar levels, and perform

at your optimum. A diet high in fi bre will help keep

your blood glucose regulated at a consistent level.

figure 3.2 Energy sources when

the body is at rest

Lactic acidATP

CO2 H2O

Anaerobic

Glucose 13

Fats 23

O2 Aerobic ATP



Energy is needed for everyday activities and exercise. The amount of energy required during exercise by the body depends on the intensity and duration of the exercise being undertaken. If you were to run around the local sports oval at a very high speed and intensity, you would only be able to sustain this effort for a relatively short period of time. But if you were to run at lower intensity, you would be able to sustain this activity for longer. We can further categorise each running activity according to the demand for ATP, the duration of each activity, the presence of oxygen (or not) and the intensity. Remember, though, that while there will always be one dominant system, the three energy systems do not function independently; all three systems ‘turn on’ at the start of exercise.

ATP-PC systemThe fi rst energy system/pathway is the ATP-PC system. It is immediately available, is the simplest and quickest system for breaking down PC (phosphocreatine) to create ATP, and is stored in muscle cells. This system does not require long chemical reactions, does not use oxygen and is used for high-intensity activities involving explosive movements such as sprinting, long jumping and throwing the javelin.

This system is dominant for the fi rst 1–5 seconds of an activity, with peak power being between 2 and 4 seconds, and is exhausted after about 10 seconds of intense activity. Once PC has been depleted, 50 per cent replenishment is achieved within 30 seconds of passive recovery; total replenishment takes 3+ minutes. The ATP-PC system is therefore linked to the fi tness components of muscular power and speed. If it were not for this system, explosive powerful activities would not be possible.

PC is not used for muscle contraction; it is mainly used for resynthesising ATP. ATP is broken down to adenosine diphosphate (ADP). As rapidly as this breakdown occurs, the remaining PC is broken down to join with the ADP to form ATP again. This is the resynthesising process of ATP. The ATP-PC system does not require oxygen, and there are no waste products produced – it does not produce lactic acid. However, this system is very short in duration.

The process cannot continue indefi nitely as the stores of PC deplete. After the initial 10 seconds when the PC stores totally deplete, the next energy system becomes dominant and there is increased reliance on the anaerobic glycolysis system to supply ATP. Remember – the energy systems contribute energy sequentially but in an overlapping way.

As discussed in Chapter 1, fast-twitch fi bres are better suited to short-duration high-intensity anaerobic work requiring performers to call upon speed, power and explosive efforts. Once these fi bres are recruited they begin to rapidly produce metabolic by-products that cause fatigue and hence can only be used for a very short time. For example, an elite 100-metre sprint runner has a predominance of fast-twitch fi bres, which have the capacity to produce a large force when activated. But the athlete will fatigue quickly; therefore, we say this athlete has a low fatigue resistance. Fast-twitch fi bres have high stores of phosphocreatine, but are low in oxidative enzymes and myoglobin. Recall that the ATP-PC system does not use oxygen, so characteristics associated with the oxygen system will not be predominant when considering fast-twitch fi bres.

It becomes easier to understand individual athletes’ fi bre make-up by considering the predominant energy system being used. The elite 100-metre sprint runner, who is able to complete the race in 10 seconds, requires speed and explosive power. This athlete will have



high numbers of fast-twitch fi bres. As the race predominantly utilises the ATP-PC system (determined by the duration and the intensity), this athlete will have high phosphocreatine stores, as this is the quickest source of immediate energy under anaerobic conditions. In short, the 100-metre sprint runner has the fi bres that are best suited to the event they perform.

Anaerobic glycolysis or lactic acid systemThe anaerobic glycolysis (lactic acid) system involves the incomplete breakdown of carbohydrates. The energy is obtained from the breakdown of glycogen; however, as oxygen is not present, glycogen is not completely broken down and pyruvic acid is formed. As with the ATP-PC system, the anaerobic glycolysis system does not require oxygen; however it does produce lactic acid (which accumulates within the muscle), which leads to an accumulation of hydrogen ions. This in turn increases muscle acidity, decreasing muscle pH and preventing the coupling of cross-bridges (see Chapter 1).

The decrease in pH also impedes the action of glycolytic enzymes, which will decrease the rate at which glycogen is broken down to form ATP. Next time you are swimming with maximum effort, visualise this happening in your muscles. This will further help you understand and explain that burning sensation or feeling of exhaustion you experience during this type of activity. The lactic acid produced is also broken down to glycogen to produce further energy.

figure 3.3 The 100-metre sprint predominantly

uses the ATP-PC system and fast-twitch fi bres.

