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Chapter 7: Energy Systems and Physical Activity
Three Key Energy Nutrients
Carbohydrates, Protein, and Fats
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Nutrients are chemical substances obtained from food and used by the body for many different processes.
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Energy nutrients are the raw materials that our bodies need to supply energy, to regulate cellular activities,
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and to build and repair tissues.
The food we take in contains three key energy nutrients that are broken down over the course of digestion:
carbohydrates, proteins, and fats.
Carbohydrates yield 4.1 Kcal per gram; Proteins yield 4.2 Kcal per gram; and Fats yield 9.3 Kcal per gram.
Of these three key energy nutrients, carbohydrates are our most important source of energy since they are easily
metabolized by our bodies.
The Central Role of Carbohydrates in Supplying Energy
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Sugar and starches are examples of carbohydrates. Carbohydrates are the most abundant organic substance in
nature, and are essential to our survival.
Carbohydrate molecules always contain carbon, hydrogen, and oxygen atoms, and are always in the same ratio of
two atoms of hydrogen for each atom of oxygen or carbon.
Within our bodies, the main functions of carbohydrates are to provide materials to build cell membranes and to
provide energy storage for organisms. They provide both short and long term energy storage for organisms.
Carbohydrates come mainly from plant sources such as vegetables, fruits, and grain-based foods like pasta and
bread.
Carbohydrates are formed when green plants undergo photosynthesis (carbon dioxide and water undergoing a
chemical reaction to produce oxygen and sugar).
Glucose is stored within skeletal muscles and within our liver as glycogen. Glucose that is stored in this way
can be broken down under conditions of stress or the demands of muscular activity, carried through the body by
the blood, and brought into action as an energy source.
ATP – The Common Energy Molecule
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Adenosine triphosphate (ATP) is the name of the common energy molecule which is essential for the
cellular processes in all living things.
In order to be able to actually use the energy from our key nutrients, the nutrients need to be reconstructed into a
universal form of energy – a “free energy” that can be used for muscle contraction and many other physiological
processes that take place in our bodies.
ATP captures the chemical energy resulting from the breakdown of food and can be used to fuel various cellular
processes.
At the molecular level, ATP consists of three phosphates attached by high-energy bonds to adenosine and ribose.
Energy is released when a trailing phosphate (Pi) is broken from the ATP molecule. This results in ADP
(adenosine diphosphate) plus energy. The chemical equation is ATP → ADP + P + Energy
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Since ATP energy is always in high demand, it is used up very quickly. The problem then becomes how to recreate new supplies of ATP to ensure that bodily functions, including physical movement, continue.
There are two methods for resynthesizing ATP:
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The anaerobic system occurs relatively quickly in the
muscle fibre, utilizing chemicals and enzymes readily for powerful but relatively short-lived physical
actions. For example, a 100 metre dash.
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Aerobic (with oxygen) System:
Anaerobic (without oxygen) System:
The aerobic system is much more complicated and takes place
in the mitochondria (the cell’s power stations). The aerobic process involves many enzymes and several
complex sub-pathways, leading to the complete breakdown of glucose. Fat and protein also entre the
cycle at this stage. For example, a marathon.
Both the anaerobic and aerobic systems co-exist, overlap, and interact in various combinations.
Three Metabolic Pathways
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Within these two energy systems (anaerobic and aerobic), there are three main metabolic pathways by which ATP
energy reserves are restored.
The three metabolic pathways allow our bodies to create sufficient energy to carry out not only movement of all
kinds, but also all of our vital processes (neural activities, organ function, breathing, etc)
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These three metabolic pathways are:
ATP-PC (anaerobic alactic):
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It is also known as the phosphagen system, and it relies on the action of phosphocreatine (a compound
normally stored in muscle and is readily accessible).
Just like ATP, phosphocreatine (PC) is a high-energy molecule in which the phosphate can be easily broken
off and used to convert ADP back to ATP.
The small amount of PC within the muscle can sustain the level of ATP required during the initial phase of
short but intense activity.
The chemical equation is: PC + ADP → ATP + creatine
This pathway draws on processes deep within the muscle fibre itself, allowing for quick, and intense muscle
contraction.
It is also referred to alactic because lactic acid is not a byproduct of this pathway.
