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Lecture 6

KIN 143 Lecture 6: Chapter 6 Notes and Study Questions


Department
Biomedical Physio & Kines
Course Code
BPK 143
Professor
Tony Leyland
Lecture
6

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ENERGY PRODUCTION
Energy:the capacity/ability to perform work. Energy is required for muscle contraction and
other biological work (e.g. digestion, nerve conduction, gland secretion etc.)
Power:The rate of change of energy or how quickly you can perform work. In this text, you
will see the term “power outputwhen referring to muscle function during exercise.
Power output: the rate at which working muscles can produce energy. If I do 50 push-ups in
four minutes and then train and improve that to two minutes, my power output for that
exercise has doubled (work done performing 50 push-ups divided by half the time period).
Adenosine triphosphate (ATP):only immediate energy source for muscle contraction. The
phosphate bonds in ATP are high-energy bonds and energy is released when one of these
bonds is broken (Figure 6.1). ATP is the only fuel a cell can use, the body must be able to
rebuild ATP as fast as it is broken down if muscle contraction and other processes are to
continue.
Anaerobic processes:Chemical processes that do not require the presence of oxygen
delivered by the blood.
Aerobic processes:Processes that do require the presence of oxygen delivered by the
blood.
Complex organic fuel molecules (such as carbohydrates, proteins, and fats) with large
amounts of energy stored in their chemical bonds are broken down into other molecules. In
this process energy is made available to produce ATP. Metabolic processes for producing
ATP can be divided into two categories:
There are 3 energy systems in total (some suggest 4). This is because there are two different
anaerobic systems and some suggest the aerobic breakdown of carbohydrates should be
considered a different system than the aerobic breakdown of fats and/or proteins.
Phosphagen System/ATP-Creatine Phosphate System(immediate/alactic energy system)
The two-way arrows shown in Figure 6.1 highlight the principle of coupled
reactions,whereby the energy released by one chemical reaction is used to drive
another chemical reaction.
The body has a limited amount of ATP available for the muscle cells to use
immediately. However, it has a higher amount of CP that can be used almost as quickly
to release energy, which can resynthesize ADP and Pi into ATP. We must continually
resynthesize ATP to supply our cells with energy.
Both ATP and CP are stored in muscle fibres, providing an immediate supply of
energy. However, the amount of ATP available from this process is very limitedonly
enough stored phosphate energy to perform an all-out power exercise (sprinting for
10s). Thus, the phosphagen system predominates in high-power, short-duration
activities, such as sprinting, jumping, throwing, and kicking.
No lactate by-productin this system. ATP is broken down into adenosine
diphosphate(ADP)and inorganic phosphate(Pi), which releases energy for work(muscle
contraction). As ATP is broken down to ADP + Pi it is quickly resynthesize using the energy
released when the chemical bonds of CP are broken. Enzymes catalyze all chemical
reactions in the body. There is a specific enzyme for almost every chemical reaction in the
body; ATPase and creatine kinase, referred to in Figure 6.1Phosphagen system and
principle of coupled reactions, are two such enzymes.
The glycolytic system is an anaerobic system and it must use glucose as a fuel.
Glucose is a source of energy and metabolic intermediate.The acidic form of lactate
cannot be formed in human tissues under normal circumstances. The glycolytic
pathway in humans indicates there are not enough hydrogen ions present in the
glycolytic intermediates to produce lactic acid.
Glycolytic System/Anaerobic Glycolysis
Lactic Acid and Lactate
Glycogen is the form in which humans/mammals store glucose. Glycogen molecules
consist of clusters of glucose molecules attached to each other. Glycogen is stored in
liver and muscle tissues. Plants store glucose molecules in long chains (starch) rather
than in clusters. Glucose accounts for 99% of all sugars circulating in the blood. Blood
glucose comes from the digestion of carbohydrate and the breakdown of liver
glycogen.
Glycolysis refers to the chemical breakdown of glycogen/glucose; this metabolic
pathway uses only carbohydrates. The breakdown of one molecule of glucose to two
molecules of lactate results in the formation of 2-3molecules of ATP, as shown
in Figure 6.2 Anaerobic glycolysis: The glycolytic system. Glycolysis produces a net
of2molecules of ATP for one molecule of glucose. If glycogen is used there is a net
production of 3ATPs as energy is released when splitting the glucose molecule from
the glycogen cluster. The reactions of anaerobic glycolysis release only about 5% of the
energy within the glucose molecule.
Glucose molecules pass from the blood through the muscle cell membrane into
the cell interior.
The glucose splits from glycogen stores in the muscle cell itself.
This system uses glucose molecules to obtain the necessary energy to produce ATP.
This glucose can be made available in the muscle cells for breakdown to lactate by two
principal methods:
You will see the terms lactic acid and lactate used interchangeably; however, they are not the
same compound. Lactic acid is an acid whereas lactate is any salt of lactic acid. When lactic
acid releases its hydrogen ion (H+), the remaining compound forms a salt. Glycolysis
produces lactic acid, but it immediately dissociates and the salt, lactate, is formed.
Anaerobic glycolysis can produce ATP rapidly to help meet energy requirements during
severe exercise, when oxygen demand is greater than oxygen supply. However, high rates
of ATP production by glycolysis cannot be sustained for very long (6090s). This is because
the acidity in the muscle cells (low muscle pH) associated with lactate
accumulation inactivates a few key enzymes in the glycolytic metabolic pathway and
interferes with the process of muscle contraction, causing muscle fatigue (local muscle
fatigue).If you have ever tried sprinting 400 metres, you will notice that, by the second
curve on a 400-metre track, your legs start to feel heavy” and ache slightly. This is local
muscle fatigue. It is not damaging, but it does mean that you are starting to fatigue and soon
you will be no longer able to sustain that power output. The glycolytic system can provide a
relatively rapid supply of ATP, but not as rapid as the phosphagen system. The glycolytic
system can resynthesize a greater quantity of ATP than the phosphagen system, but the
total amount of ATP that can be produced is still limited. Exercises that are performed at
maximum rates for between 1-3minutes, such as sprinting 400 or 800 metres, depend
heavily on the glycolytic system for ATP energy. Also, in some performances, such as
running 1,500 metres or 5,000 metres, the glycolytic system is used predominantly for the
“kickat the end of the race.