Can you think of other examples of sports that

predominantly use this system?

figure 3.4 A

gymnast performing

a pommel horse

routine predominantly

uses the anaerobic

glycolysis system and

fast-twitch fi bres. Can

you recall the colour

of the fast-twitch fi bre

type?



This system is normally used for activities that last between 10 and 75 seconds. Anaerobic glycolysis is more complicated than the ATP-PC system, involving many more chemical reactions. The energy is obtained by breaking down glucose (either stored in muscles or from the bloodstream). Peak power occurs between 5 and 15 seconds. Anaerobic glycolysis, like the ATP-PC system, is critically important to the functioning of the human body because it provides a rapid supply of ATP, thereby enabling the continuation of high-intensity effort. When the anaerobic glycolysis system is used for ATP production, there comes a time when the high-intensity effort cannot be sustained. The performer will either stop (due to fatiguing factors) or reduce the intensity of the effort. This system is typically used in 100-metre freestyle swimming at the elite level, or a pommel horse routine in gymnastics that lasts 30–45 seconds. The anaerobic energy system is linked to the fi tness components of muscular power and speed.

Like the ATP-PC system, the anaerobic glycolysis energy system predominantly recruits fast-twitch fi bres. Performers undertaking activities predominantly using the anaerobic glycolysis system benefi t from the presence of glycolytic enzymes and the high contraction speed of fast-twitch fi bres. Glycolytic enzymes will speed up glycogen breakdown during high-intensity efforts lasting beyond 10 seconds, such as a 50-metre freestyle sprint swim.

Aerobic energy systemThe aerobic energy system uses oxygen and is by far the most powerful of the three energy systems. The preferred food fuel for the aerobic energy system during activity is carbohydrates from stores in the muscle or from the blood. The aerobic energy system is also known as aerobic glycolysis – or the breakdown of glucose in the presence of oxygen. The clear difference between anaerobic and aerobic glycolysis is the complete breakdown of glycogen with no fatiguing toxic by-products. The waste products are carbon dioxide, which we breathe out (respiration), and water. In order for this system to function, oxygen must be present.

At rest the aerobic system uses fats. However, fats are also used as a food fuel by the aerobic system during activity, particularly during extended endurance exercise. Fats can produce more ATP than carbohydrates, but they require more oxygen to produce the equivalent amount of ATP.

Aerobic glycolysis occurs within the mitochondria – known as the powerhouses of the cell. The aerobic energy system is the slowest to contribute to ATP resynthesis – due in part to the many complex chemical reactions. If the intensity of the activity is not too high and the body has the necessary stores of glucose and triglycerides, then the activity can continue indefi nitely.

The aerobic energy system will be the dominant system for activities such as sitting and walking, and for sustained endurance activities such as a 5 km run, that are said to be of submaximal intensity – or less than 80 per cent of maximum heart rate (usually in the zone of 60–80 per cent). Peak power occurs between 1 minute and 1 minute 30 seconds, and this energy system is the dominant system for activities that are more than 75 seconds in total duration. A marathon runner will have a well-developed aerobic system, as would a midfi elder in Australian Rules football and a midcourt player in netball. The fi tness component linked to this system is cardiorespiratory endurance.

There are three stages in aerobic energy production, responsible for the release of energy:

• Stage 1 – the breakdown of carbohydrates and fats to produce 2 ATP molecules

• Stage 2 – Kreb’s cycle, which involves the breakdown of pyruvic acid into carbon dioxide. Further energy is released to resynthesise to ATP for a net production of 1 ATP molecule.

• Stage 3 – the electron transport stage, which involves water, heat and produces the largest yield of ATP – a total of 34 molecules



Slow-twitch muscle fi bres are best suited to aerobic, endurance-based activities. Marathon runners have predominantly this fi bre type. Slow-twitch fi bres contract slowly and repeatedly but have a low force capacity.

Slow-twitch fi bres are high in:

• oxidative enzymes

• mitochondria density

• fatigue resistance, as the muscle can continue to be called upon to work over an extended period of time

• capillary density.

Recall that the aerobic energy system relies upon the blood to deliver oxygen to the working muscle to understand why these characteristics would be high. For a more detailed discussion of fi bre types, refer to Chapter 1.

