Energy Systems

1. What are energy systems?

The body needs energy to:

• Move.

• Contract muscles.

• Maintain body temperature.

• Keep organs working.

• Perform exercise.

The immediate source of energy used by muscles is called ATP.

ATP stands for:

Adenosine triphosphate

The body has three main energy systems that can resynthesise ATP:

• ATP-PC system.

• Anaerobic glycolytic system.

• Aerobic system.

All three systems work together during exercise.

However, the contribution from each system changes depending on:

• Intensity.

• Duration.

• Type of exercise.

• Fitness level.

2. ATP

ATP is the body's immediate usable energy source.

It is found in all cells.

Muscle contraction requires ATP.

The ATP molecule contains:

• Adenine.

• Ribose.

• Three phosphate groups.

The bonds between the phosphate groups contain energy.

When ATP is broken down:

ATP → ADP + Pi + energy

Where:

ATP = adenosine triphosphate

ADP = adenosine diphosphate

Pi = inorganic phosphate

Energy is released when the final phosphate bond is broken.

This energy can be used by the muscles.

3. Why does ATP need to be resynthesised?

The muscles only store a very small amount of ATP.

The stored ATP would only provide enough energy for a few seconds of very intense activity.

Therefore the body has to continually remake ATP.

This process is called:

ATP resynthesis

The three energy systems resynthesise ATP in different ways.

4. The three energy systems

ATP-PC system

Also called:

• Phosphocreatine system.

• Phosphagen system.

• ATP-CP system.

Main features:

• Anaerobic.

• Very fast ATP production.

• Uses stored ATP and phosphocreatine.

• Does not require oxygen.

• Produces no lactate.

• Used mainly for very short, explosive activity.

Examples:

• 100 m sprint start.

• Jumping.

• Throwing.

• Weightlifting.

• Short swimming sprint.

Anaerobic glycolytic system

Also called:

• Lactate system.

• Anaerobic glycolysis.

Main features:

• Anaerobic.

• Breaks down glucose/glycogen.

• Produces ATP quickly.

• Does not require oxygen directly.

• Lactate and associated hydrogen ions are produced.

• Important during high-intensity activity lasting longer than the ATP-PC system can support.

Examples:

• 400 m sprint.

• Repeated high-intensity efforts.

• Hard swimming intervals.

• Some team-sport efforts.

Aerobic system

Main features:

• Uses oxygen.

• Produces ATP more slowly.

• Can produce large amounts of ATP.

• Uses carbohydrates and fats.

• Important during longer-duration activity.

• Produces carbon dioxide and water as end products.

Examples:

• Distance running.

• Long-distance swimming.

• Cycling.

• Longer horse-riding sessions.

5. The most important thing to understand

The energy systems do not work like three switches.

It is not:

ATP-PC ON → ATP-PC OFF → lactate ON → lactate OFF → aerobic ON

Instead:

All three systems contribute all the time.

The main difference is which system is contributing the most.

For example:

A 5-second sprint:

ATP-PC = greatest contribution

A 30–60 second maximal effort:

Anaerobic glycolytic contribution becomes much more important

A 30-minute run:

Aerobic system = greatest contribution

But the other systems still contribute.

6. ATP-PC system

The ATP-PC system is the fastest way of resynthesising ATP.

It is particularly important during very short, explosive activities.

PC stands for:

Phosphocreatine

It is stored in skeletal muscles.

7. How the ATP-PC system works

When exercise begins:

1. ATP is broken down.

2. ATP becomes ADP + Pi.

3. Energy is released.

4. ATP stores begin to decrease.

5. Phosphocreatine is broken down.

6. The phosphate from phosphocreatine is transferred to ADP.

7. ADP becomes ATP again.

8. The newly produced ATP can be used for muscle contraction.

The basic reaction is:

PC + ADP → ATP + creatine

The enzyme involved is:

Creatine kinase

8. Why the ATP-PC system is so fast

The ATP-PC system is fast because:

• It requires very few chemical steps.

• It does not depend directly on oxygen.

• The reactants are already stored in the muscles.

• ATP can be rapidly resynthesised.

This makes it ideal for explosive activity.

9. ATP-PC system and sport

The ATP-PC system is important for activities lasting roughly up to around 10 seconds, although the exact contribution depends on intensity and the individual.

Examples include:

• Sprint starts.

• Long jump take-off.