The ATP-PC system yields enough ATP for about 10-15 seconds of strenuous exercise.
In sports, the ATP-PC system plays an important role in such power events as the 50 or 100 metre dash, high
jump, or single rep max in weightlifting.
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The ATP-PC system provides the highest rate of ATP re-synthesis that cannot be matched by other energy
systems.
The ATP is used to recombine phosphate and creatine, and it takes about 2-5 minutes of recovery time.
Glycolysis (anaerobic lactic):
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This pathway involves the partial breakdown of glucose, with lactic acid as a byproduct.
It does not involve oxygen, but it allows for longer bursts of energy.
Glycolysis is the 2nd energy pathway in which ATP is produces allowing a person to engage in a high level of
performance for about 1 ½ to 3 minutes.
Since glucose is normally plentiful throughout the body, glycolysis is an ideal backup to the ATP-PC system
for medium-term physical activities, such as the 400 or 800 metre sprints, or a typical shift in hockey.
Under certain conditions, glycolysis becomes the first stage of the aerobic process.
During glycolysis, glucose is partially broken down to provide useable energy to the form of ATP.
Glycolysis involves 11 separate biochemical reactions and yields twice as much ATP.
The chemical equation is: C6H12O6 + 2 ADP + 2Pi → 2C3H6O3 + 2ATP + H2O (lactate + ATP + water)
The main product of glycolysis is pyruvate (pyruvic acid). Under aerobic conditions, pyruvate is the beginning
of the 3rd (aerobic) system that eventually leads to the complete breakdown of glucose and to very large
quantities of ATP.
In the absence of adequate oxygen supplies, the process is halted at the glycolysis stage and pyruvic acid is
converted into lactic acid – this is when exhaustion or painful muscle agony begins to set in.
The buildup of lactic acid eventually hampers the breakdown of glucose and decreases the ability of the muscle
fibres to contract.
Lactic acid removal requires 30-60 minutes of exercise recovery or 1-2 hours of rest recovery.
For example, running 400 metres; a typical shift in hockey; anything that takes around 2 minutes to complete.
Cellular Respiration (aerobic):
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This is the main source of ATP during endurance events.
It involves oxygen and the complete breakdown of glucose to yield large amounts of ATP for energy.
For any athlete to sustain intense activity longer than 90 seconds or so, a 3rd energy system must come into
play. This molecular activity occurs in the mitochondria of cells and is referred to as cellular respiration.
Fats and proteins in addition to glucose can be used as energy sources at this stage.
The ATP produced by the aerobic method far exceeds the ATP produced by the other two pathways –
ultimately producing 36 molecules of ATP for every molecule of glucose – this is nearly 20 times the number
of ATP molecules produced by the anaerobic system.
In the presence of oxygen, the aerobic system can sustain activity for a very long time, or until other
physiological limits are reached.
This is the energy pathway that our bodies depend upon most heavily to sustain endurance-type events, such as
marathon runs or long-distance swimming.
The cellular respiration pathway results in the complete breakdown of glucose.
The chemical equation is: C6H12O6 + 6O2 + 36ADP + 36Pi → 6CO2 + 36ATP + 6H2O
Cellular respiration actually involves three separate sub-pathways:
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Glycolysis:
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The 1st stage is the same as the anaerobic lactic system except that, in the presence of oxygen,
pyruvic acid is converted into acetyl CoA (rather than lactic acid).
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Krebs Cycle (citric acid cycle):
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Acetyl CoA then enters a more complicated pathway known as the Krebs cycle.
The Krebs cycle involves a series of 8 chemical reactions.
Two ATP molecules are produced at the end of this process, along with new compounds that are capable
of storing “high-energy” electrons.
These high-energy electrons are then sent to a process within the mitochondria of cells, known as the
electron transport chain.
Electron transport chain:
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During this final stage of cellular respiration, large amounts of ATP are produced, with carbon dioxide
and water as the only by-products.
The “chain” is a series of electron carriers and protein complexes that accept and donate electrons in a
sequential series.
The final electron acceptor is oxygen.