Remember that all three energy systems are activated at the start of exercise and no single system works by itself. All systems are in play all the time, no matter what you are doing. Just as the aerobic system is activated and utilised for the 100-metre sprinter in their 10-second maximum effort, so too the 1500-metre swimmer will activate all three systems as they dive Aerobic glycolysis

Glycogen Glucose

Energy

Energy

Pyruvic acid (insufficient oxygen)

Lactic acid

Glycogen Glucose Pyruvic acid (sufficient oxygen)

CO2 H2O ATP

Anaerobic glycolysis

ATPADP Pi

ATPADP Pi figure 3.6 Anaerobic

vs aerobic glycolysis


1 Summarise each energy system by listing the key characteristics of each.2 Explain why we call upon the ATP-PC energy system at the start of exercise.3 Outline the key difference between anaerobic and aerobic glycolysis.

figure 3.5 A marathon runner

predominantly uses the aerobic system.

What fi bre types are best suited to this

type of activity?

>>answers



Energy system interplay in an intermittent team sport activity

Kate plays centre for her local netball team.

The 2-minute warning before the start of the game booms over the loudspeaker, as Kate stands waiting for the game to start. Her aerobic energy system is dominant, as the intensity is low and the demands for energy production are being met by the delivery of oxygen to the working muscles.

The whistle signifi es the start of the game. Kate steps into the circle, makes a pass and then pushes off explosively, sprinting to make position to receive the next pass. To fuel this explosive movement, the dominant system is the ATP-PC system – PC is the fuel source. Kate continues to move at pace, cutting and driving, continually making position to receive the next pass. Kate has now been working for 17 seconds at high intensity, and her anaerobic glycolysis system has become dominant, so glucose is the fuel.

Kate jumps into the air to intercept an opponent’s pass, landing and passing to the goalshooter in the one action. Kate’s movement drops to a low-intensity walk; her

>>

off the starting blocks. At any given time though, there will be a dominant system. The following Real World Focus will help enhance your understanding of interplay of the energy systems in a team game situation.

Energy systems

Anaerobic power

Muscular power

LME

Speed endurance

LME

Aerobic power

Muscular strength

Speed AgilityAnaerobic power Muscular power

Anaerobic glycolysisLactic acidATP-PC Aerobic glycolysis

Aerobic

figure 3.7 The relationship

between energy systems and

fi tness components

REALWORLDFOCUS



heart rate drops, allowing her to replenish some PC and remove metabolic by-products. Breathing heavily, she walks towards the goal ring in case the opposition rebounds the shot. The goal is missed and the ball is quickly transferred out of the opposition team’s defensive third. Forced to defend, Kate pushes off to man up and defend her opponent, rapidly moving from a standing start to a sprint. This rapid transfer from a stationary position to a sprint requires immediate energy, and as her PC has been depleted (and not been fully replenished) and her movement interspersed with low-intensity activity, the anaerobic glycolysis system is again called upon to power this short effort.

Kate’s team wins back possession. Kate’s work rate drops to a cruise, before she again sprints to take possession, catching and passing in one action. This change of intensity sees Kate move between the energy systems, the anaerobic glycolysis system fuelling the explosive activities. This time Kate’s team scores a goal.

Kate moves from a stationary position (resting) to a jog to a cruise to get back in position to defend the centre pass. These movement patterns are repeated throughout the game: Kate moves from periods of continual motion at high intensity to low intensity to standing still. It will not be until quarter or half-time break (2+ minutes) when Kate can be at complete rest, and she will have the opportunity replenish her PC stores.

Kate’s movement patterns are typical of a ‘mobile’ player. A set position player such as a goalkeeper will have longer periods of low-intensity activity (and rest periods) with intermittent high-intensity efforts.

Netball could be described as an aerobic game interspersed with high-intensity activities. As the type of activity changes, so too does the contribution of the energy systems°– at any given time there will be one dominant system. For this reason we refer to this as ‘interplay’. The duration of the activity will determine which system is used primarily, but they are all used to some extent.

>>

figure 3.8 After

reading the Real World

Focus on netball, discuss

the interplay of the three

energy systems.



Energy system interplay in a continuous individual activity

Each night Steve completes a 30-minute run. As Steve takes his fi rst couple of steps, he is using the ATP-PC system. As the intensity of the run is low initially, the PC stores will not be depleted as rapidly as they would if he was sprinting. The ATP-PC system will be the dominant system at the start as it is the immediate provider of energy, PC being the chemical fuel. As Steve continues his run, his PC stores will drain away, while his anaerobic glycolysis system will be increasingly contributing to energy production through the breakdown of glycogen (as will the aerobic system).