• High jump.

• Shot put.

• Weightlifting.

• Short swimming sprint.

• Explosive jump.

10. Example – jumping

Imagine an athlete performs a vertical jump.

The movement is:

• Very short.

• Very intense.

• Explosive.

The muscles need ATP immediately.

The ATP-PC system provides ATP very quickly.

Therefore:

Stored ATP + phosphocreatine → rapid ATP resynthesis → explosive muscle contraction → jump

11. Example – sprint start

A sprinter pushes out of the starting blocks.

The movement requires:

• Very high force.

• Very high power.

• Immediate energy.

The ATP-PC system contributes significantly.

PC is broken down to help rapidly resynthesise ATP.

This allows the muscles to contract powerfully.

12. Advantages of the ATP-PC system

Advantages:

• Very rapid ATP production.

• Does not require oxygen.

• Produces ATP without producing lactate.

• Provides energy for explosive movements.

This is useful when:

• Maximum power is needed.

• There is very little time to produce energy.

• Exercise is extremely intense.

13. Limitations of the ATP-PC system

The main limitation is:

Very limited stores of ATP and phosphocreatine.

Therefore:

• It cannot provide energy at a high rate for long periods.

• PC stores become depleted.

• Other energy systems become increasingly important.

14. Recovery of phosphocreatine

After intense exercise, phosphocreatine stores need to be restored.

This happens mainly using energy supplied by the aerobic system.

Phosphocreatine can be substantially restored during the first few minutes of recovery, although the exact recovery time varies.

This is one reason rest periods are important during repeated sprint or power training.

15. Anaerobic glycolytic system

The anaerobic glycolytic system is also called:

The lactate system

or:

Anaerobic glycolysis

It is important during high-intensity exercise when ATP is needed quickly.

16. What does glycolysis mean?

Glycolysis means the breakdown of glucose.

The glucose can come from:

• Blood glucose.

• Muscle glycogen.

Glycogen is the stored form of carbohydrate.

The process breaks glucose down to produce ATP.

17. How anaerobic glycolysis works

The basic process is:

Glucose/glycogen

↓

Glycolysis

↓

Pyruvate

↓

When oxygen availability and exercise intensity mean pyruvate cannot all be processed aerobically:

Lactate-related metabolism

↓

ATP is produced quickly

The process can provide ATP much faster than aerobic metabolism.

18. Why is it called anaerobic?

Anaerobic = without oxygen

The anaerobic glycolytic pathway does not require oxygen to directly break down glucose during glycolysis.

This makes it useful when:

• Exercise is very intense.

• ATP demand is high.

• Oxygen delivery cannot meet the immediate energy demand.

19. Lactate

During high-intensity exercise, lactate production increases.

It is important to understand that lactate itself is not simply a waste product that causes muscle soreness.

Lactate can be transported and used as a fuel.

Hydrogen ion accumulation and changes in muscle conditions are more closely associated with the burning sensation and fatigue experienced during very intense exercise.

20. What happens to lactate?

Lactate can:

• Be used by muscles as fuel.

• Be transported in the blood.

• Be taken to other tissues.

• Be converted back into pyruvate.

• Be used in the liver to help reform glucose.

The body therefore does not simply throw lactate away.

21. Lactate and fatigue

During very intense exercise:

• Anaerobic metabolism increases.

• Hydrogen ion concentration can increase.

• Muscle acidity increases.

• Muscle contraction can become more difficult.

• Fatigue can develop.

This can contribute to the athlete being unable to maintain maximum intensity.

22. Anaerobic glycolysis and sport

This system is important for activities where:

• Intensity is high.

• Duration is longer than a few seconds.

• ATP is required quickly.

Examples:

• 200–400 m running.

• High-intensity swimming.

• Repeated sprinting.

• High-intensity team-sport efforts.

• Combat sports.

• Hard intervals.

23. Example – 400 m sprint

A 400 m runner needs to maintain a very high intensity.

The ATP-PC system contributes strongly at the start.

However, the event lasts too long for ATP-PC alone.

Anaerobic glycolysis becomes increasingly important.

The aerobic system also contributes.

Therefore:

ATP-PC + anaerobic glycolysis + aerobic system

all contribute.

The anaerobic glycolytic contribution is particularly important because the intensity is very high.

24. Advantages of anaerobic glycolysis

Advantages:

• Produces ATP quickly.

