Area of study 2
Physiological 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 deficit, 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.
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Whether we are waiting for the starter’s gun to sound at the Olympic 100 metre track final or
watching a sprint finish 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 fitness 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.
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The energy systems:
engines of
performance
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 specifications 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.
ATP: our energy
currency
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.
VCEPE_CH05.indd 97
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
Metabolism: the breaking
down of fuels via a series of
chemical reactions for energy
release
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Resynthesis (ATP):
the re-formation of ATP
following metabolism; ATP is
resynthesised during exercise
for muscle contraction
Muscle
contraction
Recovery
method
ATP
demand
Performance
Interplay
of energy
systems
Fatigue
mechanisms
Figure 5.1
Oxygen
kinetics
Our body’s performance
relies on the production of
ATP through the breakdown
of body fuels
Simplified structure
Energy for muscular work
ATP
Adenosine
Figure 5.2
ATP is the currency that
fuels muscle contraction
ADP
P
P
P
Adenosine
+
P
P
Pi + energy
+
P
+
energy
‘High-energy’ phosphate bond
Figure 5.2 shows a simplified 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
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The energy systems: engines of performance
CHAPTER 5
99
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 difficult 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 spinning
By 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 finger 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 France
Breakfast
250 g (dry measure) rice or pasta
115 g muesli
125 g carton of yoghurt
1 to 2 slices wholegrain bread with
homemade marmalade
Omelette made with 3 egg whites
and one yolk
1 to 2 slices ham and/or cheese
Freshly squeezed orange juice
Coffee
Feed bags (on the bike)
4 cakes or bars (rice cakes, cookies,
fruit bars)
Fruit: chunks of pineapple, sliced
banana and a quartered apple
2 or 3 finger sandwiches (ham and
cheese)
3 or 4 protein bars
One small can of Coke
Snack (in hotel)
Small bowl rice pudding and fruit
Dinner
1 to 2 slices prosciutto or ham
250 g (dry measure) pasta with
fresh tomato sauce
250 g beef steak
Ratatouille (eggplant, zucchini,
tomato stew)
Yoghurt and fresh fruit
Slice 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/
>>
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Figure 5.3
Cyclists making the
most of refuelling
opportunities
Chemical energy: energy
stored in the chemical bonds of
molecules
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 specific 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.
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
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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.
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The energy systems: engines of performance
CHAPTER 5
101
Figure 5.4
Athletes carbohydrate
loading
Adipose tissue
Blood
Triglyceride
(12 kilograms)
Glycerol
Muscle
Triglyceride
(~350 grams)
+
FFA
Mitochondria
FFA
Liver
Glycogen
(~100–120 grams)
Acetyl – CoA
Glucose
Glucose
(~25 grams)
O2
Krebs cycle
and
electron transport
O2
Glycogen
(~500 grams)
Figure 5.5
Energy storage in the body
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
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Anaerobic: a substance that
does not require oxygen to be
broken down
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Supplementation: an
intake of vitamins, minerals or
nutrients in addition to what is
gained through dietary sources
it was initially thought that increasing the amount of creatine we consume
through our diet (fish 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.
Creatine—is more better?
Figure 5.6 Creatine supplementation has been
shown to improve sprint and agility performance
Creatine stores found in human muscle may come from
two potential sources: our diet (meat or fish), 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 benefits
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.
Carbohydrate
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
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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 first
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
flour, cereals, pasta, potatoes and other vegetables.
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CHAPTER 5
103
Glycaemic index
There are many different types of carbohydratecontaining foods. How can we distinguish between
these different types of carbohydrates and which ones
are beneficial 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
Muffin (cake-style)
62
Baked potato
85
Chocolate
49
Sucrose
65
Sports drink
95
Ripe banana
52
Soft drink
68
Glucose
100
Food for fuel
Food for fuel: Olympian Phelps’ unusual diet
Breakfast: Three fried egg
sandwiches; cheese; tomatoes;
lettuce; fried onions; mayonnaise;
three chocolate-chip pancakes;
five-egg omelette; three sugar-
coated slices of French toast; bowl
of grits; two cups of coffee
Lunch: Half-kilogram (one pound)
of enriched pasta; two large ham
and cheese sandwiches with
mayonnaise on white bread; energy
drinks
Dinner: 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>
>>
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Figure 5.7
Michael Phelps, winner
of eight Olympic gold
medals in Beijing
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
Fats: an essential component
down and yields 37 kilojoules (9 kilocalories) of energy per gram, whereas
of a balanced diet; a type
CHO only yields 17 kilojoules (4 kilocalories) of energy per gram. The average
of fuel source broken down
70–75 kilogram athlete stores 10–12 kilograms of fat, with most located in
during exercise
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 enduranceFatty acid
trained athletes; whereas triglyceride stored in adipose tissue must
first be broken down into glycerol and free fatty acids (FFA) and
Fatty acid
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:
Fatty acid
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 fish is omega, which is associated with a reduced
Figure 5.8
risk of heart disease.
Triglycerides are composed
of three fatty acids and one
A summary of total energy transfer from fat breakdown is this:
Glycerol
Lipids: a large group of
water-soluble compounds
containing fats and oils
glycerol molecule
Triglyceride → 1 ⫻ glycerol ⫹ 3 ⫻ fatty acids
ATP production → 19 ATP ⫹ 3 ⫻ 147 ATP
Net ATP production per triglyceride ⫽ 19 ⫹ 441
⫽ 460
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The energy systems: engines of performance
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 VO2max as it resynthesises ATP at a faster rate than fat, resulting in
greater power output. Additional factors such as exercise duration, fitness
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. Ultraendurance 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.
