Table of contents
Page number
content
1
Table of content
2
Introduction
3
Source of energy for different types of exercise duration and
intensity
5
The high-energy phosphate system
6
Anaerobic glycolytic system
6
Aerobic oxidative system
7
The lactic acid system
7
Energy investment phase
8
Energy generation phase
11
Oxygen consumption during exercise
12
Oxygen deficit
13
Why knowing oxygen consumption is important
14
At what exercise level does the body switch to anaerobic energy
metabolism?
16
Conclusion
17
References
1
Introduction
There is no doubt that exercise physiology is an important field of science
that combines between biochemistry, anatomy and physiology. The medical
research reached great achievements in this field. In this topic we will pick a
very important section in exercise physiology which is exercise metabolism.
Exercise metabolism is essential not just for physiotherapists but also for
nutrition experts, trainers whether in a club or a gym, and many other
professions.
In this topic we will mention some points about exercise metabolism. We
will start with describing the most important three metabolic pathways that
supply our bodies with energy, which are the high-energy phosphate system,
anaerobic glycolytic system and aerobic oxidative system. So, we will show
how our bodies choose the suitable pathway according to the type of
exercise duration and intensity. Then we will move to talk about one system
of these systems in detail, which is anaerobic glycolytic system, known as
the lactic acid system. After that, we will follow oxygen uptake in our bodies
during performing exercise and mention the importance of it. Finally, we
will mention how our bodies get energy during intense exercise after energy
requirements exceeds maximal oxygen uptake.
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1- Source of energy for different types of exercise duration and
intensity.
Before discussing this topic we should mention the contribution of every
nutrient substrate. Firstly, Proteins have the minor role in this process, as
they contribute to only 2% of the nutrient substrate utilized during exercise
and this is for one hour’s duration. This role slightly increases during
prolonged exercise for three to five hours duration. During this type, the
total contribution of proteins reaches 5-10%. This is very helpful when the
other sources of energy have been depleted from our bodies.(Yamany et al,
2019-2020)
Secondly, Fats and carbohydrates are the primary sources of energy during
exercise. The energy stored in fats and carbohydrates is released when these
substrates are broken down. This energy is in the form of ATP. ATP is then
used by cells for many purposes, including muscle contraction during
exercise. Although each gram of fat stores 9 kcal, while each gram of
carbohydrates stores 4 kcal, the rate at which ATP can be formed is higher
for carbohydrates than for fats. (Thompson, 2009)
Choosing between them is dependent on several factors:
• Diet: High-fat and low-carbohydrate diet increase the rate of fat
metabolism.
• The intensity: Low-intensity exercise relies mainly on fat as an energy
source, while high-intensity exercise relies primarily on carbohydrate as an
energy source.
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• Duration of exercise: Long-term, low-intensity exercise has a progressive
increase in the amount of fat oxidized by muscles. (Yamany et al, 20192020)
Now we will mention an abstract about a very critical molecule in our topic
which is adenosine triphosphate. ATP is broken down in a process termed
ATP turnover. Water hydrolyzes the unstable chemical bonds of the
phosphate groups of the ATP molecule, so the output is an inorganic
phosphate molecule (Pi) and adenosine diphosphate molecule (ADP). The
released energy is about 9-10 kilocalories (kcal) per mole of ATP. When we
start performing work, our bodies need continuous supply of large amounts
of ATP, but the initial ATP in our muscles is depleted rapidly. Thus, we
need to reform ATP. ATP can be regenerated by the combination of Pi and
ADP. This is done through a metabolic process called ATP resynthesis:
1-ADP+ Pi + energy = ATP
It can also be regenerated through the combination of phosphocreatine and
ADP and this reaction is very fast:
2-PCr + ADP + Energy = ATP + Cr
Now let us mention the three energy sources in our bodies:
1)
The high-energy phosphagen system
2)
The anaerobic glycolytic system
3)
The aerobic oxidative system
These sources of energy for muscular contraction and other types of work
are divided into aerobic and anaerobic, according to whether they need
oxygen to provide energy. The high-energy phosphate and the anaerobic
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glycolytic systems do not require oxygen, while the aerobic oxidative system
depends on oxygen to produce energy.) Wells, Selvadurai & Tein, 2009)
The regeneration of ATP requires energy. This energy is supplied by the
breakdown of complex food molecules, mainly carbohydrates and fats in
metabolic energy systems of our bodies. (Wells et al, 2009)
Now let us discuss each source of them and the type of exercise supplied by
each one of them.
1-The high-energy phosphate system: It is also known as phosphagen
system or anaerobic alactic system. This system can be the main energetic