• Does not require oxygen directly.

• Can provide energy for longer than the ATP-PC system.

• Useful for high-intensity exercise.

25. Limitations of anaerobic glycolysis

Limitations:

• ATP production is less rapid than the ATP-PC system.

• High-intensity exercise can lead to fatigue.

• Hydrogen ion accumulation can impair muscle function.

• Carbohydrate stores are limited.

• It cannot sustain maximal intensity indefinitely.

26. Aerobic energy system

The aerobic system produces ATP using oxygen.

It is the dominant energy system during longer-duration exercise.

It can use:

• Carbohydrate.

• Fat.

Protein can also contribute to energy production, particularly when energy availability is low or exercise is prolonged, but carbohydrate and fat are the main fuels during normal exercise.

27. Why is it called aerobic?

Aerobic = with oxygen

The aerobic system requires oxygen.

Oxygen is used in the processes that allow large amounts of ATP to be produced.

This makes it very important for endurance activities.

28. Where does aerobic energy production occur?

Aerobic energy production mainly occurs in the:

Mitochondria

Mitochondria are structures inside cells.

They are often called the:

Powerhouses of the cell

because they are where much of aerobic ATP production takes place.

Muscle cells contain many mitochondria.

29. Carbohydrate and the aerobic system

Carbohydrate is broken down into glucose.

Glucose can then enter glycolysis.

The products can enter the mitochondria and continue through aerobic energy-producing pathways.

The main stages are:

Glycolysis

↓

Link reaction

↓

Krebs cycle

↓

Electron transport chain

↓

ATP

30. Glycolysis

Glycolysis is the first stage of breaking down glucose.

It occurs in the:

Cytoplasm

Glucose is broken down to produce:

Pyruvate

Some ATP is produced during glycolysis.

If oxygen is available and aerobic metabolism can continue, pyruvate can enter the mitochondria.

31. Link reaction

The link reaction connects glycolysis to the Krebs cycle.

Pyruvate is converted into:

Acetyl CoA

Carbon dioxide is also produced.

Acetyl CoA then enters the Krebs cycle.

32. Krebs cycle

The Krebs cycle takes place in the:

Mitochondrial matrix

It produces:

• A small amount of ATP.

• Reduced coenzymes such as NADH and FADH₂.

• Carbon dioxide.

The reduced coenzymes carry high-energy electrons to the electron transport chain.

33. Electron transport chain

The electron transport chain takes place on the:

Inner mitochondrial membrane

It uses electrons carried by NADH and FADH₂.

Energy is released through a series of reactions.

This energy is used to produce ATP.

Oxygen is the final electron acceptor.

Water is formed.

This stage produces the largest amount of ATP in aerobic respiration.

34. Aerobic respiration overall

For carbohydrate:

Glucose + oxygen → carbon dioxide + water + ATP

The ATP produced can then be used by working muscles.

35. Aerobic system and fat

Fat can also be used to produce ATP.

Fat is broken down into:

• Fatty acids.

• Glycerol.

Fatty acids can enter pathways that produce acetyl CoA.

They can then contribute to the Krebs cycle and electron transport chain.

Fat provides a large amount of energy.

However, it takes longer to mobilise and use than carbohydrate.

Therefore fat is particularly important during lower-intensity, longer-duration exercise.

36. Carbohydrate vs fat

Carbohydrate

Useful when:

• Exercise intensity is higher.

• ATP is needed relatively quickly.

Fat

Useful when:

• Exercise is lower intensity.

• Exercise lasts a long time.

• Large energy stores are needed.

The body can use both at the same time.

The balance changes depending on exercise intensity and duration.

37. Advantages of the aerobic system

Advantages:

• Produces large amounts of ATP.

• Can continue for a long time.

• Uses carbohydrate and fat.

• Produces ATP efficiently.

• Important for endurance exercise.

38. Limitations of the aerobic system

Limitations:

• ATP production is slower than the anaerobic systems.

• Requires oxygen.

• Cannot meet the immediate ATP demand of very explosive exercise on its own.

Therefore the aerobic system is less dominant during very short, maximal efforts.

39. Comparing the three energy systems

40. Energy system continuum

A useful way to understand the systems is as a continuum.

Very short and explosive

ATP-PC dominates

Example:

• Shot put.

• Jump.

• Very short sprint.

Short and very intense

Anaerobic glycolytic contribution becomes greater

Example:

• 200–400 m sprint.

• High-intensity interval.

Longer duration

Aerobic contribution becomes dominant

Example:

• Distance running.

• Long swimming session.

• Cycling.

But remember:

All three systems contribute at the same time.

41. Energy systems during a 100 m sprint

A 100 m sprint lasts around 10 seconds for many competitive athletes.

The athlete:

• Needs immediate ATP.

• Works at very high intensity.

• Cannot rely mainly on aerobic energy production.

The ATP-PC system makes a major contribution.

Anaerobic glycolysis also contributes.

The aerobic system contributes a smaller amount.

42. Energy systems during a 400 m sprint

A 400 m sprint lasts much longer than a 100 m sprint.

Therefore:

• ATP-PC contributes at the start.

• Anaerobic glycolysis contributes significantly.

• Aerobic metabolism also contributes.

The relative contribution changes throughout the race.

43. Energy systems during distance running

During a long-distance run:

• The aerobic system is dominant.

• Carbohydrate and fat provide fuel.

• Oxygen is required.

• ATP is produced continuously.

The ATP-PC and anaerobic glycolytic systems still contribute during:

• Starts.

• Hills.

• Sprints.

• Finishing.

44. Energy systems during swimming

Swimming can involve all three systems.

Short sprint

ATP-PC is highly important.

100–400 m event

Anaerobic glycolysis becomes more important.

Long-distance swimming

Aerobic metabolism becomes dominant.

A swimmer may also use anaerobic energy during:

• Starts.

• Turns.

• Finishing sprint.

45. Energy systems during horse riding

Horse riding can involve different energy systems depending on the activity.

During longer periods of riding:

• Aerobic metabolism provides much of the energy.

During short intense efforts:

• Anaerobic systems contribute more.

Examples:

• Long steady ride → aerobic.

• Fast gallop → greater anaerobic contribution.

• Jumping → ATP-PC contributes strongly to the explosive movement.

46. Energy systems during football

Football is an intermittent sport.

This means activity changes repeatedly.

For example:

• Walking → aerobic.

• Jogging → aerobic.

• Sprinting → anaerobic + ATP-PC.

• Jumping → ATP-PC.

• Repeated high-intensity running → anaerobic glycolysis + aerobic contribution.

• Recovery between efforts → aerobic system helps restore energy stores.

Therefore football uses all three energy systems.

47. Recovery and energy systems

The energy systems are also important during recovery.

After exercise:

• ATP stores need to be restored.

• Phosphocreatine needs to be resynthesised.

• Glycogen stores may need replenishing.

• Lactate can be processed.

• Oxygen consumption remains elevated.

The aerobic system is particularly important during recovery.

48. EPOC

After intense exercise, oxygen consumption remains above resting levels.

This is called:

Excess Post-Exercise Oxygen Consumption

or:

EPOC

It helps the body return towards resting conditions.

Oxygen is used to help:

• Resynthesise phosphocreatine.

• Restore ATP.

• Support recovery processes.

• Process lactate.

• Restore oxygen stores.

• Return body systems towards normal.

49. Oxygen deficit

At the beginning of exercise, the aerobic system does not immediately reach the level required to meet the body's full energy demand.

This creates an:

Oxygen deficit

The anaerobic energy systems provide more ATP while aerobic metabolism increases.

As exercise continues:

Aerobic contribution increases

and the oxygen deficit is reduced.

50. Oxygen deficit example

Imagine someone starts running suddenly.

At rest:

• Oxygen demand is low.

They suddenly begin sprinting.

Their muscles immediately need much more ATP.

The cardiovascular and respiratory systems take time to increase oxygen delivery.

Therefore:

ATP-PC + anaerobic glycolysis provide more energy initially

while:

Aerobic energy production increases

This initial gap is the oxygen deficit.

51. Steady state

During moderate-intensity exercise, the body can eventually reach a point where oxygen supply and energy demand are relatively balanced.

This is called:

Steady state

At steady state:

• Oxygen uptake is relatively stable.

• Aerobic metabolism provides most of the required ATP.

• Exercise can be maintained for longer.

52. Energy system adaptations to training

Training can improve the body's ability to produce and use energy.

Different types of training produce different adaptations.

53. ATP-PC adaptations

Power and sprint training can improve:

• Phosphocreatine availability.