CHAPTER 5
105
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
AW 05013
Figure 5.9
Ultra-endurance athletes at
the start line of the Hawaii
Ironman competition
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.
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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
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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.
Amino acids: the building
blocks of protein; there are
20 different amino acids in
human proteins
Protein: one of the three
major classes of foods,
it is derived mainly from
animal sources; an essential
component of a balanced diet
Assessment workout
Critical reflection
The crossover concept
CHO
Fat
60
100
90
50
80
30
70
20
60
10
50
Figure 5.10
0
40
The relationship
between exercise
intensity and fuel
utilisation
Rest
20
60
40
Aerobic Power (%)
80
100
CHO (%)
Fat (%)
Training
40
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, VO2max 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?
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CHAPTER 5
107
How long could you cycle if only using fat as a fuel?
AW 05015
Figure 5.11
The long road ahead
of an Hawaii Ironman
cycling 180 kilometres
across lava fields
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.
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 (HRmax).
Step 2 Using this value now calculate 50, 65 and 80 per cent of your HRmax.
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 HRmax.
Recorder
Record the heart rate of the runner as soon as they finish 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?
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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
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.
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CHAPTER 5
109
Figure 5.12
The sport of AFL requires
a contribution from all the
energy systems
Energy
Anaerobic
ATP-PC
(PC)
Glycolysis
(CHO)
Lactic acid
Aerobic
Glycolysis
(CHO)
Lipolysis
(Fat)
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 field
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 weightlifting is achieved when
the ATP-PC system is
predominant
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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
Biological work
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
ATP
ADP + P + energy
2 seconds of maximal-effort work (existing ATP stores) up to
6 to 10 seconds (ATP + PC). During recovery, the body will
PC
P + Cr + energy
replenish phosphocreatine stores by rejoining creatine with
phosphate but this takes time and requires the assistance of the
aerobic energy system.
Net ATP production = <1 ATP
Byproduct: a secondary
product of a reaction or
process
Figure 5.14
ATP-PC energy system
reaction
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)
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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 highintensity 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.
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Figure 5.15
Sprinters maximise speed
in a 400 m race when the
anaerobic glycolysis system
is predominant
G
G
G
Glycogen
G
G
G
Pyruvic acid
Glucose
Lactic acid
Lactate + Hydrogen ions
energy
Figure 5.16
Net ATP production = 2 ATP
Anaerobic glycolysis energy
system reaction
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
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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 fitness 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 VO2max 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.
AW 05024
Figure 5.17
Long-distance cyclists
maximise speed when
the aerobic system is
predominant
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
Oxygen-dependent: a
system or reaction in which
oxygen is a requirement
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
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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 reflect 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)
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Assessment workout
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Data analysis
Going the distance
100
Muscle triglycerides
% energy expenditure
90
80
70
Plasma FFA
60
50
40
Blood glucose
30
Figure 5.18
20
Muscle glycogen
10
0
0
1
2
Exercise time (hours)
3
4
The percentage of
energy derived from
carbohydrates and
fats during submaximal 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.
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 (H2O + CO2 + 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).
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Food fuels
FATS
CARBOHYDRATES
PROTEINS
Body fuels
FFA/triglyceride
Glucose/glycogen
Amino acids
147–460
36–39
Beta
oxidation
Pyruvic acid
ATP yield
Lactic acid
Acetyl - CoA
Figure 5.19
Krebs
cycle
The aerobic release of
energy from the breakdown
of CHO and fat
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)
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Assessment workout
CHAPTER 5
115
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
–
–
– Ironman triathlon
Aerobic lipolysis
–
–
Assessment workout
Laboratory report
Predominant energy systems
You are the coach of a national sporting team (AFL, hockey, basketball, soccer, etc.).
VCEPE_CH05.indd 115
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 specific 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.
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Energy system interplay—all systems go
Interplay: the concept that
more than one energy system
contributes to ATP resynthesis
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.
Relative contribution of energy systems to energy production at maximal effort
ATP
Power
PC
Anaerobic
glycolysis
Aerobic
glycolysis (CHO)
Aerobic
lipolysis (fat)
Figure 5.20
The relative contribution
of the energy systems at
maximal effort across time
Time
Capacity
Assessment workout
Critical reflection
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
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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 significantly (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.
Aerobic 3%
Stored ATP 10%
PC 55%
Figure 5.21
Anaerobic
glycolysis 32%
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 fitness level of the subjects
6.3%
The relative
contribution of the
energy systems for
a 3-second maximal
sprint effort
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.
8%
40%
50%
44.1%
50%
65%
92%
60%
49.6%
50%
50%
35%
6 seconds 30 seconds
ATP
PC
60 seconds 120 seconds
Anaerobic glycolysis
Aerobic glycolysis
Aerobic lipolysis
1 hour
4 hours
Figure 5.22
The relative
contribution of the
energy systems to
efforts of different
durations
>>
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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.
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 deficit–debt relationship during exhaustive exercise in humans’, Journal of Physiology, 422, 1990,
pp. 539–559
Assessment workout
Summary
World records
Complete 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 butterfly
1500 m track sprint
10 000 m run
Olympic distance triathlon
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HOT questions
Food 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
Define 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.
CHAPTER SUMMARY
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 specific 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.
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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.
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