OXIDATIVE SYSTEM
Oxidative system (aerobic energy system): Used when enough oxygen is present to
produce required ATP. Predominates in majority of daily situations. In sports,
aerobic processes predominate in lower-intensity, longer-duration activities
(>2-3minutes).
Figure 6.3.Muscle cell. Sarcolemma:the cell membrane of a muscle
cell. Sarcoplasm:a gelatin-like substance fills the space between the myofibrils; this
is where the enzymes for glycolysis are located. The enzymes for aerobic ATP
production are located in the mitochondria this is where most of the ATP in cells are
produced. In presence of adequate oxygen, the mitochondria can produce energy
from carbohydrates, fats, and proteins.
Unlike phosphagen and glycolytic systems, which are limited in the fuel they can
use, the aerobic system can use carbohydrates, fats, and proteins.
Glycolysis:Same process in both aerobic and anaerobic conditions. In
the presence of oxygen, pyruvate molecules are converted to acetyl-
coenzyme A(acetyl-CoA), which means they are not forced to lactate.
!
Krebs cycle(citric acid cycle): Series of chemical reactions occurring in
mitochondria in which carbon dioxide is produced; hydrogen ions and
electrons are removed from carbon atoms (oxidation). Acetyl-CoA
molecules pass from the sarcoplasm into the mitochondria, where they
enter Kreb’s cycle and electron transport chain(ETC),and are ultimately
broken down to carbon dioxide and water.
!
Electron transport chain (ETC)Krebs cycle is coupled with the ETC.
The hydrogen ions and electrons released during glycolysis and Kreb’s
cycle are passed through the ETC and ATP is produced. A chain of
chemical reactions occurs in which electrons and hydrogen ions combine
with oxygen to form water, and ATP is resynthesized. Because this
process requires oxygen, it is called oxidative phosphorylation.
!
The oxidative (aerobic) production of ATP from glucose involves 3processes:
Aerobic Carbohydrate Breakdown (Aerobic Glycolysis)
This process can produce up to 39 molecules of ATP from one molecule of glycogen.
If the process begins with glucose(net gain=38 molecules). The main point is that the
aerobic breakdown of carbohydrates yields many molecules of ATP and does not
result in a build-up of lactate and hydrogen ions, which can interfere with muscle
function.
The breakdown of glycogen can be summarized as:
Figure 6.4Aerobic and anaerobic breakdown of glucose shows the breakdown
of glucose in an anaerobic condition and an aerobic condition.
The main thing is how much more energy can be produced with aerobic
glycolysis (3640 ATP molecules) than anaerobic glycolysis (23 ATP
molecules).
Stored fat represents the bodys greatest source of available energy. Fuel
reserves from stored fat are approximately 70,00075,000 kcal in an
average-size male/female. Carbohydrate energy reserve is1,2002,000
kcal. 1,500 kcal/2000kcalare stored as muscle glycogen, 400 kcal as liver
glycogen, and about 80 kcal of glucose would be in the blood.
Oxidation of Fat (Aerobic Lipolysis)
Some fat is stored in cells, the most active supplier of fatty acid molecules is
adipose tissue. Some fatty acids are stored in the muscles which are an
important supplier of energy to the muscle during low power endurance
activities. Fatty acids are also released from adipose tissue into the blood and
carried to working muscles.
Fat --> fatty acids --> beta oxidation --> Acetyl-CoA--> Krebs cycle -->
electron transport chain + O2 --> CO2+ H2O + ATP.
The breakdown of fat can be summarized as:
1 gram of fat produces 9kcal of energy.
1 gram of carbohydrate produces 4kcal of energy.
1 gram of protein produces 4kcal of energy.
This shows that fat is a concentrated form of energy. For example, one
molecule of palmitic acid(a common free fatty acid) produces 129 molecules of
ATP. One 18-carbon fatty acid molecule yields 147 ATP molecules. It is this
quality that makes it the ideal way for our bodies to store energy. However, in
terms of numbers of ATP molecules produced per molecule of oxygen
consumed, carbohydrate is more efficient than fat. In aerobic carbohydrate
metabolism, 6.3 molecules of ATP are produced for each molecule of oxygen
used. With fat metabolism, however, only 5.6 molecules of ATP are produced
for each molecule of oxygen used.  So when your aerobic activity is of high
intensity (getting close to being anaerobic) you will use glucose to produce
ATP almost exclusively.
As oxygen delivery is limited by the oxygen transport system, carbohydrate is
the preferred fuel during high-intensity exercise. Glucose is the only source of
fuel for the central nervous system (CNS) under non-starvation conditions (eat
a reasonable amount of carbohydrates to ensure glycogen stores are at
optimum levels).
Glycogen --> glucose --> pyruvate --> Acetyl-CoA--> Krebs cycle --> electron
transport chain + O2 -->CO2+ H2O + ATP (39ATP produced from one molecule of
glycogen, 38ATP produced from one molecule of glucose)
Extra protein needed by strength-trained athletes to repair injuries to muscle
fibers and to remodel muscle tissue in response to strength training. However,
endurance activities also cause muscle breakdown and a small amount of
protein is used as fuel during endurance exercise. So the protein requirement
of endurance athletes is higher than those of sedentary individuals.
Although carbohydrates and fatty acids are our preferred fuels, we can
metabolize protein as an energy source. Digestion breaks protein down into
amino acids, which are the building blocks used to repair tissue, make
enzymes. If amino acids exceed the body’s biological requirements, they are
metabolized to glycogen or fat and used for energy metabolism. If amino acids
are to be used for energy, their carbon skeletons are converted to acetyl-
CoA(Amino acids contain nitrogen, and this has to be stripped from the amino
acid molecule in a process called deamination),which enters the Kreb’s cycle for
oxidation, producing ATP. The final products of protein catabolism include
carbon dioxide, water, ATP, urea, and ammonia.
Research indicates that protein breakdown during exercise of long duration,
such as a marathon or long distance cross-country skiing, can account for up to
5-10% of energy expenditure. This is because limited carbohydrate stores
become very low during events of long duration (e.g. >90 minutes) and some
protein can be converted into glucose. Fat, on the other hand, cannot be
converted into glucose. Glucose is more efficient (6.3 ATP molecules for every
O2molecule used) and is the only source of fuel for the central nervous system
under non-starvation conditions. These facts further emphasize the importance
of carbohydrate in the diet.
Protein breakdown above the resting level in both endurance and resistance
training can be considerable. This is most apparent when carbohydrate and/or
energy reserves are low. Thus, adequate carbohydrates in the body will
“spare” protein breakdown and specifically conserve muscle protein, which is
more readily utilized for energy than other proteins. The sparse upper body
musculature of marathon runners illustrates the potential for muscle
breakdown during endurance activities.
Protein --> amino acids --> deamination --> kreb's cycle --> electron
transport chain + O2 --> CO2+ H2O + ATP +urea + ammonia.
The breakdown of protein can be summarized as follows:
Some researchers actually refer to four energy systems, because the aerobic
breakdown of carbohydrate is more efficient than the aerobic breakdown of fat
and protein. In fact, we tend to burn most fat at rest and during recovery rather
than relying on it during exercise, due to it being a slow way to produce
energy.
Protein Metabolism
The power output of these systems varies, as does the length of time they can maintain
ATP generation. Some general characteristics of the phosphagen, glycolytic, and
aerobic systems are given in Table 6.1.
SUMMARY OF THE THREE ENERGY SYSTEMS
It is important to realize you never use only one energy system during activity. Table
6.2 shows an estimate of the relative contributions of the three energy systems during
maximal physical activity of various durations. Again, you must realize that this is the
maximal power output possible for the duration (times) listed. If I work at a
power output for 30 seconds, and that is the maximum time I can sustain that power
output, I will be predominantly using the glycolytic system (approximately 65% of
the energy).
It is important to realize that, when determining which energy system you are using,
Intensity (power output) is the key. If I engage in an activity that requires such a high
muscular power output that I could only sustain this power output for 1012s, most
of the energy in those 1012s will have been supplied by the Phosphagen system.
However, if I walk from my front door down the driveway to my car and that takes
1012 seconds, the predominant energy system will be the aerobic system. Why?
Because I can sustain the power output required to walk to my car for hours. This is a
key point. Although time is discussed and is an issue, it is power output that is the
determining factor as to which energy system you use.
The only exception to this rule of power output determining energy-system usage is
when you start exercising from a resting state. In this situation, even at moderate
power outputs, you will work anaerobically until your cardiorespiratory system
“ramps upto deliver enough oxygen to the working muscles. It takes time for your
respiratory rate and cardiac output to get up to speed. So although you may have
the ability to deliver enough oxygen to work aerobically, you cannot just flick a
switch and attain that cardiac output immediately. This is one reason why warm-ups
are so importantyou do not want to start working anaerobically during a race before
you absolutely have to.
You will notice that I have been careful to say things like predominant energy
system” or getting most energy from. This is because the energy systems discussed
separately are part of the energy continuum. We do not just automatically utilize
the phosphagen system for 1012 seconds, and then switch over to anaerobic
glycolysis, and then on to aerobic metabolism.
So the energy requirements for activities under the duration oftwo hourson are not
provided by one energy system exclusively, and even a marathon runner may get 1%
of his or her energy anaerobically. The energy for most high-intensity activity is
provided for by all three energy systems. As many prefer a graphical representation of
such concepts, the information in Table 6.2 is shown graphically in Figure6.5
Logarithmic graph of energy system contribution. Be careful when looking at the x-
axis values as the scale is logarithmic.
Please note a few things about this graph. The author separated
the phosphagen system into just ATP and ATP-PC. This is not uncommon as strength-
power activities like shot-putting or vertical jump are so short that they don’t even rely
on resynthesizing ADP and PC into ATP. On the other hand, activities like the 100-
metre sprint are termed sustained-power activities, where PC must be used.
Table 6.3 shows the maximum power output associated with the three energy
systems. In this example, moles of ATP” is used to quantify energy. However, you
should realize that in the table, power is the rate at which the energy system can
produce ATP, and capacity is the total amount of ATP that system can produce. You
can see that the phosphagen system can supply energy over twice as fast as anaerobic
glycolysis and around 4x faster than the aerobic system. In reality, this depends on
aerobic training level; So the power available via each system is very different. Notice
also that the energy available (capacity) in the phosphagen system is 1/2 that of the
glycolytic system. When you combine the phosphagen system’s high rate of power
with its small energy availability, you get approximately 10s of peak power output!
Table 6.4 ranks the power and capacity of these systems. Fast glycolysis is just another
term for the breakdown of glucose in the absence of oxygen (glycolytic system). Slow
glycolysis is the breakdown of glucose when oxygen is present (this is part of the
aerobic energy system). The oxidation of carbohydrates is not stated as a separate
process by many working in this field so do not worry about distinguishing between
this and slow glycolysis. While you do not need to know the numbers from Table 6.3,
you should know that the problem with ranking systems is that you don’t get a sense
of the differences between the variables. The obvious example of this is to look at the
capacity of the aerobic system in comparison to the capacity of the anaerobic systems.
Table 6.5 shows metabolic power, mechanical power, predominant energy system
utilized, and time to exhaustion for a healthy male subject on an ergometer where
movement velocity was held constant.
1,000 watts = 1.36 horsepower = 864 kcal per hour.
The efficiency of converting metabolic (chemical) power to mechanical power (output)
is assumed to be 23%.
Only the predominant energy system is listed, but we use all 3 at most power levels.
For example, exercise intensity resulting in exhaustion in six minutes would require
approximately 20% of energy to be obtained from anaerobic systems, and a 14-second
sprint would obtain approximately 10% of its energy from the oxidative system.
Elite athletes can easily surpass the mechanical power outputs or times to exhaustion
listed in Table 6.5. A specialized heavyweight Olympic lifter can generate 6,000 watts
of mechanical power in a single lift, and Lance Armstrong could ride up mountains in
France generating close to 500 watts of mechanical power for 20 minutes. This is
something the subject in Table 6.5 above managed for only six minutes, and a typical
25-year-old could do for only 30seconds.
Once you are working in the aerobic energy system, you theoretically can sustain that
power output for hours. Unless you completely deplete your glycogen, are injured, or
just get bored, you can keep going for hours because you have such large stores of
fatty acids in terms of ATP production.
Another way to present this information is to look at the power outputs of
selected activities on a continuum as a percentage of an individual’s
maximum power output, rather than a specific power output in watts
(Figure 6.6Percentage of maximal power output expended during various
activities)
ENERGY PRODUCTION IN SPORT
Athletes rely on all 3 energy systems to some extent, although marathon and ultra-
marathon athletes may obtain 99%+ of the energy from the oxidative (aerobic)
system. However, many marathon runners do shorter training runs that may use
the glycolytic system to some degree. When you look at steady-state activities like
a short sprint or a distance run, where the athletes power output is relatively
constant, it is easy to determine the predominant energy system. For example, a
400-metre race, despite some tactics, is basically flat out at an intensity you can
maintain for 45+seconds, so we know the predominant energy system will be the
glycolytic system. However, when you look at a team sport or a tennis game, you
will see that the athletes power output varies throughout the game and it becomes
difficult to classify what exactly is happening.
As an example of the variation in power output in a sport is soccer. A soccer game
lasts 90minutes and many players cover around 1012 km at the elite level. For
aconditioned athlete, covering 10 km in 90 minutes averages out to a very slow
jogging-walking pace (only 15 minutes a mile). But there is nothing average about
the movement patterns during a soccer game. A soccer player is often not near the
ball, as the field is large and there are 21 other players on the field. During this
time, the player is usually standing, walking, or doing some light jogging to get
into position. As the play (the ball, or somebody the player wishes to defend
against or get away from) comes closer, the player may cruise (medium-pace
running) to get closer to a desired position. When the player feels he or she can
break into space to receive a pass or urgently have to chase down an opponent, he
or she will sprint. The sprint portion of this activity is very short. A forward may
sprint 10 or 20 metres, at most, to get into space and break clear of the defense. A
defensive player usually has the better position on the attacker and can get away
with a shorter sprint. In addition to these activities, the players are often striking
the ball, jumping up to head the ball, being tackled, bracing themselves against
contact, falling down, and so on. These events all require short bursts of high
power output and hence rely on the phosphagen system.
Table 6.6 is a study of the time and intensity of motion in the English 1st Division.
However, if you look at the average (you can see that walking and jogging make
up 62% of all activity, while sprinting accounts for only 11%. Keep in mind that the
vast majority of backing up and cruising will still utilize the aerobic energy
system, and you can see that this sport requires a blend of short-power bursts and
overall aerobic endurance. The pace of the game has increased since this study,
which is why I said elite players are closer to the 1012 km rather than the 810 km
range shown in this study.
Very rarely are you involved in a play or series of plays that last long enough
to elicit substantial muscle acidity. You are tired at the end of the game, but
that is due to the short sprints and other bursts of energy required in
conjunction with the requirement to cover 812 km during the course of the
game. Admittedly, soccer players quite often run at a power output that can
be sustained for, 90 seconds, and hence they are using the aerobic glycolysis
system. However, it is extremely rare that they have to sustain this power
outputlong enough to accumulate a high muscle acidity and high blood
lactate levels. It is one thing to work at an intensity youcouldsustain for 90
seconds; it is another thing to keep going for 90seconds and completely
deplete the glycolytic system. For this reason, soccer players should not over-
emphasize training to enhance tolerance of high muscle acidity.
Table 6.7 shows information provided by the training group the soccer club
Ajax Amsterdam. This data is more current than the English data and shows
a trend to more sprinting in the modern game. The value of 18% for sprinting
is for elite players in some matches.
Researchers and coaches often break down energy systems into smaller sub-
groups. Figure6.7 Predominant energy pathways is one such example. The
point here is that throwers and jumpers, for example, require strength-power
and need to train for very short bursts of high-intensity power
generation, whereas 100-metre sprinters need a longer sustained-power
output and hence train differently. This is one reason that some prefer to call
the phosphagen system the ATP-PC system, as this highlights the fact
there are two chemical fuels used. Coaches working with short-duration
power athletes really have to focus on training that is specific to ATP
strength-power and ATP-PC sustained power.
Another way some coaches view the energy continuum is shown in Table
6.8. Here there are five predominant areas within the continuum. The middle
time frame of 30 seconds to two minutes will, in fact, increasingly involve the
oxidative system as the time edges toward two minutes. It shows the
difficulty of trying to define set areas in a continuum.
This discussion and Table 6.8 showing the major pathways of energy
production and should allow you to train more specifically for your
sport. Remember though that all types of metabolism are used to some extent
in all activities. Only the primary or near-primary energy systems are shown
in this table.
If you look at the maximal oxygen uptake of various athletes, you will see
that those who have the highest VO2max participate in sports where the
predominant energy system is the aerobic system. The best anaerobic work
to improve VO2max are intervals in the 3090s range. Very short duration
events (strength-power) with long rest periods between exercise bouts (in
sports like baseball and football) will not be very effective at improving
VO2max compared with repeated 400-metre runs. This is because the longer
anaerobic work in the 3090 second range stresses the aerobic system for a
considerable amount of time and ensures a very high oxygen debt.
One of the greatest running backs of all time was found to have a VO2max of
only 45.2 mL/kg×min. This further indicates the specificity of training, as
this athlete will have spent the majority of his training time doing very short-
duration anaerobic drills, which develop neuromuscular coordination,
timing, quickness, speed, and explosive power. It is estimated that 80%+ of
the energy requirements for a volleyball match are anaerobic, while 99%+ of
the energy in a marathon is from aerobic sources (see Table 6.9 for a
comparison of various sports and activities).
The energygraphs/table on ExRx.netare slightly different; this indicates
that the percentage of involvement of an energy system is not absolute for
any individual. Even within sports, different athletes play the same game
very differently. However, you do need to be able to identify the primary
energy system used in a sport when given information on intensity and
duration.
Notes:
TRAINING ALL THREE METABOLIC PATHWAYS
Should you train in all three of these pathways? Obviously, if your sport involves
all three energy systems, the answer is yes (examples include wrestling, MMA,
rugby, soccer, lacrosse, and ice hockey). However, focusing on longer duration
anaerobic work will improve aerobic capacity. I wouldn’t train any of these
athletes with long slow-distance work (maybe one long run every 2-3weeks).
OXYGEN CONSUMPTION DURING RECOVERY: THE OXYGEN DEBT
Once you stop exercising, your breathing and heart rate remains elevated for some time.
Therefore, you must be using more oxygen than you would at rest (even though you are
at rest).This oxygen uptake during recovery from exercise, which is in excess of the
oxygen uptake normally observed during a rest period of similar duration, is called the
excess post-exercise oxygen consumption(EPOC). EPOC is a measurably increased rate
of oxygen uptake following exercise intended to erase the body's "oxygen debt."
During the first few minutes of light to moderate exercise, oxygen consumption
increases progressively until the body reaches a steady state. At this point, oxygen
supply equals oxygen demand. This situation is shown in Figure6.8Oxygendeficit at the
start of aerobic exercise Notice, however, that for a period of time you are exercising at
a level where your oxygen uptake (thick black line) does not meet the exercise needs.
This region is referred to as the oxygen deficit(non-shaded area).It is a simple
conceptyou cannot push your respiratory rate and heart rate to the required levels
immediately at the start of exercise.
In heavy exercise (above maximum capacity), oxygen consumption increases until is
reached, but a steady-state situation is not achieved. Thus, oxygen demand continues to
exceed oxygen supply throughout the entire workout and the oxygen deficit is quite
large. The accumulation of hydrogen ions causing high muscle acidity means that the
duration of such exercise is limited before muscle fatiguesets in. This situation is shown
in Figure6.9Oxygen deficit during anaerobic work
For each situation (aerobic/anaerobic), you have incurred an oxygen deficit whereby
you have had a period during which you did not have enough oxygen available and
therefore produced energy anaerobically. You would have used stored substrates, such
as ATP, CP, and muscle glycogen. These fuels must be replenished after exercise.
Depending on the length and intensity of your exercise, you will also generate excess
heat. So after your exercise, you have to restore normal resting physiological levels.
You continue to breathe heavily for some time after a bout of exercise. You may notice
that, your respiratory frequency may be back to normal (or close to it) relatively quickly,
it can take a considerable length of time before your heart rate returns to resting levels.
All this time (which can be over an hour), you are burning more energy than you would
at rest (i.e. a rest period not following exercise). You are, in fact, consuming more
oxygen than you normally would at rest. TheEPOC refers to this excess oxygen uptake
(oxygen debt). It is shown in Figure6.10 Oxygen debt after exercise(shaded area).
So why do we have this excess oxygen uptake after exercise? There is only a moderate
relationship between the sizes of the oxygen deficit and the EPOC. EPOC is not simply
repaying the oxygen deficit. The graph also shows two definite phases of EPOCa
rapid recovery phase and a slow recovery phase.
Replenishment of muscle phosphagen stores (ATP and CP) and
reloading hemoglobin and myoglobin with oxygen. This results in the rapid
recovery phase.
Body temperature remaining elevated for a long time after cessation of strenuous
exercise. This has a stimulating effect on the rate of chemical reactions in the cells
of the body. This effect of elevated temperature on metabolism probably accounts
for the greater part of the slow recovery phase of oxygen debt.
The residual effects of hormones such as epinephrine and thyroxine released during
exercise. This may continue to increase the metabolism for a long time during
recovery.
The energy needed for tissue repair and redistribution of ions (sodium, potassium,
calcium) in the body.
The extra oxygen needed for heart and respiratory muscles, since heart rate and
minute ventilation remain elevated during recovery.
The major portion (approximately 75%) of the lactate produced during exercise is
oxidized during recovery to provide energy in organs such as the heart, liver,
kidneys, and skeletal muscle. Thus, the main source for re-establishing pre-
exercise glycogen levels is the post-exercise carbohydrate in the diet, not
resynthesized lactate.
The excess post-exercise oxygen consumption during recovery from exercise (oxygen
debt) is caused by the following factors:
Many people look at calories burnt after exercising on a stair master, treadmill that
predicts calories expended, and feel they have not burnt much energy. Energy
expenditure must be viewed as cumulativeover a month you may burn thousands of
calories, and you expend additional calories during recovery. Do not focus on the
calories expended during a workout; being fit has many health benefits, including that
you will burn more calories during the day because you are fit. Aerobic machines only
estimate calorie expenditure for the exercise period. Metabolism stay elevated for some
time after exercise.
RECOVERY TIME
The exact time you need to recover from exercise varies. For steady-state aerobic
exercise (less than 5060%for non-endurance athletes) or maximum-intensity work for
up to 1015s (ATP-CP System), very little muscle acidity and lactate accumulates.
Therefore, recovery is rapid and passive recovery procedures are best. If body
temperature has not been significantly elevated, EPOC will be small. Table 6.10 shows
some recommended recovery times for various metabolic processes.
In extended, non-steady-state, intense exercise, the glycolytic system is activated to
asignificant extent. After such exercise, you will have high muscle acidity and
high muscle and blood lactate levels. You can accelerate the return of muscle
acidity to normal levels with active aerobic recovery exercise. For running, this
would involve jogging at 4060% VO2max during the recovery period. Active
recovery speeds increase the rate of blood flow throughout the body, resulting in
faster normalization of muscle acidity. Also, this sustained blood flow helps
dissipate lactate and increase the rate of catabolism of lactate from the working
muscles due to the elevated metabolic rate. The metabolized lactate will then be
available to provide energy to help the recovery process. After
accumulating muscle acidity, your first impulse may be to rest-recovery, but you
are better to do some light jogging (exercise-recovery).
Some recovery times are affected by diet (need carbohydrates to replenish
glycogen stores). After a bout of prolonged exercise (half/full marathon), it can
take nearly 2days to replenish muscle glycogen. Many studies have shown that
athletes who perform prolonged exercise on consecutive days (e.g., a 1015 km
run) will eventually see a drop in performance, due in part to glycogen depletion
(fatigue and tissue damage are also factors).
If you eat a high-fat, high-protein diet, your glycogen stores may not be
replenished in the time frame shown in Table 6.10. Many athletes eat too much
carbohydrate at the expense of protein and fat. A balance is important.
BLOOD LACTATE
Blood lactate signals anaerobic work. During moderate levels of exercise, energy
demands are adequately met by reactions that use oxygen, and muscle acidity remains
relatively normal. During exercise that involves 60% or more of the untrained subject’s
vo2max, lactate levels in the blood begin to rise. Although lactate is used as a fuel
(when enough oxygen is available), it still signals that you are working anaerobically
and that muscle acidity is rising. At a certain point, the level reaches something called
the onset of blood lactate accumulation(OBLA).The term lactate threshold refers to the
highest intensity of exercise that is not associated with a rise in blood lactate above
resting levels. Lactate threshold is often used interchangeably with OBLA, although
their exact points are different, as shown in Figure 6.11 Lactate threshold and the onset
of blood lactate (OBLA). As exercise intensity increases above the lactate
threshold toward Vo2max, blood lactate rises sharply. For trained endurance athletes,
the lactate threshold occurs at a higher percentage of VO2maxapproximately 75%.
This favourable response could be caused by an endurance athletes genetic endowment
(a high percentage of slow-twitch fibres) or by specific local adaptations within the
muscle that help buffer the hydrogen ions and reduce muscle acidity. The quicker you
remove lactate, the liver converts it to blood glucose, and the longer you will likely be
able to continue that exercise intensity. There is some independence between OBLA and
because of parameters such as fibre type, capillary density, muscle mass used, enzyme
density, and so on. However, in general, it is
true to say that increases in area companied by a higher OBLA.
BIOCHEMICAL ADAPTATIONS TO EXERCISE CONDITIONING
Systemic changes (increased stroke volume) results from aerobic conditioning. We
can look at further adaptations to exercise that occur at the cellular level.  The lists
below are obviously the outcome of specific training as discussed when I
introduced the principle of specificity.
Increases in resting levels of anaerobic substrates (ATP and CP), and to a lesser
extent glycogen content.
Increases in the quantity and activity of key enzymes controlling
the phosphagen system (ATPase and creatine kinase). The largest alterations in
anaerobic enzyme function occur in Type Iix fast-twitch muscle fibres.
Selective hypertrophy of fast-twitch fibres (particularly Type IIx).
The metabolic changes that occur due to activities that demand a high level of
anaerobic metabolism (sprinting, heavy weight training, and power-type training)
include:
Changes Due to Phosphagen System Training
Increases in resting levels of anaerobic substrates, such as ATP, CP, and
glycogen.
Increases in the quantity and activity of key enzymes controlling the anaerobic
phase of glucose breakdown. The largest alterations in anaerobic enzyme
function and increases in size occur in Type IIa fast-twitch muscle fibres.
Selective hypertrophy of fast-twitch fibres (particularly Type IIa).
Increased ability to tolerate high muscle acidity during all-out exercise,
resulting in increased anaerobic exercise capacity.The large amounts of ATP
being produced and hydrolyzed in a short period of time during anaerobic
work overcomes the buffering systems, causing pH to fall and creating a state
of acidosis. Anaerobic training allowsathletes to tolerate acidity levels that are
2030% higher than in untrained subjects. Their greater tolerance is probably
caused by enhanced levels of stored glycogen and glycolytic enzymes, as well
as improved motivation and pain tolerance during fatiguing exercise.
The metabolic changes that occur due to activities that demand a high level of
sustained anaerobic metabolism (30-90s of intense exercise) include:
Changes Due to Gylcolytic System training
Higher capillary density in trained muscle, which allows for greater blood
delivery to the muscle’s cells.
Increase in both size and number of mitochondria and a substantial increase in
the level of aerobic system enzymes. Therefore, mitochondria from trained
skeletal muscle have a greatly increased capacity to generate ATP aerobically.
Increase in skeletal muscle myoglobin content.
Increase in the trained muscle’s ability to mobilize and oxidize fat. Thus, at any
sub-maximal work rate, a trained person uses more free fatty acids for energy
than an untrained person. This glycogen-sparing effect is beneficial to
endurance athletes, who could deplete their limited stores of glycogen in a
long race.
Greater capacity to oxidize carbohydrate in trained muscle. This finding is
consistent with the increased oxidative capacity of the mitochondria as well as
increased glycogen storage in trained muscles.
Biochemical adaptations also occur with aerobic training. These are different from
the cardiovascular and respiratory adaptations (such as increased stroke volume and
decreased resting heart rate).
The changes above occur in all muscle fibres. The basic fibre type does not change to
any great extent, but all fibres maximize their aerobic potentials. Slow-twitch fibres
hypertrophy selectively; in endurance athletes, slow-twitch fibres are larger than
fast-twitch fibres in the same muscle.
Aerobic System Changes
MUSCLE FATIGUE
The appearance of lactate in the blood signals that the body is unable to supply enough
oxygen to the working muscles for them to work aerobically. If anaerobic work
continues for an extended period of time, the muscles will fatigue and be unable
to maintain that level of power output.It is easy to associate fatigue with blood lactate.
This is incorrect, even though rising blood lactate does, signal that Type II fibres are
being recruited and fatigue is imminent if this power output continues.
Lactic acid quickly releases a hydrogen ion and dissociates to form lactate. Lactate
productionis beneficial because it allows the regeneration of a coenzyme that ensures
that energy production is maintained and exercise can continue.Lactate released from
the muscle into the blood is converted in the liver to glucose, which is then used as an
energy source. So rather than causing fatigue, lactate helps to delay a possible lowering
of blood glucose concentration, which is characteristic of a condition called
hypoglycaemia that will cause an athlete to feel weak and fatigued.