• Ability to rapidly resynthesise ATP.

• Neuromuscular power.

• Ability to produce force quickly.

This can improve performance in:

• Sprinting.

• Jumping.

• Throwing.

• Power movements.

54. Anaerobic adaptations

High-intensity interval and anaerobic training can improve:

• Anaerobic capacity.

• Ability to tolerate high-intensity exercise.

• Ability to buffer changes in acidity.

• Ability to clear/use lactate.

• Muscle glycogen availability.

• Enzyme activity involved in glycolysis.

This can improve performance during repeated high-intensity activity.

55. Aerobic adaptations

Regular aerobic training can improve:

• Mitochondrial density.

• Capillarisation.

• Myoglobin.

• Aerobic enzymes.

• Oxygen delivery.

• Oxygen extraction.

• Ability to use fat as a fuel.

• Glycogen storage/use.

• VO₂ max.

These adaptations allow ATP to be produced more efficiently during endurance exercise.

56. Muscle glycogen

Glycogen is the stored form of carbohydrate.

It is stored mainly in:

• Muscles.

• Liver.

Muscle glycogen is particularly important during exercise.

During exercise:

• Glycogen can be broken down.

• Glucose is produced.

• Glucose is used to produce ATP.

57. Glycogen depletion

During prolonged or intense exercise, muscle glycogen stores can become depleted.

This can contribute to fatigue.

This is particularly important in:

• Long-distance running.

• Long cycling events.

• Long swimming sessions.

• Endurance events.

Adequate carbohydrate intake can help maintain glycogen stores.

58. Carbohydrate loading

Carbohydrate loading is a nutritional strategy used before some prolonged endurance events.

The aim is to:

• Increase muscle glycogen stores.

• Provide more available carbohydrate for prolonged exercise.

It may be useful for events lasting long enough for glycogen depletion to become a significant concern.

Examples:

• Marathon.

• Long-distance cycling.

• Long-distance swimming.

59. Energy systems and fatigue

Fatigue can occur when:

• ATP demand is greater than ATP resynthesis.

• Phosphocreatine stores become depleted.

• Glycogen availability decreases.

• Hydrogen ion accumulation affects muscle function.

• Dehydration occurs.

• Body temperature becomes too high.

Different types of exercise create different causes of fatigue.

60. Energy systems and intensity

As exercise intensity increases:

The body relies more heavily on:

• Carbohydrate.

• Anaerobic energy production.

At lower intensities:

• Aerobic metabolism contributes more.

• Fat can provide a greater proportion of energy.

This does not mean one fuel is completely switched off.

The body uses a mixture of fuels and energy systems.

61. Energy systems and duration

Very short duration

Greater ATP-PC contribution.

Short/high-intensity

Greater anaerobic glycolytic contribution.

Longer duration

Greater aerobic contribution.

However:

Duration and intensity interact.

A short exercise performed maximally can have a very different energy demand from a longer, low-intensity exercise.

62. Diabetes and energy metabolism

Diabetes affects how the body regulates blood glucose.

Insulin is an important hormone involved in regulating blood glucose.

People with diabetes need appropriate management of:

• Blood glucose.

• Food intake.

• Medication/insulin where prescribed.

• Exercise.

Exercise can affect blood glucose levels.

63. Hypoglycaemia

Hypoglycaemia means blood glucose is too low.

It can cause:

• Shaking.

• Sweating.

• Dizziness.

• Weakness.

• Confusion.

• Reduced performance.

For people at risk, appropriate management before, during and after exercise is important.

64. Children and the energy systems

Children have developing physiological systems.

Their responses to exercise can differ from adults.

In general:

• Children have a lower anaerobic capacity than adults.

• They may rely more on aerobic metabolism.

• Their ability to produce and tolerate high levels of anaerobic metabolism develops with maturation.

Training should therefore be appropriate to:

• Age.

• Development.

• Experience.

• Ability.

65. Energy systems and the muscular system

The energy systems provide ATP.

The muscular system uses ATP.

Therefore:

Energy systems → ATP → muscle contraction → movement

Without ATP:

• Cross-bridge cycling cannot continue normally.

• Muscles cannot continue producing force.

66. Energy systems and the cardiovascular system

The cardiovascular system transports oxygen.

This is particularly important for the aerobic system.