system supplying the muscles in the initial 1-15 seconds of high-intensity
exercise. The substrates of this system are the adenosine triphosphate (ATP)
and phosphocreatine (PCr). On the onset of the exercise, ATP is broken
down by the enzyme ATPase, and PCr is broken down by the enzyme
creatine kinase to supply Inorganic phosphate for the process of ATP
resynthesis as mentioned in the equations (1 and 2) before. (Wells et al,
2009)
This system is able to produce large amounts of energy in short duration
(from 2.4 mmol/kg/s in sedentary people to 10-15 mmol/kg/s in athletes).
The total muscles stores of ATP (3.5-7.5 mmol/kg) and and PCr (16-28
mmol/kg) are very small and consumed quickly during high-intensity
exercise. Thus, the ability of an individual to do short-term, high-intensity
exercise is dependent on the initial concentrations of high-energy
phosphates. (Wells et al, 2009)
This system is sufficient in short-term intense events such as weight lifting,
100-meter run or 25-meter swim. These high power output events need high
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rate of power production in short intervals. If individuals are to perform
activities lasting more than 15 seconds, the PCr stores are depleted and ATP
resynthesis must take place through other pathways at a lower power output.
(Wells et al, 2009)
2-Anaerobic glycolytic system: Our bodies depend primarily on this system
as a source of energy to perform high-intensity exercise greater than 12-15
seconds and less than 3 minutes. This system is used mainly in sports events
such as 800-meter swim, 1500-meter speed skating and single sprints during
football or hockey games. Energy production in this system is done via
glycolysis in the cytoplasm of skeletal muscles by the catabolism of
carbohydrates in the form of blood glucose or glycogen to pyruvic acid
which may be converted into lactic acid through 10 separate but linked steps
of chemical reactions (these steps will be mentioned in detail in the
following section). (Wells et al, 2009)
3-Aerobic oxidative system: This system is the primarily supplier of a
broad range of activities, so it is a very important system in our bodies. It is
the energy source of the exercise performed at an intensity lower than the
anaerobic threshold (will be mentioned in detail in the last section). As the
duration of intense exercise increases, the contribution of this system to the
total energy production increases. This system is predominant in prolonged
exercise provided that:
1-There is sufficient number of mitochondria in the working muscles to meet
energy requirements.
2-Sufficient amount of oxygen is delivered to the mitochondria
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3-There is no enzymes or intermediates limit the rate of energy production
by the Krebs cycle or electron transport chain (the bioenergetics pathways
that are responsible for production of ATP in the mitochondria). (Wells et al,
2009)
2-The lactic acid system
It is also known as anaerobic glycolysis and it happens in the cytoplasm of
eukaryotic cells. Anaerobic glycolysis is the process of taking carbohydrate
in the body and putting it through a series of chemical reactions to release
adequate energy to rephosphorylate ADP and reform ATP. The final product
of this series of chemical reactions is often lactic acid, so this energy system
is often called the lactic acid system. Lactic acid is a weak acid which is
rapidly dissociated under normal conditions in the body, separating into
lactate and a hydrogen ion (H+). (Doyle and Dunford, 2011)
Carbohydrates consumed are converted to and used as glucose, or are stored
as glycogen within the muscle and liver for later use. Glycogen is a large
molecule formed of lots of smaller molecules of glucose. Glycolysis is the
breakdown of glucose through a selected series of chemical steps to
rephosphorylate ADP and reform ATP. When the substrate is glycogen, the
method is named glycogenolysis. (Doyle and Dunford, 2011)
The reactions of anaerobic glycolysis can be considered as two distinct
phases:
1- An energy investment phase
In this phase, there are 2 phosphates added to glucose. Glycolysis begins
with hexokinase phosphorylating glucose into glucose-6 phosphate (G6P).
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This is often the primary transfer of a phosphate group and where the
primary ATP is used (this step is an irreversible step). This phosphorylation
save the glucose molecule within the cell as it cannot cross the cell
membrane. From there, phosphoglucose isomerase isomerizes G6P into
fructose 6-phosphate (F6P). After that, the second phosphate is added by
phosphofructokinase (PFK-1). PFK-1 uses the second ATP to phosphorylate
the F6P into fructose 1,6-bisphosphate (This step is irreversible and for that
it is the rate-limiting step). Within the following step, fructose 1,6bisphosphate is lysed into 2. Fructose-bisphosphate aldolase lyses it into
dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate
(G3P). Dihydroxyacetone phosphate is converted into G3P by
triosephosphate isomerase. DHAP and (G3P) are in eqilibrium with one
another. (Chaudhry & Varacallo, 2019)
2-An energy generation phase
It is important to know that in this phase there are a total of 2-3 carbon
sugars for every one glucose molecule in the beginning. The enzyme,
