Syllabus coverage
- Outline the aerobic respiration equation: glucose + oxygen → carbon dioxide + water
- Outline the anaerobic respiration equation: glucose → lactic acid
- Link duration and intensity to the use of aerobic and anaerobic respiration: longer, low-intensity activities require aerobic; shorter, intense activities require anaerobic; with examples of aerobic and anaerobic energy demands in physical activities
- Recovery after exercise, with reference to Excess Post-exercise Oxygen Consumption (EPOC), also known as oxygen debt — caused by anaerobic exercise producing lactic acid and requiring a high breathing rate after exercise to remove it — and factors affecting recovery time
- Short-term effects of exercise: heart rate increases, breathing rate increases, red skin / heat control / sweating, fatigue (feeling tired), nausea / feeling light-headed
- Long-term effects of exercise on heart size (hypertrophy), resting pulse rate (bradycardia), stroke volume, and the ability to tolerate lactic acid
Part of the IGCSE PE 0413 syllabus 0413 / 0995.
Two ways to release energy: aerobic and anaerobic
Energy is released from glucose in the muscles in a process called respiration, and Cambridge wants two equations for it. Learn them in words, exactly as the syllabus writes them — a symbol equation or a half-remembered version loses the mark.
The difference is oxygen. Aerobic means with oxygen; anaerobic means without oxygen, and it leaves a waste product behind.
| Aerobic respiration | Anaerobic respiration | |
|---|---|---|
| Equation | glucose + oxygen → carbon dioxide + water | glucose → lactic acid |
| Oxygen needed? | Yes | No |
| How long it can supply energy | As long as oxygen and glucose keep arriving — so it can carry a performer through a long race | Only a short time at high intensity, because lactic acid builds up in the muscles |
| Amount of energy released | A lot of energy, released steadily | A smaller amount of energy, but released very quickly |
| Waste products | Carbon dioxide (breathed out) and water | Lactic acid, which is removed after exercise |
Duration and intensity decide which system you use
Intensity is how hard the performer is working; duration is how long they work for. Together they decide whether the muscles can be supplied with enough oxygen to keep working aerobically, or whether they run short and have to switch to anaerobic.
The rule to learn: longer, low-intensity activities require aerobic respiration; shorter, intense activities require anaerobic respiration. