Area of study 2 - Macmillan Publisherscdn-media.macmillan.com.au/mea/downloadpdfs/9781420229813.pdfArea of study 2 Physiological responses to physical activity Key knowledge This knowledge

Area of study 2Physiological responses to physical activity

Key knowledge

This knowledge includes:

the mechanisms responsible for the acute responses to exercise in the cardiovascular,

respiratory and muscular systems

characteristics and interplay of the three energy systems (ATP–CP, anaerobic glycolysis,

aerobic system) for physical activity, including rate of ATP production, the capacity of each

energy system and the contribution of each energy system

fuels (both chemical and food) required for resynthesis of ATP during physical activity and

the utilisation of food for energy

relative contribution of the energy systems and fuels used to produce ATP in relation to the

exercise intensity, duration and type

oxygen uptake at rest, during exercise and recovery including oxygen defi cit, steady state,

and excess post-exercise oxygen consumption

understanding of the multi-factorial mechanisms (including fuel depletion, metabolic

by-products and thermoregulation) associated with muscular fatigue, as a result of varied

exercise intensities and durations

passive and active recovery methods to assist in returning the body to pre-exercise levels.

Key skills

These skills include the ability to:

describe, using correct terminology, the interplay and relative contribution of the energy

systems in different sporting activities

participate in physical activities to collect and analyse data relating to the range of acute

effects that physical activity has on the cardiovascular, respiratory and muscular systems of

the body

perform, observe, analyse and report on laboratory exercises designed to explore the

relationship between the energy systems during physical activity

explain the role the energy systems play in enabling activities to occur as well as their

contribution to active and passive recovery

explain the multi-factorial mechanisms associated with fatigue during physical activity and

sporting events resulting from the use of the three energy systems under varying conditions

compare and contrast suitable recovery strategies used to counteract fatigue and promote

optimal performance levels.

VCEPE_CH05.indd 95VCEPE_CH05.indd 95 5/17/10 4:49:13 PM5/17/10 4:49:13 PM

Whether we are waiting for the starter’s gun to sound at the Olympic 100 metre track fi nal or

watching a sprint fi nish during a stage of the Tour de France, there is one question universally

asked more often than any other—who will win? When it comes to understanding why an

athlete or team produces a superior performance there is usually not just one explanation. It

may be related to superior genetics, physiology and fi tness or it may be more closely linked

to an athlete’s technique, skill and decision-making ability. Whatever the combination, sports

scientists and coaches alike recognise the importance of an athlete being able to supply energy

for muscle contraction in order to maximise event power, speed, agility or endurance.

Chapters 5 to 7 focus on understanding differences between the three distinct pathways

that provide energy for muscle contraction known as the energy systems. How the body

systems work to supply oxygen and nutrients essential for ATP production and the interplay

between the three energy systems are also discussed. This Area of Study concludes with an

extensive review of the potential fatigue mechanisms that limit physical activity and the passive

and active recovery methods used by athletes.


The energy systems:

performance

engines of

To understand energy systems the easiest approach is to liken a system to a car engine. Using this simple analogy it is not surprising to learn that the three distinct energy systems differ in both power and capacity in much the same way as the specifi cations of car engines differ. It is important to note that, although the energy systems differ with respect to power and capacity, all three in fact serve a common purpose. That purpose is to provide the human body with a continual supply of chemical energy in the form of the energy-rich compound known as adenosine triphosphate, or ATP. This process is essential for maintaining many complex cellular functions including muscle contraction. We rely on ATP to maintain our everyday lives and ensure that our respiratory, cardiovascular and muscular body systems function. It is the molecule that gives us life.

Energy systems: chemical

pathways in our body that

resynthesise adenosine

triphosphate (ATP) for everyday

activities

Power (energy system): the

rate of adenosine triphosphate

(ATP) resynthesis, related to

exercise intensity

Capacity (energy system): the yield of adenosine

triphosphate (ATP) resynthesis,

related to the exercise duration

Body systems: systems

within the body that respond to

physiological changes (during

exercise); the three principal

body systems involved in

physical activity are respiratory,

cardiovascular and muscular

So why is it that we need to store energy in the form of ATP within the human body? The reason is simple: the energy released from the breakdown or metabolism of food is not able to be transferred directly to the cells to be used for biological work. Therefore, it is critical that we can capture this energy in a form that can be used by the body. The ATP molecule offers an effective storage solution for potential energy and can be thought of as our universal ‘energy currency’. However, the problem we face is that ATP can only be stored within the body in limited amounts. In fact, the amount is so small that it is only enough to fuel approximately 2 seconds of performing at maximal effort. As most sporting events last longer than a couple of seconds, ATP must continually be replenished or resynthesised . This is achieved from the breakdown of fuel sources by three different systems, known as our energy systems. Sporting success depends on the ability of these energy systems to supply ATP for muscle contraction to generate force and power for the duration of the event.

Metabolism: the breaking

down of fuels via a series of

chemical reactions for energy

release

ATP: our energy currency


98 Macmillan VCE Physical Education 2 (Units 3 & 4)

Performance

Interplay

of energy

systems

ATP

demand

Muscle

contraction

Recovery

method

Fatigue

mechanisms

Oxygen

kinetics

Figure 5.1

Our body’s performance

relies on the production of

ATP through the breakdown

of body fuels

Figure 5.2 shows a simplifi ed structure of ATP. As considerable energy is released upon breaking the outermost phosphate bond, ATP is also often referred to as a high-energy phosphate. During exercise, the energy released in this reaction is used as the immediate source for muscle contraction to generate force and power.

Fuelling performance Professional cyclists competing in the Tour de France are required to sustain high power outputs for up to 7 hours in the pursuit of a stage win. In doing so, they will expend large amounts of energy per day. Researchers have calculated the average daily expenditure to be about 6500 kilocalories with

Resynthesis (ATP): the re-formation of ATP

following metabolism; ATP is

resynthesised during exercise

for muscle contraction

Figure 5.2

ATP is the currency that

fuels muscle contraction

‘High-energy’ phosphate bond

P P PAdenosine P P + +PAdenosine

Energy for muscular workSimplified structure

ATP ADP + Pi + energy

energy


99CHAPTER 5The energy systems: engines of performance

some cyclists expending up to 9000 kilocalories during longer race stages. To sustain this work rate, their skeletal muscles need a continual supply of energy in the form of ATP.

It is diffi cult for us to appreciate the daily challenge faced by these athletes when their energy requirements are reported in kilocalories. So when researchers expressed these requirements as the number of cheeseburgers a Tour de France cyclist would need to consume per day, it certainly put things into perspective. Imagine having to eat up to 27 cheeseburgers a day for three weeks to meet your energy needs: 15 cheeseburgers across the morning and afternoon with an additional 12 cheeseburgers for dinner! When put in this way the practical energy problem faced by elite endurance athletes becomes immediately obvious.

Food requirements for athletes

The food that keeps the wheels spinningBy Bonnie DeSimone, Globe correspondent

Because they can’t stop for lunch, cyclists are handed the food, and they eat as they ride. Team soigneurs, who double as masseurs and errand runners, prepare the musettes, or feed bags, that riders get at the start and in the middle of each stage during the three-week race. Soigneurs stand by the side of the road with the food, and riders slow down to pick it up. Feed-bag fare tends to be utilitarian—the ultimate fi nger food, since it’s being consumed in motion—and includes cut-up sandwiches, fruit, fruit or protein bars, and Coke.

Italian riders in the Tour de FranceBreakfast250 g (dry measure) rice or pasta115 g muesli

125 g carton of yoghurt1 to 2 slices wholegrain bread with homemade marmaladeOmelette made with 3 egg whites and one yolk1 to 2 slices ham and/or cheeseFreshly squeezed orange juiceCoffeeFeed bags (on the bike)4 cakes or bars (rice cakes, cookies, fruit bars)Fruit: chunks of pineapple, sliced banana and a quartered apple2 or 3 fi nger sandwiches (ham and cheese)3 or 4 protein barsOne small can of CokeSnack (in hotel)Small bowl rice pudding and fruitDinner1 to 2 slices prosciutto or ham250 g (dry measure) pasta with

fresh tomato sauce250 g beef steakRatatouille (eggplant, zucchini, tomato stew)Yoghurt and fresh fruitSlice of cake (optional)Water

Source: Adapted from B DeSimone (2006), ‘The food that keeps the wheels spinning’, The Boston Globe, 19 July, www.boston.com/ae/food/articles/2006/07/19/the_food_

that_keeps_the_wheels_spinning/

>>



So although most of us do not have the same energy requirements as an elite endurance athlete we still need to fuel our daily physical activities. This is achieved from the breakdown of different fuel sources by the three energy systems in order to maintain muscular ATP stores. The chemical energy trapped within the bonds of a fuel source is extracted via a series of complex reactions specifi c to an energy system. While a car engine uses one type of fuel (petrol or diesel), skeletal muscle can obtain energy to resynthesise ATP from breaking down as many as four different fuel sources:

phosphocreatine (PC) carbohydrate (CHO) lipid (fat) protein.

The majority of these fuels come from dietary sources (food), with some being produced internally by the body. These fuels are stored in our muscles, liver and adipose tissue . The amount of each fuel stored differs along with the amount of oxygen required for breakdown and the resulting rate and yield of ATP that is resynthesised. The predominant fuel source used, and the relative importance of each energy system, will ultimately be determined by the intensity and duration of the exercise performed. Figure 5.5 displays the sites of energy storage in the body and the approximate quantities of each.

Chemical energy: energy

stored in the chemical bonds of

molecules

Adipose tissue: a kind of

body tissue containing stored

fat that serves as a source of

energy; it also cushions and

insulates vital organs

Predominant fuel source: the type of fuel that

contributes the majority of

chemical energy for a bout of

exercise

Figure 5.3

Cyclists making the

most of refuelling

opportunities



Phosphocreatine

The most important fuel when undertaking maximal-effort exercise lasting a few seconds (1 to 6 seconds) is phosphocreatine (PC), also referred to as creatine phosphate (CP). It is stored in small amounts in skeletal muscle and does not require oxygen (that is, it is anaerobic ) to be broken down. Even though our diet does contain creatine, our daily intake is relatively small. So

Anaerobic: a substance that

does not require oxygen to be

broken down

Figure 5.5

Energy storage in the body

Mit

och

on

dri

a

Adipose tissue

Liver

Blood Muscle

Triglyceride

(12 kilograms)Triglyceride

(~350 grams)

Glycogen

(~500 grams)

Glucose

Glycerol

FFA

+

Glycogen

(~100–120 grams)

FFA

Acetyl – CoA

O2

O2

Glucose

(~25 grams)

Krebs cycle

and

electron transport

Figure 5.4

Athletes carbohydrate

loading



Carbohydrate

Carbohydrates (CHO) are an important fuel for maximal efforts lasting from a few seconds to a few minutes. Unlike fat, CHO can be broken down with (that is, in an aerobic process) or without oxygen (anaerobic), and is able to provide energy at a much faster rate. This makes it an important fuel source for sporting events that are short in duration requiring maximal effort (for example, a 400 metre track sprint), as well as longer events performed above 65 per cent of maximal aerobic power (for example, a marathon). CHO, also called ‘sugars’, are stored in limited quantities in skeletal muscle and liver tissue as glycogen (see Figure 5.5 ). Muscle glycogen can be broken down to glucose and used directly by the muscle to fuel contraction; whereas, glycogen stored in the liver must fi rst be broken into glucose and then transported to the muscle via the bloodstream.

Most forms of CHO come from plant foods. These forms include glucose, maltose, fructose and sucrose with the exception, lactose, found in milk. Some example foods are fruits, syrups, honey and lollies, as well as grain fl our, cereals, pasta, potatoes and other vegetables.

Carbohydrates: an essential

component of our everyday

diet, they are a fuel source that

is broken down during exercise

Aerobic: a substance that

requires oxygen to be

broken down

Glycogen: a stored form of

glucose (CHO) found in muscle

tissue and the liver

it was initially thought that increasing the amount of creatine we consume through our diet (fi sh and meat) would lead to an increase in the amount of PC we stored in muscle. However, researchers have shown that creatine supplementation (approximately 20 grams per day), in addition to a normal dietary intake, is the most effective way to increase muscular PC availability. Interestingly, recovery between repeated short-duration, high-intensity efforts is also thought to be enhanced when an athlete has larger stores of PC due to an increased rate of PC resynthesis between sprints.

Supplementation: an

intake of vitamins, minerals or

nutrients in addition to what is

gained through dietary sources

Creatine—is more better?

Creatine stores found in human muscle may come from two potential sources: our diet (meat or fi sh), and/or our body (produced internally). What we don’t get from our diet, we can easily make in our liver and kidneys from a few amino acids (glycine, arginine and methionine). Did you know that, on average, a 70 kilogram adult has approximately 120 grams of creatine stored in skeletal muscle? Of this amount, approximately 2 grams is turned over each day, with our diet and what our body produces contributing equally. An increased dietary intake of creatine appears to reduce the internal production of creatine via a feedback mechanism. Research has shown that creatine supplementation (creatine monohydrate) offers short-term benefi ts to power but not endurance athletes. However, a study performed by the Australian Institute of Sport showed that acute creatine supplementation improved performance in some repeated sprint and agility tasks in female national-level soccer players during a simulated soccer match. This occurred despite an increase in body mass (weight) of approximately 1 kilogram. Whether this practice is harmful in the long term has yet to be fully determined.

Figure 5.6 Creatine supplementation has been

shown to improve sprint and agility performance



Food for fuel

Food for fuel: Olympian Phelps’ unusual dietBreakfast: Three fried egg sandwiches; cheese; tomatoes; lettuce; fried onions; mayonnaise; three chocolate-chip pancakes; fi ve-egg omelette; three sugar-

coated slices of French toast; bowl of grits; two cups of coffeeLunch: Half-kilogram (one pound) of enriched pasta; two large ham and cheese sandwiches with

mayonnaise on white bread; energy drinksDinner: Half-kilogram of pasta, with carbonara sauce; large pizza; energy drinks.

Source: BBC News (2008), ‘Food for fuel: Olympian Phelps’ unusual diet’, BBC News, 15 August, <http://news.bbc.co.uk/2/hi/7562840.stm>

>>

Glycaemic index

There are many different types of carbohydrate-containing foods. How can we distinguish between these different types of carbohydrates and which ones are benefi cial for exercise? The glycaemic index (GI) was developed by nutritionists in order to classify CHO-containing foods in terms of their rate of digestion and subsequent effect on blood glucose concentration when compared to ‘glucose’. A low GI food is given an index value <55, moderate GI foods are 55–70 and high GI foods have a rating >70. Glucose is the reference value and has a value of 100, as can be seen in Table 5.1 .

When foods are digested, the glucose compounds within them are absorbed into the blood. Foods with a high glycaemic index, such as jelly beans, result in a rapid elevation of blood glucose and therefore provide energy to the working muscles more quickly. Foods with a low glycaemic index, such as lentils, take longer to digest and the energy from these sources is not as readily available for the muscles during activity.

Table 5.1 shows the GI values for a variety of foods. Which ones are surprising to you?

Table 5.1: GI values for a range of foods

Low GI (<55) Moderate GI (55–70) High GI (>70)

Fructose 23 Mango 55 Weet-Bix 70

Lentils 26 Basmati rice 59 White bread 70

Unripe banana 30 Orange juice 57 Watermelon 72

Orange 44 Ice-cream 61 Coco Pops 77

Porridge 49 Muffi n (cake-style) 62 Baked potato 85

Chocolate 49 Sucrose 65 Sports drink 95

Ripe banana 52 Soft drink 68 Glucose 100



Lipids

Lipids (fat) are an important fuel for sub-maximal exercise that lasts several hours. Compared to CHO, fat is stored in much larger quantities and is a more compact form of stored energy. Fat requires oxygen (aerobic) to be broken down and yields 37 kilojoules (9 kilocalories) of energy per gram, whereas CHO only yields 17 kilojoules (4 kilocalories) of energy per gram. The average 70–75 kilogram athlete stores 10–12 kilograms of fat, with most located in

adipose tissue and a small amount stored in muscle. Muscle triglycerides can be used directly by the muscle to fuel contraction and are known to be stored in larger quantities for endurance-trained athletes; whereas triglyceride stored in adipose tissue must fi rst be broken down into glycerol and free fatty acids (FFA) and then transported to the muscle via the bloodstream. Fatty acids are the basic unit of fat and can be categorised as one of three types: saturated, polyunsaturated and monounsaturated. Our body is not able to make all the fatty acids it needs. Those that we obtain through foods are called essential fatty acids. One important

essential fatty acid found in fi sh is omega, which is associated with a reduced risk of heart disease.

A summary of total energy transfer from fat breakdown is this:

Lipids: a large group of

water-soluble compounds

containing fats and oils

Fats: an essential component

of a balanced diet; a type

of fuel source broken down

during exercise

Figure 5.7

Michael Phelps, winner

of eight Olympic gold

medals in Beijing

Figure 5.8

Triglycerides are composed

of three fatty acids and one

glycerol molecule

Gly

cero

l

Fatty acid

Fatty acid

Fatty acid

→ 19 ATP � 3 � 147 ATP

Net ATP production per triglyceride � 19 � 441

� 460

→ 1 � glycerol � 3 � fatty acids

ATP production

Triglyceride



In almost all aerobic events, fat and CHO are broken down simultaneously to meet energy demands. However, the relative contribution of each fuel is determined by the intensity of exercise. CHO is the principal fuel used by athletes when working above an exercise intensity of 65 per cent VO 2max as it resynthesises ATP at a faster rate than fat, resulting in greater power output. Additional factors such as exercise duration, fi tness level, diet and pre-event nutrition also play a role in the fuel selection. Events of longer duration (greater than 2 hours), and by necessity lower intensity, rely more heavily on fat fuels due to the vast amount we have stored in the body compared to our CHO stores, which are limited. Ultra-endurance athletes will train to maximise fat use during their event in order to ‘spare’ their limited muscle glycogen stores. This will enable them to work at a higher intensity towards the latter part of the race and hopefully improve performance. However, a diet high in CHO and/or a CHO snack or meal eaten up to 2 hours before an event leads to reduced fat use and increased CHO breakdown.

The breakdown of fat and CHO differs with respect to oxygen requirements or ‘fuel economy’. It is more economical to use CHO as a fuel source during exercise as less oxygen is required in the oxidation (aerobic breakdown) process. Fats require 50 per cent more oxygen and take substantially longer to break down, leading to a slower rate of ATP production. However, the advantage of using fats as a fuel is that they provide a far greater yield of ATP compared to that derived from CHO.

VO2max: the maximal amount

of oxygen an individual can use

per minute during exercise

Ultra-endurance: a sporting

event that lasts longer than

4 hours

Figure 5.9

Ultra-endurance athletes at

the start line of the Hawaii

Ironman competition

AW 05013

The crossover concept The crossover concept shows the relative contributions of carbohydrate and fat as fuel for exercise of increasing intensity. As intensity increases, the contribution of carbohydrate will increase and the utilisation of fat will decline. The crossover point represents the intensity at which CHO takes over from fat as the principal fuel source due to the need to resynthesise ATP at a faster rate.

Crossover concept: a

theoretical way to understand

the effects of exercise intensity

and endurance training on the

balance of carbohydrate (CHO)

and lipid metabolism during

sustained exercise



Amino acids

Amino acids released from the breakdown of muscle protein stores can be used as a fuel during exercise when CHO availability is low. However, they play a small role (1 to 5 per cent) compared to the contribution from CHO and fat fuels to the overall energy demand.

Protein: one of the three

major classes of foods,

it is derived mainly from

animal sources; an essential

component of a balanced diet

Amino acids: the building

blocks of protein; there are

20 different amino acids in

human proteins

The crossover concept

Assessment workout Critical refl ection

1 For the athlete in Figure 5.10 , at what percentage of maximal aerobic power will the principal

fuel source become carbohydrate?

2 What type of fuel would you expect to be dominant in an ultra-endurance event such as

‘Around the Bay in a Day’? Compare this to a cycling criterion race lasting 1 hour.

3 Specialised laboratory testing (that is, VO 2max testing) is used to determine an individual’s

crossover point to establish racing and training intensities. Explain what would happen to

the intensity at this crossover point if an athlete consumed a sports drink immediately before

performing their test.

4 Figure 5.10 shows that training will shift the crossover point to the right, meaning that

an athlete will be able to work at a higher intensity while relying on fat as a principal fuel

source. What advantage does this provide to an endurance athlete?

Figure 5.10

The relationship

between exercise

intensity and fuel

utilisation

Rest

Fat

(%)

CH

O (%

)

0

10

20

30

40

50

60 100

90

80

70

60

50

4020 40

Aerobic Power (%)

Training

CHO

Fat

60 80 100



Practical application How to use your heart rate in training Work in groups of three students. Divide the following jobs among yourselves: runner, timer and recorder. Step 1 Use the formula ‘220 – your age’ to calculate your estimated maximal

heart rate (HR max ). Step 2 Using this value now calculate 50, 65 and 80 per cent of your HR max . Step 3 Place a heart-rate monitor across your chest. Ensure that the electrodes

are wet for conductance. Press ‘start’ on your watch to get a reading of your heart rate.

Timer

Using a stopwatch, time the runner so that they are performing 3 minutes of running at each of the following intensities: 50, 65 and 80 per cent HR max .

Recorder

Record the heart rate of the runner as soon as they fi nish each bout of exercise and also record a rating of perceived exertion (Borg scale) from 1 to 10, where 1 = rest and 10 = maximal effort. The rating of perceived exertion is a subjective assessment of how hard someone is working or how they are feeling during exercise. 1 Why do we use heart rate to monitor athletes and what feedback does it

provide the coach during a training session? 2 What is the principal fuel used at each of the intensities in this activity? 3 Explain why the principal fuel source changes with increasing exercise

intensity. Use correct terminology. 4 Discuss how the crossover point would change for an Ironman triathlete. Why?

How long could you cycle if only using fat as a fuel?

The amount of energy stored in the body as fat for a male (70 kilogram) and female (55 kilogram) cyclist with average body composition is 400 000 kilojoules (100 000 kilocalories) and 500 000 kilojoules (125 000 kilocalories) respectively. So based on the calculated

energy requirement of cycling at 40 kilometres per hour, if only fat were used as a fuel you would theoretically have 110 hours of continuous cycling in the bank. Compare this to only around 90 minutes of cycling time at this pace if relying solely on CHO fuel sources.