A Comparison of the Three Metabolic Pathways in the Human Body
Name of Metabolic
Pathway
ATP-PC
(Anaerobic
Alactic System)
Glycolysis
(Anaerobic Lactic
System)
Cellular Respiration
(Aerobic System)
Location of
activity
Energy source
Uses oxygen or not
Cytoplasm
Cytoplasm
Mitochondria
Creatine phosphate
Glucose (glycogen)
Glycogen, fats, proteins
Anaerobic (without
oxygen)
1 molecule
Anaerobic (without oxygen)
Aerobic (with oxygen)
10-15 seconds
2 molecules per molecule of
glucose
15 seconds to 3 minutes
36 molecules per molecule of
glucose
120 seconds and beyond
1-2
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Glycolysis, Krebs cycle, and the
electron transport chain
ATP
Duration of
activity
Number of
chemical reactions
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By-products
Type of physical
activities
Types of exercise
that rely on this
system
Advantages
Limitations of the
energy system
None
Lactic acid
Water and carbon dioxide
Power surges, speed
events
Intermediate activities, sprint
finishes
Prolonged activities
Sprints, jumping,
weightlifting
200-800 metre runs, a shift in
hockey
Marathons
Very quick surge of
power
Short duration; muscles
store small amounts of
ATP and creatine
phosphate
Quick surge of power
Long duration, complete
breakdown of glucose
Slow; requires large amounts of
oxygen
Buildup of lactic acid causes
pain and fatigue
Fats and Proteins as Energy Sources
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During physical exercise, the primary sources are carbohydrates and fats.
Proteins typically contribute a relatively small percentage of the total energy used, since it less accessible.
Fats:
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Fats are macromolecules that are a rich energy source and provide twice as much energy as equal masses of
carbohydrates or proteins.
The primary types of fats found in muscle cells and adipose tissue are known as “fatty acids”. These fatty
acids are stored in the body as triglycerides.
The point at which fatty acids enter the energy system is in the Krebs cycle.
Before entering the energy supply chain, fatty acids first need to be converted to acetyl-CoA. This is achieved
through a process known as beta oxidation.
Beta oxidation of fats occurs within the mitochondria of cells and involves four chemical reactions, ultimately
yielding acetyl-CoA. Acetyl-CoA then enters the Krebs cycle and becomes a primary energy source for the
production of ATP within the electron transport chain.
Proteins:
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As a fuel source, protein contains approximately as much potential energy as carbohydrates. However, unlike
carbohydrates and fats, there are no “protein reserves” in the body.
All proteins are part of existing body tissue or are actively engaged as component parts of the metabolic
system.
Proteins are long chain-like biological molecules composed of many smaller molecules known as amino
acids (there are approximately 20 amino acids) and are used by the body to form the various body tissues.
Nine of these (called essential amino acids) cannot be made by the body and must be consumed as food.
To be utilized as an energy source, protein must first be broken down into separate amino acids.
For example, the amino acid called alanine is the main contributor – it is converted in the liver to glycogen,
which is then transported s glucose through the bloodstream to working muscles.
Proteins play an important role in endurance-type activities as well as chronic conditions when glycogen
reserves are significantly reduced. In the absence of adequate levels of other energy sources, the body may
draw upon protein as a kind of energy backup.
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Contrary to popular belief, eating large amounts of high-protein foods (ie: lean meats) or taking protein
supplements, does not automatically result in an increase of muscle mass, a lean body, or an increased ability
to perform better at sports. It may actually lead to serious bodily harm by putting excessive strain on the liver
and kidneys.
Muscle Fibre Types and Energy Systems
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In general, there are two kinds of muscle fibres – slow and fast twitch muscle fibres.
Slow-Twitch Muscle Fibres:
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Slow-twitch muscle fibres generate and relax tension relatively slowly but can maintain a lower level
of tension for long durations.
They are red or dark in colour.
They feature a type of enzyme called myosin ATPase found attached to the thick filaments (myosin) in the
muscle fibre.
The body uses myosin ATPase to provide instant energy for muscle contraction.
These fibres contain low levels of what are known as glycolytic enzymes, which permit the release of glycogen
within muscles and contain high levels of oxidative enzymes.
Since slow-twitch muscle fibres produce lower tension and/or contraction levels over a longer period of time,
they are ideal for activities such as long-distance swimming, cycling, or running.
Fast-Twitch Muscle Fibres:
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Fast-twitch muscle fibres have the ability to tense and relax quickly and can generate large amounts of
tension with relatively low endurance levels.