Lactate also does not cause an increase in acidity (acidosis) within
the muscle.Theacidosisassociated with increases in lactate concentration during heavy
exercise arises from a separate reaction. When ATP is broken down to release energy for
muscular contraction, hydrogen ions are released. ATP-derived hydrogen ions are
primarily responsible for the increase in acidity within the muscle. During high-
intensity exercise, large amounts of ATP are produced and hydrolyzed in a short period
of time and tissue-buffering systems are overcome, causing pH to fall and acidity to
increase. This natural process facilitates the easier dissociation of oxyhemoglobin and
allows easier transfer of oxygen from the blood. Lactate is a source of fuel and does not
cause post-exercise muscle soreness. However, working out anaerobically causes high
muscle acidity, and discomfort.
There are great benefits to anaerobic work; It is the only way to substantially increase
VO2max.  It is believed that high muscle acidity is one factor, contributing to acute
muscular discomfort experienced during and shortly after intense exercise. However,
research suggests that it may not be acidity that causes muscles to fatigue, but that
fatigue is caused by calcium leaking into muscle cells from the release channels within
the muscle. One of the functions of calcium is to help control muscle contractions. After
extended high-intensity exercise, small channels in the muscle cells begin to leak
calcium, leading to weakened muscle contractions. This leaked calcium also stimulates
an enzyme that attacks muscle fibres, and also leads to fatigue. Very high acidity can
cause damage to the cells, so the calcium leaks may even be a protective mechanism to
prevent muscle cell damage due to excessive acidity.
DELAYED ONSET MUSCLE SORENESS
Delayed onset muscle soreness (DOMS) is not caused by high lactate levels or by
high muscle acidity. High forces/sustained repetitive forces (eccentric muscle
contractions)causes muscle soreness. If we take a blood sample from a runner the
day after a marathon, we find that the levels of an enzyme called creatine kinase
are very high. This is a marker of muscle damage as this particular enzyme "leaks"
from damaged muscle.  The "damage" is in the form of minuet tears or ruptures of
the muscle fibres. We can see this trauma if a sample of muscle is examined
microscopically. However, he or she would experience a huge amount of physical
trauma (force) from around 1,000 foot strikes per mile. The force of each foot strike
can be around 3xbody weight and relying on the muscles, bones and connective
tissue to absorb the impact; and then the muscles have to drive the runner forward
each time. It is not just the muscle that is damaged. By measuring hydroxyproline,
it is possible to show that the connective tissue in and around the muscles is also
disrupted. What these studies show is that DOMS results from muscle damage
and breakdown of connective tissue.
NEURAL FATIGUE
The issue of central nervous system (CNS) fatigue. Neurotransmitters are
involved in cell signalling. During intense, repeated bouts of strenuous exercise,
these neurotransmitters get depleted, resulting in reduced physical and cognitive
performance. However, all voluntary muscle activities are controlled by the CNS
through nerve connections; hence, CNS fatigue is an integral part of rest-recovery
cycles during bouts of exercise. We know that neurotransmitter depletion can
cause CNS fatigue, reduced motivation, loss of motor control, and can affect
memory.
Neural fatigue (weakness) can be both central and peripheral. Central muscle
weakness manifests as an overall, bodily, or systemic sense of energy deprivation,
whereas peripheral weakness manifests as a local, muscle-specific incapacity to do
work.
The central component of muscle fatigue results in a reduction in the neural drive to
working muscles, which in turn causes a decline in the output of force. It has been
suggested that reduced neural drive during exercise may be a protective mechanism
to prevent organ failure if muscular power output is continued at the same level.
The exact mechanisms of central fatigue are unknown.
Central Nervous System
Nerves are responsible for controlling muscle contraction, and determining the
number, sequence, and force of contraction. Nervous fatigue is seldom an issue for
everyday or low-power output activities, such as marathon running. However, for
extremely powerful contractions that are close to the upper limit of a muscle’s ability
to generate force, nervous fatigue can be a limiting factor, especially in untrained
individuals. In novice strength trainers, the muscle’s ability to generate force is most
strongly limited by the nerve’s ability to sustain a high-frequency signal. After a
maximum contraction/several contractions near maximum (e.g. 5-RM set), the
nerves signal is reduced in frequency and the force generated by the contraction
diminishes. There is no sensation of pain or discomfort; the muscle appears to
simply stop listening” and gradually ceases to produce force. As the accumulated
stress on the muscles and tendons is quite low compared to the number of
contractions required to run a marathon, there will often be no delayed-onset muscle
soreness following the workout.
Part of the benefit of strength training is to increase the nerve’s ability to generate
sustained, high-frequency signals, which allows muscles to contract with greatest
force. It is this neural training” that initially causes several weeks worth of rapid
gains in strength despite very little muscle hypertrophy. These strength gains level
off once the nerve becomes accustomed to generating maximum contractions. Past
this point, training effects increase muscular strength through myofibrillar or
sarcoplasmic hypertrophy, and metabolic fatigue (lack of fuel and/or increased
acidosis due to hydrogen ion concentrations and possibly calcium channel leakage
e.g. glycogen depletion) becomes the factor that most limits contractile force.
Peripheral Neural Factors
THE NUMEROUS FACTORS INVOLVED IN FATIGUE
CNS fatigue (neurotransmitter depletion).
My cardiovascular system would have been focusing on delivering a large
percentage of the cardiac output to the running muscles and wouldn’t be able to
switch immediately to opening up capillary beds in the muscles required for the
pull-up.
There is also a psychological factorI would come back into the gym bursting a
lung, and starting pull-ups would be very tough because I would feelfatigued.
My mind would say, Stop doing this!
Glycogen stores depleted
Fatigue has many components. Many things will affect that pull-up set after a hard run:
Anaerobic processes:Chemical processes that do not require the presence of
oxygen delivered by the blood.
Aerobic processes:Processes that do require the presence of oxygen
delivered by the blood.
Define the terms aerobic and anaerobic.1.
ATP (adenosine triphosphate): only immediate energy source for
muscle contraction. The phosphate bonds in ATP are high-energy
bonds and energy is released when one of these bonds is broken. ATP
is the only fuel a cell can use, the body must be able to rebuild ATP as
fast as it is broken down if muscle contraction and other processes are
to continue.
What is ATP?2.