AW 05015

Figure 5.11

The long road ahead

of an Hawaii Ironman

cycling 180 kilometres

across lava fi elds



Our body’s three energy systems—ATP-PC (alactacid), anaerobic glycolysis (lactic acid) and aerobic glycolysis (lipolysis)—operate like engines to enable us to move at different speeds for a given duration. Of these, the ATP-PC and anaerobic glycolysis systems are able to resynthesise ATP in the absence of oxygen (anaerobic). In contrast, the aerobic system can only resynthesise ATP in the presence of oxygen (aerobic).

Take a moment to consider the term ‘energy systems’ and the purpose of all three engines becomes immediately clear. It is to provide energy for ATP resynthesis that is then used by the muscle during contraction. This is achieved through the breakdown of one or more body fuels. Although all three systems have the same important goal, they differ considerably in their power and capacity: that is, the rate (power) and yield (capacity) of ATP they are able to produce with the energy released from breaking down a fuel source. It is important to understand that these characteristics in turn affects the intensity (rate) and duration (yield) of exercise that can be performed.

Let’s now take a closer look at each energy system to understand how these performance engines differ with respect to power, capacity, fuel use and ATP resynthesis.

HOT questions Food and fuel sources

KNOW 1 List the fuel sources used for ATP resynthesis.

COMPREHEND 2 Explain the pros and cons of using carbohydrate compared to fat as

a primary fuel source during exercise.

APPLY 3 Discuss the trade-off between fuel sources in terms of the rate and

yield of ATP resynthesis for a discus thrower versus a 400 metre

hurdler.

SYNTHESISE

EVALUATE

ANALYSE

4 For each principal fuel source, provide an example of a sport and

describe the effects of the fuel source on exercise intensity and

duration.

5 Discuss the limitations facing an endurance athlete in terms of fuel

availability, and make recommendations on a strategy that can be

used to prolong the point of fatigue. Refer to the glycaemic index in

your answer.

The energy systems



Figure 5.12

The sport of AFL requires

a contribution from all the

energy systems

Energy

Anaerobic Aerobic

Lipolysis

(Fat)

Glycolysis

(CHO)

Glycolysis

(CHO)

Lactic acid

ATP-PC

(PC)

ATP-PC system—off the line The ATP-PC (alactacid) energy system is the predominant ‘engine’ used by athletes competing in short-duration, high-intensity ‘power’ events such as the 100 metre track sprint, weight-lifting or fi eld events. Not surprisingly, it is our most powerful system and therefore has the fastest rate of ATP resynthesis. We can liken this engine to a Formula 1 racing car in power as it allows us to work at our highest intensity for a few seconds when predominant. Unfortunately, the limitation of this system is its capacity . Compared to the other energy systems, it fatigues rapidly, and research supports that it can only be used as the predominant system for 6 to 10 seconds. As a consequence, it supplies the smallest yield of ATP during exercise (see Table 5.2 ).

Figure 5.13

Maximal force production

in the sport of weight-

lifting is achieved when

the ATP-PC system is

predominant



The ATP-PC system resynthesises ATP through the breakdown of the body fuel phosphocreatine (PC). Figure 5.14 shows that during this process the phosphate group is removed from PC and donated or added to the adenosine

diphosphate (ADP ) molecule using the energy released from the PC bond. The replenished ATP can then be used immediately to supply energy for muscular contraction.

The byproduct of this process is creatine (Cr). There are enough PC stores to maintain ATP levels for a few more seconds. So at this point we’ve moved from approximately 2 seconds of maximal-effort work (existing ATP stores) up to 6 to 10 seconds (ATP + PC). During recovery, the body will replenish phosphocreatine stores by rejoining creatine with phosphate but this takes time and requires the assistance of the aerobic energy system.

Anaerobic glycolysis system—hitting top gear The anaerobic glycolysis (lactic acid) energy system is the predominant ‘engine’ used by athletes competing in short-duration, high-intensity ‘speed’ events such as the 400 metre track sprint, 100 metre freestyle, or the kilo track cycling event. We can liken this engine to a V8 supercar in power as

it is ranked second to the ATP-PC system. Nevertheless, it is able to resynthesise ATP at a fast rate and is rapidly activated at the start of intense exercise. It is generally accepted that the lactic acid system will be predominant during maximal-intensity efforts of 30 to 60 seconds. Although this duration is slightly greater than that of its anaerobic counterpart (ATP-PC), the lactic acid system is also characterised by a limited capacity to resynthesise ATP (see Table 5.2 ).

The anaerobic glycolysis process, also referred to as ‘fast glycolysis’, derives its name from the oxygen-independent pathway that produces energy from the breakdown of carbohydrates, namely

muscle glycogen and blood glucose. Figure 5.16 details the anaerobic release of energy through glycolysis, resulting in the production of lactic acid (lactate + hydrogen ions). Lactic acid is the metabolic byproduct of this system and is formed in contracting muscles both at rest and during high-intensity exercise. In comparison to the ATP-PC system, this process is more complex as it requires a greater number of steps. This also explains why the relative rate of ATP resynthesis is slower when the duration of a maximal effort extends beyond 6 to 10 seconds, causing a reduction in muscular power and exercise intensity.