glyceraldehyde-3-phosphate dehydrogenase metabolizes the G3P into 1,3diphosphoglycerate by reducing NAD+ into NADH. Then, the 1,3diphosphoglycerate loses a phosphate group by the method of
phosphoglycerate kinase to make 3-phosphoglycerate and makes an ATP
through substrate level phosphorylation. At this point, there are two ATP
created, one from each 3-carbon molecule. The 3-phosphoglycerate is
converted into 2-phosphoglycerate by the action of phosphoglycerate
mutase, and then enolase converts the 2-phosphoglycerate into
phosphoenolpyruvate (PEP). In the final step, pyruvate kinase converts
phsphoenolpyruvate into pyruvate and phosphorylates ADP into ATP
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through substrate-level phosphorylation, therefore making two more ATP
(This step is irreversible). So we can say that, the input for one glucose
molecule is two ATP, and the output is four ATP (net gain=two ATP) and
two NADH and two pyruvate molecules. (Chaudhry & Varacallo, 2019)
It is also important to mention that, the net gain glycolysis when glycogen is
the substrate is three ATP, as glycogen does not require phosphorylation by
ATP, but is phosphorylated by inorganic phosphate (Pi) instead. (Yamany et
al, 2019-2020)
After that hydrogen ions (H+) are removed frequently from nutrient
substrates and carried by two biologically important carrier molecules .The
first one is nicotinamide adenine dinucleotide (NAD), the second is flavin
adenine dinucleotide (FAD). (Yamany et al, 2019-2020)
Both of NAD and FAD carry hydrogen ions and their associated electrons to
generate ATP through aerobic glycolysis lately by Kreps cycle and electron
transport chain in the mitochondria.
If the process is to continue, it is critical that adequate amounts of NAD
must be available to accept free hydrogen ions from glyceraldehyde 3phosphate and
there is two methods that the cell follows to restore NAD
from NADH:
1-First, if there is sufficient amount of oxygen, the hydrogen ions from
NADH can be transported to mitochondria of the cell and contributes to
aerobic production of ATP.
2-Second, if oxygen is not available or the cell lacks mitochondria; pyruvic
acid can accept the ions and form lactic acid. This reaction is catalyzed by
lactate dehydrogenase enzyme (LDH) with end result of formation of lactic
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acid and reformation of NAD. It is obvious that recycling of NAD is the
reason of accumulation of lactic acid during exercise. (Yamany et al, 20192020)
As a conclusion to this we can say that the microenvironment of the cell
determines what will happen to the pyruvate following the steps of the
glycolysis. If there is no oxygen available, there are no mitochondria in the
cell, or energy demand has rapidly increased to be above the rate at which
oxidative phosphorylation can provide adequate ATP, pyruvate can be
converted to lactic acid by the enzyme lactate dehydrogenase. This step
includes the oxidation of NADH to NAD+, allowing glycolysis to continue
through the glyceraldehyde-3-phosphate dehydrogenase reaction as
mentioned before. (Melkonian & Schury, 2019)
(Figure I)
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3-Oxygen consumption during exercise
It is also called Oxygen uptake (VO2), it is the amount of oxygen taken in
and used by the body per minute to produce massive amounts of ATP to the
muscle cells via aerobic oxidative system. Although there are processes that
can produce ATP without oxygen (O2), these anaerobic systems (The lactic
acid system and high-energy phosphate system) are limited in their capacity
to produce adequate amounts of ATP. (Thompson, 2010)
The importance of oxygen is that it is the last receptor in the electron
transport chain, so the availability of oxygen in the muscle cells is critical to
continue the process of aerobically generating massive amounts of ATP (32
ATP per glucose molecule and 33 ATP if the substrate is glycogen as
mentioned before) to continue the process of generating force. (Yamany et
al, 2019-2020)
Now let us talk about the pathway of consuming oxygen during exercise:
The pathway for oxygen transport includes three main structures; the lungs,
the circulation, and the muscle. Oxygen follows a linear pathway without
branches. Respiration is a regulated process in line with the demands of
aerobic metabolism: as muscle demands for ATP increases, oxygen demand
is increased proportionally. Increases in oxygen transport that occur during
exercise are considered as a result of:
1-Increase in ventilation and uptake of oxygen into the blood.
2-Increase in cardiac output and the transport of oxygen by the blood.
3-An increased extraction of oxygen by the muscles due to increased oxygen
metabolism in the mitochondria.
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Respiration is a limited function, and the limit of the maximal rate of oxygen
uptake is termed VO2 max. Any additional energy requirements above that
intensity will be fulfilled by the anaerobic glycolytic system. Surely, VO2
max is higher in athletes than nonathletes, and to a certain extent it can be
elevated by training. (Wells et al, 2009)
The rate of oxygen use (VO2) is sometimes expressed in liters of O2 per
minute (liters per minute) or it can be expressed as a function of body