When the energy released by one chemical reaction is used to drive another
chemical reaction. E.g. The body has a limited amount of ATP available for
the muscle cells to use immediately, however, it has a high amount of CP
that can be used almost as quickly to release energy, which can resynthesize
ADP and Pi into ATP. We must continually resynthesize ATP to supply our
cells with energy.
Describe the Principle of Coupled Reactions.3.
Intensity/power output is important because If I engage in an activity
that requires such a high muscular power output that I could only
sustain this power output for 10–12s, most of the energy in those
10–12s will have been supplied by the Phosphagen system. However,
if I walk from my front door down the driveway to my car and that
takes 10–12 seconds, the predominant energy system will be the
aerobic system. Why? Because I can sustain the power output required
to walk to my car for hours. This is a key point and one that is often
confused. Although time is discussed and is an issue, it is power
output that is the determining factor as to which energy system you
use.
What is the most important variable in determining which energy system is
being utilised? Why?
4.
Aerobic: Glycogen --> glucose --> pyruvate --> Acetyl-CoA--> (Now
inside mitochondrial wall) Krebs cycle --> electron transport chain +
O2 -->CO2+ H2O + ATP (39ATP produced from one molecule of
glycogen, 38ATP produced from one molecule of glucose)
Anaerobic: Glucose --> 2pyruvate--> Lactate +2-3ATP
Compare and contrast the aerobic and anaerobic breakdown of muscle
glycogen. Include as much detail as possible in your answer.
5.
Small organelles that contain enzymes for aerobic ATP production, this
is where most of the ATP in cells are produced. In presence of adequate
oxygen, the mitochondria can produce energy from carbohydrates, fats,
and proteins.
What are mitochondria?6.
Fat --> fatty acids --> beta oxidation --> Acetyl-CoA--> Krebs cycle -->
electron transport chain + O2 --> CO2+ H2O + ATP.
1gram of fat produces 9kcal of energy
Summarise the steps in the oxidation of fat.7.
Extra protein needed by strength-trained athletes to repair injuries to
muscle fibers and to remodel muscle tissue in response to strength
training. However, endurance activities also cause muscle breakdown
and a small amount of protein is used as fuel during endurance exercise.
So the protein requirement of endurance athletes is higher than those of
sedentary individuals.
Although carbohydrates and fatty acids are our preferred fuels, we can
metabolize protein as an energy source. Digestion breaks protein down
into amino acids, which are the building blocks used to repair tissue,
make enzymes. If amino acids exceed the body’s biological requirements,
they are metabolized to glycogen or fat and used for energy metabolism.
If amino acids are to be used for energy, their carbon skeletons are
converted to acetyl-CoA(Amino acids contain nitrogen, and this has to be
stripped from the amino acid molecule in a process called
deamination),which enters the Kreb’s cycle for oxidation, producing ATP.
The final products of protein catabolism include carbon dioxide, water,
ATP, urea, and ammonia.
Protein breakdown above the resting level in both endurance and
resistance training can be considerable. This is most apparent when
carbohydrate and/or energy reserves are low. Thus, adequate
carbohydrates in the body will spare protein breakdown and
specifically conserve muscle protein, which is more readily utilized for
energy than other proteins.
Protein --> amino acids --> deamination --> kreb's cycle --> electron
transport chain + O2 --> CO2+ H2O + ATP +urea + ammonia.
Briefly describe the role protein plays in exercise metabolism.8.
It is important to realize you never use only one energy system during
activity. If I work at a power output for 30 seconds, and that is the maximum
time I can sustain that power output, I will be predominantly using the
glycolytic system (approximately 65% of the energy).
It is important to realize that, when determining which energy system you
are using, Intensity (power output) is the key. If I engage in an activity that
requires such a high muscular power output that I could only sustain this
power output for 1012s, most of the energy in those 1012s will have been
supplied by the Phosphagen system. However, if I walk from my front door
down the driveway to my car and that takes 1012 seconds, the predominant
energy system will be the aerobic system. Why? Because I can sustain the
power output required to walk to my car for hours. Although time is an
issue, it is power output that is the determining factor as to which energy
system you use.
How can one determine which energy systems predominate in any sport or
activity?
9.
marathon running: low phosphagen system, low anaerobic glycolysis,
and high aerobic metabolism
Baseball: Phosphagen system high, anaerobic glycolysis low and
aerobic metabolism is not really used
springboard diving: high phosphagen system, low anaerobic
glycolysis and aerobic metabolism is not really used
Soccer: high phosphagen system, moderate anaerobic glycolysis, and
moderate aerobic metabolism
Volleyball: high phosphagen system, moderate anaerobic glycolysis
and aerobic metabolism is not really used
What are the primary metabolic demands of the following sports? Use the
terms high, medium and low to describe the involvement of each energy
system.
10.
Chapter 6: Energy Systems and Implications for Athletic and Sport Training
Coaches working with short duration power athletes have to take this into
consideration, such as sprinting
13.   What are the five predominant areas of the energy continuum discussed in
lecture? Give the approximate duration of these areas and name one event or
sport that relies heavily on this area.
The three energy systems are usually broken into smaller sub groups.
Throwers and jumpers, for example, require strength-power and need to
train for very short bursts of high-intensity power generation. They need to
do a lot of strength training, whereas 100-metre sprinters need a longer
sustained-power output and hence train differently. This is one reason that
some prefer to call the phosphagen system the “ATP-PC system,” as this
highlights there are two chemical fuels used. Coaches working with short-
duration power athletes really have to focus on training specific to ATP
strength-power and ATP-PC sustained power.
14.   Why do we talk about three energy systems but then also discuss numerous
areas of the energy continuum?
Replenishment of muscle phosphagen stores (ATP and CP) and
reloading haemoglobin and myoglobin with oxygen. This results in
the rapid recovery phase.
Body temperature remains elevated for a long time after cessation of
strenuous exercise. This has a stimulating effect on the rate of chemical
reactions in the cells of the body. This effect of elevated temperature
on metabolism accounts for the greater part of the slow recovery phase
of oxygen debt.
The residual effects of hormones such
as epinephrine and thyroxine released during exercise. This may
continue to increase the metabolism for a long time during recovery.
The energy needed for tissue repair and redistribution of ions
(sodium, potassium, calcium)