They are paler in colour than slow-twitch muscle fibres.
They have a different type of myosin ATPase in comparison to slow-twitch muscle fibres and contain high
levels of glycolytic enzymes.
They can activate at a rate of two to three times faster than slow-twitch muscle fibres, making them ideal for
the fast and powerful muscle contractions needed for activities such as short sprints, powerlifting, and
explosive jumps.
The Importance of Myoglobin:
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The differences in muscle fibre types are due mainly to the extent to which a particular muscle relies on
oxygen in the production of energy.
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The protein myoglobin is the oxygen storage unit that delivers oxygen to working muscles, thereby
enabling energy-producing biochemical reactions to be sustained over a long period of time.
The more a muscle utilizes processes for energy production, the more it is able to sustain longer-term activity.
Slow-twitch muscle fibres are high in myoglobin and are ideal for endurance activities.
Fast-twitch muscle fibres are low in myoglobin and are ideal for shorter bursts of effort.
Fast-twitch muscle fibres also are able to perform a muscle contraction more efficiently and more quickly
(remember that a muscle contraction involves: a) the transmission of an impulse through the transverse
tubulae system; b) the release of calcium into the sarcoplasm; and c) the attachment and detachment of myosin
and actin filaments).
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Three Muscle Fibre Types:
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If you combine the tension-generating features and the metabolic properties of muscle fibres, you actually have
three types of muscle fibres:
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Type I or Slow-Oxidative (SO):
These fibres generate energy slowly and are more fatigue-resistant,
and primarily depend on aerobic processes.
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Type IIA or Fast-Oxidative Glycolytic (FOG):
These intermediate-type muscle fibres allow
for high-speed energy release as well as glycolytic capacity.
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Type IIB or Fast-Glycolytic (FG):
These fibres store lots of glycogen and sufficiently high levels
of enzymes necessary for quick contraction without requiring oxygen.
The main reason for the subdivision within the Type II fibres is that Type IIB fibres can, as a consequence of
aerobic endurance training, become Type IIA fibres.
Characteristics of Different Human Muscle Fibre Types
Colour
Fibre diameter
Contraction speed
Force production
Energy efficiency
Myoglobin content
Myosin ATPase
Fatigue resistance
Aerobic capacity
Anaerobic capacity
Type I Slow
Oxidative
Type IIA Fast
Oxidative Glycolysis
Type IIB Fast
Glycolysis
Red
Red/white
White
Small
Medium
Large
Slow
Fast
Very fast
Low
Intermediate
High
High
Low
Low
High
Moderately high
Low
Slow
Fast
Fast
High
Moderate
Low
High
Moderate
Low
Low
High
High
Muscle Fibre Types and Athletic Performance
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For elite athletes in various sports, differences in muscle fibre types are often very pronounced.
Olympic sprinters tend to have as much as 70-80% fast-twitch muscle fibres, whereas those who excel in
marathon-style events possess an equivalent in slow-twitch muscle fibres.
Implications for Training
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To maximize performance, an athlete should match training methods to energy needs.
In training for strength, an athlete’s goal is to increase the load-bearing capacity or explosiveness of the muscles.
During training, athletes such as football lineman, shot putters, or sprinters seek to increase the power they will
require in competition.
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Short powerful training exercises lasting a few seconds will result in increased creatine phosphate in muscle
fibres. Intense exercise lasting up to 90 seconds will increase glycogen stores and enhance the ability to convert
glucose quickly. Both will delay the point at which lactic acid buildup begins to impede physical performance.
A moderate anaerobic routine involving weights, sprints, or plyometrics (hopping and jumping) are useful at
improving muscle strength.
Endurance training is less concerned with building large and powerful muscles. The emphasis is on developing
muscles that can hold up over the long run.
Training involves working to improve the oxygen-processing capacity of the lungs and blood.
Aerobic training will enhance endurance activities by increasing the number of mitochondria in muscle cells,
increasing the amount of the oxygen-storing myoglobin molecules, and enhancing the ability of enzymes within
the muscle cells to utilize this oxygen in the complete breakdown of glucose.
This higher level is achieved by engaging in exercise that raises the heart rate to well above normal for long
periods of time.
Because of the demands it places on the heart and lungs, aerobic exercise is usually recommended for reducing
the risk of heart disease and increasing endurance.
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