After 25 seconds, oxygen has still not made its way to the working muscle, so he is still working anaerobically – the anaerobic glycolysis system would become the main energy supplier. The intensity of the effort is low, and although fatiguing by-products are being produced as a result of the incomplete breakdown of glycogen, the amount of lactic acid being produced is not enough to slow Steve down or°cause discomfort.

Steve is fi nding breathing diffi cult initially because he is in oxygen defi cit – a period of transition where his body is able to provide the energy necessary for muscle contraction without oxygen, until such time as oxygen can be delivered to the working muscle to enable the complete breakdown of glycogen. During this temporary oxygen shortage, lactic acid will not reach lactate infl ection point (LIP) levels, so he will not need to reduce the intensity at which he is running while the aerobic energy system is increasing its contribution, eventually allowing for a seamless transition between the last of the anaerobic systems and the oxygen system, around the 30-second mark.

lactate infl ection point (LIP)the moment when the body is unable to prevent the accumulation of the hydrogen ions associated with the conversion of lactic acid to lactate in the working muscles

figure 3.9 Running for 30 minutes is

predominantly an aerobic activity – it is

submaximal and endurance based – and relies

on the effi cient delivery of oxygen to the

mitochondria of the working muscle for the

complete breakdown of glucose in the muscle.

Can you identify which muscle fi bre type would

be predominantly recruited?

>>

The following Real World Focus looks at the energy system interplay involved in a 30-minute run.

REALWORLDFOCUS




Explain how the energy system interplay varies between the two Real World Focus examples.

Peak power for the aerobic energy system will occur between 1 minute and 1 minute 30 seconds.

As oxygen is now being delivered to the working muscle through the bloodstream Steve settles into a comfortable running pattern; his demands are being met aerobically, his breathing is constant. It is at this point that Steve reaches a steady state. While there is minimal replenishment of his PC stores, there is not enough to allow for this system to be called upon again. It will not be until Steve completes his run and his body has been at complete rest for at least 3 minutes that his PC will be totally replenished.

Steve has now been running for around 12 minutes. A slight hill forces him to work harder. His heart rate becomes elevated in direct relation to the increased workload (during this short period Steve will experience another oxygen defi cit period), and although his anaerobic glycolysis system increases its contribution, the aerobic system remains the major contributor of energy. After 90 metres, the terrain levels out again, and Steve returns to a steady state, the demands of the run being met aerobically. He will encounter several more hills on his run, which will see an increased contribution from the anaerobic glycolysis system, but not enough to make it the most dominant system.

With 250 metres to go to the end of his run, Steve wants to fi nish strongly, so he increases his stride length. At this point, there is almost a linear increase in oxygen consumption to match the increase in exercise intensity. Even as Steve powers home, and again the anaerobic glycolysis system increases its contribution to energy production, the aerobic system will remain the dominant supplier of energy.

The run fi nished, Steve walks around breathing heavily. His body has already gone into recovery phase or excess post-exercise oxygen consumption. ATP is being resynthesised, PC is being restored, lactic acid is being oxidised and his core temperature is returning to its normal values.

As Steve continues his cool-down walk, munching on his watermelon, the smell of spaghetti bolognaise titillates his senses; and tomorrow he will do it all again.

>>

steady statethe point during exercise when oxygen supply equals oxygen demand

excess post-exercise oxygen consumptionan increased rate of oxygen following strenuous exercise

>>answers



figure 3.10 Athletes

running the beep test

stu

de

nt

stu

de

nt activ

ityactiv

ity The 20-metre multistage shuttle-run test

AIMTo gain an understanding of the contribution and interplay of the energy systems.

EQUIPMENTSuitable fl at area with two lines 20 metres apart, CD player and instruction CD of 20-metre multistage shuttle-run test

METHOD1 This test requires you to run continuously

between two lines. It can be done as a whole-class activity. Starting from one line, on the fi rst beep, run to the other line.

2 Wait for the next beep before running again. Upon hearing the beep, pivot and run in the reverse direction, reaching the other line in time for the next beep.

3 At each beep, you must have reached one of the 20-metre lines.

4 As the test proceeds, the interval between beeps reduces, so that you have to increase speed in order to continue the test, until the you fi nd it impossible to reach the line in time.