The process is:

Lungs

↓

Oxygen enters blood

↓

Heart

↓

Pumps oxygenated blood

↓

Blood vessels

↓

Transport oxygen to muscles

↓

Muscle cells

↓

Oxygen used for aerobic ATP production

67. Energy systems and the respiratory system

The respiratory system:

• Brings oxygen into the body.

• Removes carbon dioxide.

The aerobic energy system:

• Uses oxygen.

• Produces carbon dioxide.

Therefore the two systems work together.

68. Complete energy system chain

Exercise begins

↓

ATP is used

↓

ATP → ADP + Pi

↓

ATP must be resynthesised

↓

ATP-PC / anaerobic glycolytic / aerobic systems contribute

↓

ATP is produced

↓

Muscles use ATP

↓

Muscle contraction

↓

Movement

↓

Sporting performance

69. Quick comparison

ATP-PC

Very fast

Very short

Very high intensity

No oxygen

Phosphocreatine

No lactate production

Power/sprint

Anaerobic glycolytic

Fast

Short/high intensity

No oxygen directly

Glucose/glycogen

Lactate production increases

400 m/high-intensity activity

Aerobic

Slower ATP production

Long duration

Oxygen required

Carbohydrate + fat

Large ATP production

Endurance

70. Easy memory trick

Think:

ATP-PC

POWER

Very short and explosive.

Anaerobic glycolytic

PRESSURE

Hard, high-intensity activity that can only be maintained for a relatively short period.

Aerobic

ENDURANCE

Longer-lasting activity.

71. Exam question – compare the energy systems

Compare the three energy systems used during exercise.

A good answer:

• The ATP-PC system provides ATP very rapidly.

• It uses stored ATP and phosphocreatine.

• It does not require oxygen.

• It is important for very short explosive activities.

• The anaerobic glycolytic system breaks down glucose or glycogen to produce ATP quickly.

• It does not require oxygen directly.

• It becomes important during high-intensity activity lasting longer than the ATP-PC system can support.

• Lactate production increases during high-intensity anaerobic exercise.

• The aerobic system requires oxygen.

• It produces ATP more slowly but can continue for much longer.

• It uses carbohydrate and fat.

• It is the dominant system during longer-duration endurance exercise.

• All three systems contribute during exercise, but the relative contribution changes depending on intensity and duration.

72. Exam question – 100 m sprint

Explain which energy system is most important during a 100 m sprint.

A good answer:

• A 100 m sprint is a very high-intensity activity.

• The muscles need ATP immediately.

• The ATP-PC system can resynthesise ATP very rapidly.

• It uses stored phosphocreatine in the muscles.

• It does not require oxygen.

• Therefore it makes a major contribution during the sprint.

• Anaerobic glycolysis also contributes as the sprint continues.

• The aerobic system contributes a smaller amount.

• The three systems work together rather than switching on and off separately.

73. Exam question – long-distance swimming

Explain why the aerobic energy system is important for long-distance swimming.

A good answer:

• Long-distance swimming requires energy for a prolonged period.

• The aerobic system can produce large amounts of ATP.

• It uses oxygen.

• Carbohydrate and fat can be used as fuels.

• Oxygen is delivered to the working muscles by the cardiovascular system.

• The respiratory system supplies the oxygen.

• The aerobic system allows the muscles to continue producing ATP for a long time.

• The ATP is used for muscle contraction and swimming movement.

• Anaerobic systems still contribute during starts, turns and finishing efforts.

74. Exam question – ATP

Explain why ATP is important for muscle contraction.

A good answer:

• ATP is the immediate usable energy source for muscle cells.

• During contraction, ATP is broken down into ADP and inorganic phosphate.

• Energy is released.

• This energy allows the myosin heads to work during cross-bridge cycling.

• ATP is also required to detach and reset myosin heads.

• Only a small amount of ATP is stored in the muscles.

• Therefore ATP must be continually resynthesised using the three energy systems.

75. Exam question – energy systems working together

Explain why all three energy systems are used during a game of football.

A good answer:

• Football involves repeated changes in intensity.

• During walking and jogging, the aerobic system provides most of the energy.

• During short explosive sprints, the ATP-PC system makes a large contribution.

• During repeated high-intensity efforts, anaerobic glycolysis contributes more.

• The aerobic system also helps during recovery between intense efforts.

• Therefore all three systems are used throughout a match.

• Their relative contribution changes according to the intensity and duration of each activity.