Oxygen-independent: a

system or reaction that does

not depend on, or can occur

without, oxygen

Metabolic byproduct: a

substance that is produced as

a result of fuel metabolism or

breakdown (i.e. lactic acid)

Figure 5.14

ATP-PC energy system

reaction

energy

energy

Net ATP production = <1 ATP

Biological work

ATP ADP + P +

PC P + Cr +

Byproduct: a secondary

product of a reaction or

process



Aerobic system—going the distance This energy system is the main engine used by endurance athletes, such as distance runners, road cyclists and triathletes. These athletes use a ‘pay as you go’ system, which is very different from power and speed athletes. ATP and PC concentrations are maintained within a tight range during endurance exercise because the rate of ATP resynthesis meets the rate of ATP breakdown under conditions of steady state. In our car analogy, we can liken this engine to a family sedan in power as it is ranked last and, as such, has a considerably slower rate of ATP resynthesis than the other systems.

Although this limitation corresponds to a reduction in power output and speed, the aerobic

Figure 5.15

Sprinters maximise speed

in a 400 m race when the

anaerobic glycolysis system

is predominant

Figure 5.16

Anaerobic glycolysis energy

system reaction

energy

G G G G G G Pyruvic acid Lactic acid

Glycogen Lactate + Hydrogen ionsGlucose

Net ATP production = 2 ATP



system is characterised by an unlimited capacity to resynthesise ATP (see Table 5.2 ). This system is responsible for the majority of ATP resynthesis at rest and is commonly referred to when describing the fi tness requirements of athletes who perform longer endurance events. However, it is interesting to note that the aerobic system is also activated at the onset of high-intensity exercise and becomes the predominant system when the effort exceeds approximately 1 minute. As well as being the major provider of energy for exercise that ranges between sub-maximal and VO 2max intensity, this engine also plays a critical role in the recovery of both anaerobic energy systems. This is particularly important for performance in team sports that are intermittent in nature.

Figure 5.17

Long-distance cyclists

maximise speed when

the aerobic system is

predominant

AW 05024

In contrast to the anaerobic energy systems, the aerobic system has the unique ability to break down more than one type of fuel source to meet the energy demands of exercise. Under most exercise conditions, carbohydrates and lipids are used for ATP resynthesis, with proteins having a minimal contribution to energy requirements. It generally accepted that when CHO and lipids become depleted, the contribution of protein as a fuel for ATP resynthesis is increased.

The aerobic system is often described by two names to refl ect the dominant fuel used during exercise. The aerobic glycolysis system takes its name from the oxygen-dependent breakdown of CHO (muscle glycogen and blood glucose). It predominates during high-intensity exercise lasting longer than 1 to 2 minutes and up to 3 hours; whereas, during ultra-endurance events that last more than 4 hours it is the aerobic lipolysis system and the oxygen-dependent breakdown of fat (muscle triglycerides and plasma FFA)

Glycolysis: a metabolic

process that breaks down

carbohydrates through a

series of reactions to either

pyruvic acid or lactic acid, and

releases energy for the body in

the form of ATP

Lipolysis: a metabolic

process that breaks down

lipids through a series of

reactions to fatty acids and

glycerol to release energy for

the body in the form of ATP

Oxygen-dependent: a

system or reaction in which

oxygen is a requirement



that predominates. Both systems contribute equally to the energy demand of activities that last approximately 4 hours.

Figure 5.19 details the aerobic release of energy from the breakdown of CHO and fat, resulting in the production of water, carbon dioxide and heat (H 2 O + CO 2 + heat). In comparison to the anaerobic systems, this process is far more complex as it requires three stages of metabolism to be completed to yield the full amount of ATP. In fact, the largest yield of ATP during this process is generated from the last stage—the electron transport chain (ETC).

Assessment workout Data analysis

Going the distance

Figure 5.18

The percentage of

energy derived from

carbohydrates and

fats during sub-

maximal exercise

1 At the 1-hour point on Figure 5.18 , what are the relative contributions of fat and CHO fuels to

energy expenditure?

2 List two sporting events that would predominantly rely on the aerobic lipolysis energy

system and two that predominantly would be considered aerobic glycolytic events.

3 Compare and contrast the two aerobic systems using correct terminology. Which energy

system is more economical?

4 Discuss the relative contribution from CHO and fat fuels during exercise that lasts up to

4 hours. Use Figure 5.18 to explain your answer.

5 Describe what would happen to the fuel use in this event if an athlete consumed a sports

drink throughout exercise? (Note: studies have shown that the rate of muscle glycogen

depletion does not change with CHO ingestion.) Include in your answer the effect on blood

and liver CHO stores.

% e

ner

gy

exp

end

itu

re100

90

80

70

60

50

40

30

20

10

00 1 2 3 4

Exercise time (hours)

Muscle triglycerides

Plasma FFA

Blood glucose

Muscle glycogen



Table 5.2: Summary of energy systems

Anaerobic systems Aerobic systems

Characteristics ATP-PC (alactacid) Anaerobic glycolysis

(lactic acid)

Aerobic (glycolysis) Aerobic (lipolysis)

Duration

(predominant

system)

6–10 seconds 30–60 seconds 2–3 hours >4 hours

Intensity

(% HRmax)

Not applicable Not applicable >75–100% Rest – 75%

Intensity

(% VO2max)

Not applicable Not applicable >65–100% (VO2max) Rest – 65%

Perceived exertion Maximal Maximal Moderate–very hard Very light–moderate

Fuel source(s) Creatine phosphate Carbohydrate

(muscle glycogen)

Mostly carbohydrate

Fat

Mostly fat

Carbohydrate

ATP yield

(per molecule)

<1 2 36–39 >100

Products Cr + P Lactic acid H2O + CO2 + heat H2O + CO2 + heat

[Blood lactate]

(mM)

Not applicable >6 2–16 <2

Training effect Alactic power Alactic power

Alactic capacity

Lactic power

Lactic capacity

Aerobic power

Aerobic capacity

Aerobic power

Aerobic capacity

Fat oxidation

Typical events 100 m track sprint 400 m track sprint

100 m freestyle

10 000 m run

40 km TT (cycling)

Ironman triathlon

Road cycling (4 + hours)

Figure 5.19

The aerobic release of

energy from the breakdown

of CHO and fat

Pyruvic acid

Acetyl - CoA

Krebs

cycle

Lactic acid

PROTEINSFATSFood fuels CARBOHYDRATES

FFA/triglycerideBody fuels

ATP yield

Glucose/glycogen Amino acids

147–460 36–39

Beta

oxidation



Assessment workout Laboratory report

Predominant energy systems You are the coach of a national sporting team (AFL, hockey, basketball, soccer, etc.).

