Energy systems


Energy transfer in the body

In the body, the energy used from muscle contractions comes from adenosine triphosphate (ATP)

Adenosine triphosphate (ATP)- the only usable form of energy in the body

The energy we derive from foods, such a carbohydrates, is broken down to release energy used to from ATP. 

ATP consists of one molecule of adenosine and three (tri) phosphates

The energy stored in ATP is released by breaking down the bonds that hold this compound together. 

Enzymes are used to break down compounds, and the ATP-ase is the ensyme used to break down ATP, leaving adenosine di-phosphate (ADP) and an inorganic phosphate (Pi).

ATP-ase breaks down ATP to produce ADP + Pi + energy

The body constantly rebuilds ATP by converting the ADP and Pi back into ATP.

The conversion of fuels into energy takes place through: aerobic, ATP-PC, anaerobic glycogic.

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Aerobic system

The aerobic energy system uses oxygen in energy production. 

Advantage- yields large numbers of ATP molecules compared to either anaerobic energy systems. This makes it ideal to provide energy for endurance activities.

Disadvantage- releases large quantities of energy involves more chemical reactions. This makes the system slower, and unsuitable for anaerobic activity as it cannot produce enough energy that is required for intense activity. 

Examples- long distance cycling, running/jogging, swimming.

Stored fats and carbohydrates are used as a fuel source for this energy system. They're broken down into glycogen, glucose and fatty acids. There are three stages with this system:

1. Glycolysis- breakdown of glucose into pyruvic acid. Indentical to anaerobic glycolysis. However due to the prescene of oxygen, pyruvate is broken down later rather then forming lactate. Two ATP molecules are produced. 

2. Kreb cycle- takes place in the mitochondria. The pyruvate forms Acetyl-CoA, which is broken down using oxygen to form carbon dioxide and hydrogen. Two ATP molecules are released.

3. Electron transport chain- hydrogen from kreb cycle combines with oxygen to form H20 as a waste product and 34 molecules of ATP.

Recovery time-  few hours/ 2 to 3 days depending on intensity or duration of exercise e.g. marathon

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The ATP-PC system

This energy system is the first one recruited for exercise and it is the dominant source of muscle energy for high intensity explosive exercise that lasts for 10 seconds or less. It can provide energy immediately, it does not require any oxygen and it does not produce any lactic acid.

ATP is the chemical form of energy that are used from muscle contractions. There is sufficient ATP in the muscles for approximately 2-3 seconds of work, after this ATP needs resynthesising. In the ATP-PC system the energy required to resynthesise ATP is provided by phosphocreatine (PC)

PC is made up of molecules of phosphate and creatine. There is enough PC in the muscle cell to continue to resynthesise 

Creatine supplementation is a method used to extend the duration of effectiveness of the alactic anaerobic energy system. 

Advantages- energy is released quickly and no waste products are formed (lactic acid)

Disadvantages- limited stores of PC and takes 2-3 minutes to recover these stores. This means there is insufficient recovery time during play in many sporting activities to recover PC stores once they've been stored.

Examples- 100m sprint, weightlifting

Recovery time- Once the supply of PC has been broken down to resynthesis ATP, energy is needed from other energy systems to resynthesis the PC stores. This energy is provided from the aerobic system. 

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The lactate system

Dominant source of muscle energy for high intensity exercise activities that last up to approximately 90 seconds. Essentially, this system is dominant when your alactic anaerobic energy system is depleted but you continue to exercise at an intensity that is too demanding for your aerobic energy system to handle. This system is also anaerobic and so it does not require any oxygen. 

Anaerobic glycolysis

  • Two ATP molecules are used to provide energy for the breakdown of glycogen and glucose (from carbohydrate)
  • Pyruvate is formed 
  • Without oxygen, pyruvate is converted into lactate (lactic acid)
  • Four ATP are produced, giving a net gain of two ATP molecules for energy for high-intensity exercise. 

Advantages- produces energy quickly, good for short-duration high-intensity activities. 1-2 minutes.

Disadvantages- produces waste products

Example- 400 m sprint or uphill climb in a cycling race

Recovery time- Lactate will accumulate unless there is oxygen available to break it down. This accumilation will change the acidity of the blood, causing muscle fatigue and decreasing the efficiency of muscle contractions.Takes 8 minutes to recover.

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Energy used in exercise

Each energy system is used to generate energy for physical activity. The amount used will depend on the intensity and duration of the activity.

For example, in a game of football, the aerobic system provides energy for the majority of the match.

However, when a short sprint, a powerful kick or explosive jump is required, one of the anaerobic systems will be used.

During a quiet spell in the game where intensity is low the anaerobic systems can be partially recovered from the aerobic system.