76. Important things to remember

ATP

ATP = immediate energy

ATP resynthesis

Energy systems remake ATP

ATP-PC

Fastest

Power

Anaerobic glycolytic

Fast

High intensity

Glucose/glycogen

Aerobic

Oxygen

Long duration

Carbohydrate + fat

Important rule

All three systems contribute all the time.

The dominant system changes depending on the exercise.

77. Key terms

ATP
Adenosine triphosphate. The immediate usable energy source for cells.

ATP resynthesis
The process of producing ATP again after it has been broken down.

ADP
Adenosine diphosphate.

Phosphocreatine (PC)
A high-energy compound stored in muscle that helps rapidly resynthesise ATP.

ATP-PC system
Anaerobic system that rapidly resynthesises ATP using phosphocreatine.

Anaerobic
Without oxygen.

Aerobic
With oxygen.

Glycolysis
Breakdown of glucose.

Glycogen
Stored form of carbohydrate.

Lactate
A substance produced during anaerobic metabolism that can be transported and used as a fuel.

Mitochondria
Cell structures where much aerobic ATP production occurs.

Krebs cycle
A stage of aerobic energy production.

Electron transport chain
Stage of aerobic respiration where most ATP is produced.

Oxygen deficit
The gap between the oxygen required at the start of exercise and the oxygen actually being taken up.

EPOC
Excess Post-Exercise Oxygen Consumption after exercise.

Steady state
A state where oxygen uptake and energy demand are relatively stable during moderate exercise.

VO₂ max
Maximum rate of oxygen uptake and use.

78. Final exam checklist

Make sure you can:

• Define ATP.

• Explain why ATP is needed.

• Explain ATP breakdown.

• Explain ATP resynthesis.

• Name all three energy systems.

• Explain the ATP-PC system.

• Explain phosphocreatine.

• Explain creatine kinase.

• Give sporting examples of ATP-PC use.

• Explain anaerobic glycolysis.

• Explain glycolysis.

• Explain glucose and glycogen.

• Explain lactate production.

• Explain what happens to lactate.

• Give sporting examples of anaerobic glycolysis.

• Explain the aerobic system.

• Explain the role of oxygen.

• Explain mitochondria.

• Explain glycolysis.

• Explain the link reaction.

• Explain the Krebs cycle.

• Explain the electron transport chain.

• Explain how carbohydrate is used aerobically.

• Explain how fat is used aerobically.

• Compare carbohydrate and fat.

• Compare all three energy systems.

• Understand intensity and duration.

• Understand that all three systems work together.

• Explain the energy systems during swimming.

• Explain the energy systems during horse riding.

• Explain the energy systems during running.

• Explain the energy systems during football.

• Explain oxygen deficit.

• Explain EPOC.

• Explain steady state.

• Explain energy system adaptations.

• Understand muscle glycogen.

• Understand glycogen depletion.

• Understand carbohydrate loading.

• Understand fatigue.

• Understand basic links with diabetes and hypoglycaemia.

• Understand differences in children and adults.

79. One-page energy system summary

ATP-PC

Fuel: ATP + phosphocreatine

Oxygen: No

Speed: Very fast

Duration: Very short

Main activity: Explosive power

Examples: Jumping, throwing, lifting, sprint start

Anaerobic glycolytic

Fuel: Glucose/glycogen

Oxygen: No direct requirement

Speed: Fast

Duration: Short/high intensity

Main activity: High-intensity exercise

Examples: 200–400 m sprint, hard intervals

Aerobic

Fuel: Carbohydrate + fat

Oxygen: Yes

Speed: Slower

Duration: Long

Main activity: Endurance

Examples: Distance running, long swimming, cycling

80. The biggest picture

The entire process can be remembered as:

Exercise

↓

Muscles need energy

↓

ATP is broken down

↓

ATP → ADP + Pi

↓

ATP stores need replacing

↓

ATP-PC + anaerobic glycolytic + aerobic systems resynthesise ATP

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ATP powers muscle contraction

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Muscles produce movement

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Movement produces sporting performance

And the systems work together:

Respiratory system

→ supplies oxygen

↓

Cardiovascular system

→ transports oxygen

↓

Energy systems

→ use oxygen/nutrients to produce ATP

↓

Muscular system

→ uses ATP

↓

Skeletal system

→ allows movement

↓

Sporting performance