5 Once you cannot reach within two strides of the line twice in a row, fi nish the test. The

last number announced before fi nishing is your score.

RESULTSComplete a laboratory report utilising the data you have collected.

DISCUSSION1 Which muscle fi bre type would you be most

reliant upon in this test? Briefl y justify your response.

2 List two characteristics of the fi bre type used when performing this test, other than colour.

3 Name three sports that you believe this test to be specifi c to. Justify your response with specifi c movement patterns from these sports.

4 The 20-metre multistage shuttle run test predominantly relies upon carbohydrates as food fuel. Fats can provide more ATP than carbohydrates, yet they are not our preferred exercise fuel. Briefl y discuss why.

5 With specifi c reference to the data obtained, clearly discuss how all three energy systems work together (interplay) to supply energy throughout the test. Explain your response by referring to duration and intensity.

practical activity



3ChapterSummary

• Food is the body’s primary source of energy, but cannot be used directly. Digestion is the body’s way of breaking down food into nutrients, which are then absorbed.

• Foods contain nutrients, which are chemical substances. Carbohydrates, fats, proteins, vitamins, minerals and water are essential to our diet.

• One of the roles of nutrients is to provide energy. Carbohydrates, fats and proteins are used as fuel for reactions in the body – they provide us with energy or the capacity to do work or physical activity. To utilise these nutrients for muscle action, the body converts the nutrients to a common ‘energy compound’ called adenosine triphosphate (ATP).

• Carbohydrates are the sugars and starches found in fruit, cereal, bread, pasta and vegetables. Carbohydrates serve as major food fuels for the production of ATP.

• Fats or lipids are found in the body in the form of triglycerides, stored in the fat cells (adipose tissue) located throughout the body and in skeletal muscle. Triglycerides are broken down into free fatty acids, which are broken down aerobically to provide energy for movement.

• Proteins are more complex and have larger molecules than either carbohydrates or fats. Their role in the body includes growth and repair of tissue. Proteins are sometimes referred to as the building blocks of the body.

• The breakdown of ATP releases energy for muscular contraction. Stored within the muscle is a limited quantity of ATP, responsible for movement. ATP is continually being used and resynthesised, which

enables the body to keep moving.• There are three energy systems responsible

for the manufacture of ATP and there are essentially two mechanisms for producing ATP – the aerobic and anaerobic pathways.

• The two types of anaerobic systems are the ATP-PC system, also known as the alactacid or phosphocreatine system, and the anaerobic glycolysis system, also known as the lactic acid or lactacid system.

• There is only one aerobic system – the aerobic glycolysis or oxygen system.

• The ATP-PC system is immediately available, is the least complicated and quickest system for breaking down PC to create ATP, and is stored in muscle cells.

• The anaerobic glycolysis (lactic acid) system involves the incomplete breakdown of carbohydrates. The energy is obtained from the breakdown of glycogen. However, as oxygen is not present, glycogen is not completely broken down and pyruvic acid is formed.

• The aerobic energy system uses oxygen and is by far the most powerful of the three energy systems. The preferred food fuel for the aerobic energy system during activity is carbohydrate from stores in the muscle or from the blood. The aerobic energy system is also known as aerobic glycolysis – the breakdown of glucose in the presence of oxygen.

• All three energy systems are activated at the start of exercise and no single system works by itself. All systems operate all the time, but at any given time, there will be a dominant system. >>



Multiple choice

1 Which of the following nutrients are used as fuel for reactions in the body?A Carbohydrates, vitamins and proteinsB Carbohydrates, minerals and fatsC Proteins, fats and mineralsD Carbohydrates, fats and proteins

2 The fi rst energy pathway is the:A lactacid systemB aerobic glycolysis systemC alactacid systemD oxygen system.

3 Which of the following predominantly uses the anaerobic glycolysis system?A 100-metre freestyle swim at elite levelB 100-metre sprint in athleticsC Golf swingD Marathon run

Short answer

4 Explain the difference between anaerobic glycolysis and aerobic glycolysis.

5 The game of soccer predominantly relies upon the food fuel of carbohydrates. Fats can provide more ATP than carbohydrates, yet they are not the preferred exercise fuel. Briefl y discuss why.

6 Explain the term ‘energy system interplay’.

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Aerobic and anaerobic pathways – an introduction to the ... · r w > ISBN 9780170186261 CHAPTER...

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