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ATP generation of fast and slow twitch fibres

Slow twitch (type I)

  • The main pathway for ATP production is in the aerobic system
  • It produces the maximum amount of ATP available from each glucose molecule
  • Production is slow but these fibres are more endurance based, so less likely to fatigue

Fast twitch (type II) 

  • The main pathway for ATP production is via the lactate anaerobic system (during glycolysis)
  • ATP production in the absence of oxygen is not efficient- only 2 ATP molecules produced per glucose molecule. 
  • Production of ATP this way is fast but cannot last for long as these fibres have least resistance to muscle fatigue.
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Oxygen consumption during exercise

When we exercise, the body uses oxygen to produce energy (re-synthesise ATP). 

Oxygen consumption is the amount of oxygen we use to produce ATP and is usually referred to as VO2. 

When we start to exercise, insufficient oxygen is distributed to the tissues for the energy to be provided aerobically as it takes the body time to respond to exercise (increase demand of oxygen). 

As a result, energy is provided anaerobically to satisfy the increase in demand for energy until the body can cope. This is referred to as submaximal oxygen deficit.

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EPOC- the amount of oxygen consumed during recovery above that which would have been consumed at rest during the same time.

Recovery involves returning the body to its pre-exercise state. When a perfromer finishes exercise, oxygen consumption remains high in comparison to oxygen consumption at rest. This extra oxyge is needed to be taken in and help the performer recover. This breathlessness is EPOC.

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Fast replenishment stage of EPOC

Fast replenishment stage

  • Uses extra oxygen that is taken in during recovery to restore ATP and PC and re-saturate myoglobin. 
  • Complete restoration takes 3 minutes, with 50% completed after 30 seconds, 
  • Myoglobin has a high affinity for oxygen. After exercise, stores of oxygen are limited. The surplus oxygen from EPOC helps replenish these stores, taking up to 2 minutes.


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Slow replenishment stage of EPOC

Removal lactic acid

  • Cool-down- exercise keeps the metabolic rate of muscles high and keeps capillaries dilated, meaning oxygen can flush through and remove lactic acid. 
  • oxidation into carbon dioxide and water in the inactive muscles and organs and used by the muscle as an energy source
  • Cori cycle- lactic acid is transported by the blood to the liver where it is converted to blood glucose and glycogen
  • Sweat and urine
  • converted into protein

Maintenance of breathing and heart rates

  • Requires extra oxygen to provide the energy needed for the respiratory and heart muscles. This assits the removal of lactic acid and returning the body its pre-exercise state

Increase in body temperature

  • When temperature is high, respiratory rate remains high and will help the perfromer take in more oxygen during recovery. 
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Energy transfer during short/high intensity exerci

During short-duration/high-intensity exercise, energy needs to be produced rapidly. The aerobic system is too complicated to be used quickly, so the body relies on the ATP-PC system and anaerobic gylcolytic system. However, these systems cannot produce energy for long periods of time.

Lacatate accumulation

  • Lactate and lactic acid are not the same thing, but the terms are often used interchangeably. 
  • Using the glycolytic system produces the by-product lactic acid as a result of glycolysis.
  • The higher the intensity, the more lactic acid
  • The lactic acid breaks down, releasing hydrogen ions, with the remaining compound then combines with sodium or potassium ions to form lactate
  • As lactate accumulates in the muscles, more hydrogen ions are present which increases acidity
  • This causes muscle fatigue
  • The lactate produced in the muscles diffuses into the blood and blood lactate can be measured.
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Lactate threshold and onset of OBLA

As exercise intensity increases, the body moves from working aerobically to anaerobically. This crossing from aerobic to anaerobic is known as the Lactate threshold- the point at which lactic acid rapidly accumilates in the blood.

The body is unable to produce enough oxygen to break down lacatate when exercising at high intensity, which leads to OBLA- the point when lactate levels go above 4 millomoles per litre. 

Measuring OBLA gives an indication of endurance capacity. Some performers can work at higher intensities than others before OBLA and can delay when the threshold occurs. 

Lactate threshold is expressed as a percentage of VO2 max- the maximum amount of oxygen that can be utilised by the muscles per minute. 

As fitness increases, lactate threshold becomes delayed. Average performers may have a lactate threshold that is 50-60% of their VO2 max, whereas elite performers may have a lactate threshold that is 70-90% of their VO2 max

Training has a limited effect on our VO2 max as it is genetically determined. Lactate threshold plays a bigger difference- the fitter we are, the higher the lacate threshold as a % of our VO2 max and hence the harder we can work.

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Factors affecting the rate of lactate accumulation

  • Exercise intensity- the higher the intensity, the greater demand for oxygen (ATP) and the faster OBLA occurs equalling the formation of lactic acid.
  • Muscle fibre type- slow twitch produce less lactate than fast twitch fibres. When slow twitch use glycogen as fuel, due to the presence of oxygen, the glycogen can be broken down more effectively and with little lactate produced.
  • Rate of blood lacate removal-  if the rate of lactate removal is lower than the rate of lacate production, lacate will start to accumulate in the blood until OBLA is reached.
  • The respiratory exchange ratio-  when the ratio has a value close to 1.0 glycogen becomes the preferred fuel and there is a greater chance of the accumulation of lacate. 
  • Fitness of the performer- a person who trains regularly will be in a better position to delay OBLA, as adaptations occur to trained muscles. 
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Lactate-producing capacity in power performers

Elite sprinters and power athletes will have a better anaerobic endurance that non-elite sprinters. This is because their body has adapted to cope with higher levels of lactate. 