weight, as milliliters of O2 per kilogram of body weight per minute
(milliliters per kilogram per minute). Expressing VO2 as a function of body
weight is important when comparing the different sizes of people during
exercises such as running. (Thompson, 2010)
When we start the exercise our bodies take one or two minutes to reach the
steady state (the steady state refers to the plateau in oxygen uptake that is
reached following a few minutes of exercise) so there is lag in oxygen
uptake at the beginning of exercise and this lag is termed oxygen deficit.
Definition of Oxygen Deficit: “the difference between oxygen intake by the
body during early stages of exercise and an equal time period after steady
state has been reached.” (Yamany et al, 2019-2020)
As shown in (Figure II), Oxygen deficit is represented as the shaded area on
the left. Surely the duration of oxygen deficit in trained subjects is shorter
than untrained subjects, as trained subjects reach steady state of VO2 fast.
This is because trained subjects have better-developed aerobic bioenergetic
capacity, resulting from either cardiovascular or muscular adaptations
induced by endurance training as shown in (Figure III). (Yamany et al,
2019-2020)
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(Figure II)
(Figure III)
Why knowing oxygen consumption (VO2) is important?
VO2 shows energy expenditure, meaning that measuring VO2 provides an
estimate of calories burned. Approximately 5 kcal of energy are expended
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for every liter of O2 consumed. Thus, a person with a VO2 of 1 L/min is
burning 5 kcal each minute. As one exercises harder, VO2 rises, meaning
that energy expenditure is higher. The relationship between the type and
intensity of activity, O2 consumed, and energy used allows researchers to
estimate the caloric expenditure for different tasks. This information can be
used to shape exercise plans to make appropriate programs of weight control
goals. (Thompson, 2010)
4-At what exercise level does the body switch to anaerobic
energy metabolism?
We mentioned in the last section that any energy requirements above VO 2
max is fulfilled by the anaerobic glycolytic system, now let us explain this in
more detail. The exercise intensity at which lactic acid starts to accumulate
in the blood and muscles has been termed anaerobic threshold. Anaerobic
threshold is the point during exercise when the individual starts to feel
discomfort and the burning sensation in his muscles. It can be identified
during clinical incremental exercise tests as it is the point when carbon
dioxide production (VCO2) exceeds oxygen uptake (VO2). In exercise
physiology lactic acid accumulation is a measured variable and its typical
values range from 2 mmol/L at rest to 4 mmol/L in moderate exercise to 16
mmol/L during maximal anaerobic exercise (2-minutes maximal sprint).
Lactic acid is removed from type II muscle fibers (fast glycolytic fibers),
where it is primarily to produced, to type I (oxidative fibers) where it is
oxidized in mitochondria allowing individuals to continue training for longer
periods. But when the energy requirements exceeds maximal oxygen uptake,
the body will make a transition from aerobic to anaerobic metabolism to
supply the body during intense exercise. Thus, high amounts of lactic acid
14
will accumulate within our blood and muscle fatigue takes place to oblige us
to stop exercising. (Wells et al, 2009)
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Conclusion
Finally, we can conclude that our bodies supply us with different sources of
energy during our daily activities and exercises accurately, according to the
level of intensity and duration. And we saw that however the lactic acid
system is a very useful system in short-duration, high-intensity exercises as
it is takes short time, using it in high rates can make us suffer from
metabolic acidity. Then, we knew the importance of oxygen during exercise
and how it is used. Then, we highlighted an important point that in very high
intensities of exercise, our bodies shift from aerobic to anaerobic
metabolism. So, anaerobic metabolism is important as it can fill the gaps of
the aerobic system (oxygen deficit and production of ATP is less than
required during intense exercises).
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References

Chaudhry, R., & Varacallo, M. (2019). Biochemistry, Glycolysis. In StatPearls
[Internet]. StatPearls Publishing.

Dunford, M., & Doyle, J. A. (2011). Nutrition for sport and exercise. Cengage
Learning.

Hassan, A.I.,Yamany, A.M., Shehata,S., Abutaleb,E. and El-Sayyad,M.(20192020). Principles of exercise physiology. Cairo-Egypt:Al-Resala press.

Melkonian, E. A., & Schury, M. P. (2019). Biochemistry, Anaerobic Glycolysis.
In StatPearls [Internet]. StatPearls Publishing.

Thompson, D. L. (2009). Fitness Focus Copy-and-Share: The Crossover Concept.
ACSM's Health & Fitness Journal, 13(1), 4.

Thompson, D. L. (2010). Fitness Focus Copy-and-Share: What is Oxygen
Consumption?. ACSM's Health & Fitness Journal, 14(1), 4.

Wells, G. D., Selvadurai, H., & Tein, I. (2009). Bioenergetic provision of energy
for muscular activity. Paediatric Respiratory Reviews, 10(3), 83–90.
doi:10.1016/j.prrv.2009.04.005
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