1 Decide on a sport. What energy system(s) would be predominant for the athletes in your

chosen sport?

2 Design a training session for your sport and specifi c energy systems. Include some different

components within the session and give reasons to support why you have chosen these

activities.

3 Discuss why team-sport athletes should include a variety of training intensities and

durations. Refer to energy systems and their rate and/or yield of ATP in your answer.

Assessment workout Summary

Energy systems Complete the following table by adding any missing components of each energy system.

Energy system Fuels Intensity Duration Sports examples

ATP-PC ATP and PC

–

–

–

Anaerobic glycolysis Very high

–

–

–

Aerobic glycolysis 1–90 minutes

–

–

–

Aerobic lipolysis

– Ironman triathlon

–

–



Energy system interplay—all systems go During everyday activities ATP is constantly required for muscle contraction. Our energy systems enable us to move at different intensities and durations like the gears of a car. In this way we can use a gear that is appropriate to the needs of the activity and not waste valuable fuel sources or accumulate unnecessary byproducts that may lead to fatigue. All activities require an energy contribution from at least two of our three systems. However, only one system will be predominant, as it resynthesises the largest yield of ATP. Which system that is depends on the intensity, duration and type of exercise performed. Figure 5.20 shows the interplay (or interaction) between the three systems and highlights how they differ in power (y-axis) and capacity (x-axis) compared to the muscle’s currency, ATP. All systems are activated at the onset of exercise when performing a maximal effort in order to resynthesise ATP at the fastest rate to ensure the highest power output and intensity.

Interplay: the concept that

more than one energy system

contributes to ATP resynthesis

Assessment workout Critical refl ection

Ranking the energy systems 1 Rank the energy systems referred to in Figure 5.20 according to their power and capacity.

Use ‘1’ to represent the greatest and ‘4’ to represent the lowest ranked system.

2 Discuss any differences between these systems in ATP production using correct terminology,

and relate these to exercise intensity and duration.

The energy system trade-off between the rate and yield of ATP resynthesis can be seen by looking at the differences in the peak (ATP rate) and breadth (ATP yield) of each energy system displayed on the graph in Figure 5.20 . The ATP-PC system is capable of resynthesising ATP at a very high rate; however, it is unable to produce a large yield as it fatigues rapidly after activation. If the duration of our effort is extended beyond 6 to 10 seconds, the anaerobic glycolysis or ‘fast glycolytic’ system will become predominant. This in turn will reduce our rate of ATP resynthesis (that is, a lower peak), and therefore

Figure 5.20

The relative contribution

of the energy systems at

maximal effort across time

Relative contribution of energy systems to energy production at maximal effort

Pow

er

Capacity

ATP

PC

Anaerobic

glycolysis Aerobic

glycolysis (CHO)Aerobic

lipolysis (fat)

Time



exercise intensity, but will allow us to keep exercising for at least another minute. During this time the aerobic (glycolysis) energy system will become predominant. Research suggests that this will occur somewhere between 30 and 60 seconds after the start of a maximal effort. However, the trade-off is a reduction in the rate of ATP resynthesis and resulting exercise intensity. The advantage of this system is that it provides us with a greater exercise capacity (represented by a larger curve breadth) due to the enormous yield of ATP it can produce.

Anaerobic glycolysis … undervalued or overrated?

An interesting study conducted by Australian sports scientists has shown that the anaerobic glycolysis system contributes signifi cantly (about 32 per cent) to ATP resynthesis during a maximal effort sprint of only 3 seconds. This research suggests that anaerobic glycolysis plays an important role in the initial seconds of high-intensity exercise as well as maximal efforts

lasting 30 to 60 seconds (see Figure 5.21 ). This is also supported by Figure 5.22 , which shows that almost 50 per cent of the total energy requirements is met by the anaerobic glycolysis system when performing a maximal effort of 6 seconds. This contribution increases to 60 per cent when the effort duration is extended to 30 seconds.

The differences in the time ranges reported for each system (that is, predominance) by various research groups causes much confusion in understanding the concept of energy system interplay. This often arises due to differences in both the exercise modality (cycling versus running) and the fi tness level of the subjects

being tested (moderately trained versus elite). In most cases, researchers will ask athletes to provide a maximum effort for a given duration under laboratory conditions. Figure 5.22 summarises the results of these studies, graphing the relative contribution of an energy system for a maximal effort of 6 seconds to 4 hours.

Figure 5.22

The relative

contribution of the

energy systems to

efforts of different

durations

>>

Figure 5.21

The relative

contribution of the

energy systems for

a 3-second maximal

sprint effort

Stored ATP 10%

PC 55%

Anaerobic

glycolysis 32%

Aerobic 3%

ATP

PC

Anaerobic glycolysis

Aerobic glycolysis

Aerobic lipolysis

6 seconds 30 seconds 60 seconds 120 seconds 1 hour 4 hours

6.3%

40%

60%50%

50%

65%

92%

8%

50%

50%

35%

44.1%

49.6%



Assessment workout Summary

World recordsComplete the following table by inserting the current world record for each event, along with

the predominant energy system and fuel source(s).