Through a process called buffering, they have an increase volume of lactate removal and consequently have a lower lactate level. 

Buffering works like a sponge mopping up the lactate. 

This means elite performers will be able to work at higher intensities for longer before fatigue sets in. 

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Factors affecting VO2 max

The higher the V02 max, the greater the endurance capacity of a perfromer. This enables a performer to work at higher intensities for longer as they can utilise this oxygen in the muscles more effectively and therefore delay OBLA.


  • Increased maximum cardiac output
  • graeter heart rate range
  • incraesed lactate tolerance 
  • less oxygen being used at the heart so more availble for the muscles

Lifestyle- smoking, sedentary lifestyle, poor fitness can reduce VO2 max values

Body composition- higher % of body fat decreases VO2 max 

Gender- males have 20% higher VO2 max than women

Differences in age- as we get older, our VO2 max declines as our body systems become less efficient.

Genetics- VO2 max is genetically determined which limits impact of training/equipment

Training- VO2 max can be improved by upto 10-20% following a period o aerobic training ( continous, fartlek)

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Measurements of energy expenditure

  • Indirect calorimetry-  provides an accurate estimate of energy expenditure through gas exchange. It measures how much carbon dioxide is produced and how much oxygen is consumed at both rest and aerobic exercise. Calculating the gas volumes also enables us to find the main substrate being used (fat or carbs). Very reliable test and gives precise calculation of VO2 max.
  • Lactate sampling-  accurate measure of the level of lactate in the blood. Can also be used as a means of measuring exercise intensity (the higher the exercise intensity at which the lactate threshold occurs, the fitter the athlete is). Allows performer to select reletative training zones. Regular lactate testing allows the performer/coach to see whether improvements have occured. If the results show lower lactate levels at the same intensity, this shows has an increase peak in speed/power, time of exhausion, improved recovery heart rate and higher lactate threshold.
  • VO2 max test- multi-stage fitness test/ bleep test. Individual performs 20-metre shuttle runs until they reach exhausion. The level reached can be compared to a table. A sports science lab can produce more valid results using direct gas analysis- concentration of oxygen inspired and concentration of carbon dioxide expired. Tests using this method involve increasing intensities on a treadmill, cycle ergometer or rowing machine.
  • Respiratory exchange ratio (RER)- ratio of carbon dioxide produced compared to oxygen consumed and used as a measure of exercise intensity. It calculates energy expenditure and provides information about the use of fats and carbohydrates during exercise. 
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Impact of specialist training methods on energy sy

  • Altitude training - done over 2500 metres above sea level, where the partial pressure of oxygen is lower- meaning that not as much oxygen can be diffused into the blood, so haemyglobin is not fully staturated with oxygen. This results in lower oxygen carrying capacity of the blood, meaning less can be delivered to the working muscles. Thus, a reductionin aerobic performance and VO2 max and a quicker onset of anaerobic respiration.

Advantages- Increase in the number of red blood cells, concentration of haemyglobin, blood viscosity, capillarisation, lactate tolerance, enhanced oxygen transport

Disadvantages- Expensive, altitude sickness, detraining as the training intensity has to be reduced the first time the perfromer experinces altitude training, benefits can soon be lost when returning to sea level, away from home

  • High-intensity interval training- used for both aerobic and anaerobic training. Periods of short maximum intensity  interspersed with recovery periods. 4 minutes of intense exercise made up of 8 x 20 seconds maximum effort work intervals, each followeed by 10 seconds of rest. Work= anaerobic Rest = aerobic. Pushing your body to the max during maximum work increases the amount of calories burnt, improving fat buring, glucose metabolism and endurance.
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Impact of specialist training methods on energy sy

  • Plyometrics- repeated rapid stretching and contracting of muscles to increase muscle power. Involves high-intensity explosive activities such as hopping, bounding, depth jumping using fast twitch fibres. Works on the concept of muscles can generate more force after theyve been strecthed. Consists of three phases:

Phase 1- eccentric phase, on landing the muscles performs a eccentric contraction, where it lengthens under tension

Phase 2- amortisation phase, time between eccentric and concentric contractions. Needs to be short so the energy stored from the eccentric contraction is not lost. 

Phase 3- concentric or muscle contraction phase which uses the stored energy to increase the force of the contraction. 

  • Speed, agility, quickness (SAQ)- aims to improve multi-directional movemnt through developing the neuromuscular system. Drills include zig-zag runs and foot ladders, and often a ball is included so passing occurs throughout, mkaing the drill are more sport specific. Energy is provided anaerobically as maximum force is used at high speeds. 
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