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ENERGY SYSTEMS
Energy from the food we eat is made available for use by the body’s cell through a molecule
called ATP (Adenosine triphosphate).
ATP is the body’s energy currency. ATP is made up of one adenosine and three phosphate
molecules. The potential energy stored within an ATP molecule is used for all energy
requiring processes within a cell. When ATP is split it releases energy that can be used for
muscular contractions.
ATP→ADP + P + energy
Our cells only have enough ATP stored to last for a very short period of time (only a few
seconds). Once this has been used more ATP needs to be produced. Fuel molecules from
our foods can be broken down to provide energy which can be used to join adenosine
diphosphate and phosphate to form adenosine triphosphate.
ADP + P + energy from food fuels →ATP
Our energy systems break down the food fuels to reproduce ATP. There are three energy
systems at work in our body. These systems operate together however the extent to which
one takes over depends on the intensity and duration of the activity.
Energy in our body can be produced two ways
Anaerobically - WITHOUT oxygen present.
Aerobically - WITH oxygen present.
Alactic or phosphogen system.
This system uses the reserve fuel creatine phosphate which is stored in muscle fibres. ATP is
broken down into ADP + P then resynthesized with the expense of the phosphate from the
creatine phosphate molecule. The following reactions occur:
ATP → ADP + P + energy (for cross bridging)
CP →Cr + P + energy
Energy (from CP above) + ADP + P → ATP
Energy produced via the alactic system is utilised in activities of high intensity involving
explosive movements of a short duration (up to 10 seconds), as our muscles only store a
small amount of ATP and CP the duration of this system in limited. ATP and CP stores are
replenished during recovery.
If we continue to exercise beyond 10 seconds (Eg a longer sprint), we would utilise the lactic
or glycolytic system.
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Lactic or Glycolytic System
Carbohydrates are the main fuel used by this system which dominates in activities of
maximal intensity and limited duration.
Glycogen (form of carbohydrate stored in muscles)
Glucose
ADP + P
2 ATP
Pyruvic acid
Insufficient oxygen
Lactic acid (by product of anaerobic glycolysis)
This system is used in events lasting 2-3 minutes. The limiting factor of this system is the
accumulation of lactic acid in the muscle causing it to fatigue.
Aerobic System
The aerobic system dominates during longer events and endurance activities. Remember
that a person’s fitness level has a role in determining which system can override the others.
The aerobic system is used in sub-maximal activities that are performed for long periods.
The main fuels for this system are carbohydrate and fats.
Glycogen (form of carbohydrate stored in muscles)
ADP + P
Glucose
2 ATP
36 ATP + Carbon dioxide + Water
Pyruvic acid
Sufficient oxygen
NB – 38 molecules of ATP are produced from
the breakdown of one glucose molecule
Kreb cycle (this takes place in the mitochondria of the cell)
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Anaerobic Threshold
This is the point where the body cannot supply oxygen to the working muscles as fast as it is
needed so the body is required to supply more of its energy from the anaerobic system. This
in turn causes the onset of blood lactate accumulation. The fitter the individual is, the
longer it will take to reach the anaerobic threshold.
Removal of Lactic acid from blood and muscle
Lactic acid accumulates in the muscles during intense exercise, causing muscle fatigue and
forcing the performer to stop exercise. The greater the effort, the more lactic acid will be
accumulated.
Lactic acid is removed from the blood and muscles during recovery, with the removal being
faster when a light jog or warm down is performed. The lactic acid enters the blood stream
and goes to the liver where it is converted back into glycogen.
Skeletal muscles are also able to oxidise lactic acid to produce carbon dioxide, water and
ATP and so continued activity will mean that greater amounts of lactic acid will be removed.
Perceived Rate of Exertion (PRE)
This can be used to assess the level of intensity a person is working at. Generally a scale of
one to ten is used. The person is asked to identify where on the scale they are during a
specific activity. One is the lowest intensity (e.g. very slow walking) while ten is the highest
intensity (e.g. sprinting). The lower the PRE the longer the person should be able to
maintain the activity.
Steady state
This occurs when the heart rate reaches the optimal level to meet the specific demands of
the activity. The heart rate then remains steady.
Maximal aerobic capacity
During exercise the energy demands for our body increase. This causes a greater demand
for oxygen. An increase in ventilation occurs in response to this. Heart rate and blood flow
also increases.
Maximal Oxygen Consumption or VO2 max is the maximal volume of oxygen consumed per
minute during exercise. This is also the amount of oxygen capable of being transported and
consumed by the working muscles.
Two main factors determine VO2 max:
 The cardio respiratory system’s ability to take in and transport oxygen.
 The oxygen extraction capabilities of the muscle, e.g. the muscle’s ability to use the
oxygen.
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Oxygen debt
The difference between oxygen demand and oxygen consumed is called oxygen deficit.
Once exercise ceases extra oxygen is still consumed to repay the oxygen deficit that occurs
before steady state is reached. This is known as Excess Post Oxygen Consumption (EPOC).
The extra oxygen is used for a number of tasks:
To replenish oxygen reserves
To convert lactic acid into pyruvic acid
To replace glycogen stores
To replenish creatine phosphate stores
Re-oxygenation of venous blood.
EPOC has two components – a fast component and a slow component
Fast component: This is the initial component and utilizes up to 4 liters of oxygen. The main
functions of this component are:



Replenish myoglobin and haemoglobin stores.
Restore ATP and CP stores to resting levels.
Provide energy for increased ventilation and elevated heart rate.
Slow component: This component may take hours or up to one day. It utilizes between 5-14
liters of oxygen. The main functions are:
 Convert lactic acid to pyruvic acid and glycogen.
 Replenish glycogen stores.
 Repair tissue damage.
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