Event Current world

record time

Energy system Fuel source(s)

100 m track sprint

200 m butterfl y

1500 m track sprint

10 000 m run

Olympic distance triathlon

Table 5.3: The progressive change in the contributions of aerobic and anaerobic power systems to maximal exercise lasting two minutes

Time period Anaerobic (%) Aerobic (%)

First 30 seconds 80 20

Second 30 seconds 60 40

Third 30 seconds 42 58

Last 30 seconds 33 66

Source: Adapted from Bangsbo et al. ‘Anaerobic energy production and O2 defi cit–debt relationship during exhaustive exercise in humans’, Journal of Physiology, 422, 1990,

pp. 539–559

duration. From these results we can see that the aerobic energy system makes a far greater contribution to the energy demand early in exercise than was previously thought. When performing a maximal effort lasting 60 seconds, approximately 50 per cent of the total energy requirements is met by the aerobic system. This contribution increases to 65 per cent for a 2-minute maximal effort and 92 per cent when exercising for 1 hour. It would be easy to relate these values to the time an athlete should then spend training each system for an event of a particular duration. However, this approach will not work, as the training process is more complex than breaking up time into percentages.

The problem with the values reported in Figure 5.22 is that it assumes the contribution of an energy system

will be the same throughout the entire effort. However, this approach is very simplistic and does not account for progressive changes in an energy system’s contribution across an event. Table 5.3 represents the work of Dr Jens Bangsbo, a pioneer in this area of research, and shows that the relative contributions of each system will change throughout the exercise period in relation to the time segment in question. So although approximately the same average contributions are made by the energy systems to maximal effort exercise of 30 seconds to 2 minutes as those reported in Figure 5.22 , the relative demand of each system will vary throughout an event, as shown in Table 5.3 .



Physical activity and sport place an increased demand on our daily energy requirements. However,

it is important to understand that it is the intensity and duration of exercise that ultimately

determines the rate and yield of ATP resynthesis and the fuel sources utilised.

Our three different energy systems provide the human body with a continual supply of chemical

energy in the form of the energy-rich compound known as adenosine triphosphate, or ATP.

We only have enough stored ATP to fuel approximately 2 seconds of maximal-effort exercise;

after this, ATP must continually be resynthesised to meet energy demands.

The chemical energy trapped within the bonds of a fuel source is extracted via a series of

complex reactions specifi c to one of three energy systems: ATP-PC, anaerobic glycolysis and

the aerobic system.

There are four primary fuel sources that our body uses to resynthesise ATP for exercise and

activities of daily living: phosphocreatine, carbohydrate, lipid and protein.

Our fuels are stored in our muscles, liver and adipose tissue in different quantities. The amount

of each type will vary with the training status and dietary intake of an individual.

In almost all aerobic events, fat and carbohydrate are broken down simultaneously to meet

energy demands. However, the relative contribution of each fuel is determined by the intensity

of exercise.

CHAPTER SUMMARY

HOT questionsFood and fuel sources

KNOW 1 List the energy systems used to resynthesise ATP during exercise

and provide an alternative name for each system.

COMPREHEND 2 Defi ne the terms ‘power’ and ‘capacity’ and relate these to the

exercise intensity and duration of sporting events.

APPLY 3 Provide a sporting example for each energy system (that is,

predominant) and show differences in the rate and yield of ATP

resynthesis on a graph.

SYNTHESISE

EVALUATE

ANALYSE

4 Discuss the concept of interplay between the energy systems and

insert the predominant time range for each system on the x-axis

of the graph created for Question 3. Comment on how these times

may change for an individual and include the crossover concept in

your answer.

5 Explain, using correct terminology (rate versus yield), why an athlete

is not able to sustain the same intensity for a marathon as a 100

metre sprint event. Include fuel sources and the concepts of power

and capacity in your answer.



The duration and intensity of exercise will be determined by the predominant energy system,

which will also determine both the yield and rate of ATP resynthesis.

A large yield of ATP will lead to a greater exercise capacity, whereas a faster rate of ATP

resynthesis will result in a higher exercise intensity.

The crossover point is the intensity of exercise at which an individual will rely more on

carbohydrate as a primary fuel and less on fat due to increasing exercise intensity. This is

normally around 65 per cent of maximal power output.

The ATP-PC energy system (Formula 1 engine) is our most powerful system and produces ATP

at the fastest rate. However, this system has a very limited capacity and can only remain the

predominant system for up to 6 to 10 seconds.

Anaerobic glycolysis, also known as the lactic acid system (V8 supercar engine), is

predominantly used in short-duration, high-intensity ‘speed’ events such as the 400 metre track

sprint. This system is ranked second in power and has a fast rate of ATP resynthesis for up to

30 to 60 seconds. However, like the ATP-PC system, it has a limited capacity.

The aerobic energy system (family sedan engine) has the least power or the slowest rate of

ATP resynthesis. Its advantage over the anaerobic energy systems is that it has a much larger

capacity and is able to supply energy for hours rather than seconds.

The aerobic system can be further divided into aerobic glycolysis (predominantly uses CHO

fuels) and aerobic lipolysis (predominantly uses fat fuels).

All activities require an energy contribution from at least two energy systems, and under

maximal-effort conditions all three systems are activated at the start of exercise. This concept is

referred to as ‘energy system interplay’.

Although two or more energy systems operate at any one time, only one will hold the title of

the predominant system as it resynthesises the largest yield of ATP.


Documents

Area of study 2 - Macmillan Publisherscdn-media.macmillan.com.au/mea/downloadpdfs/9781420229813.pdfArea of study 2 Physiological responses to physical activity Key knowledge This knowledge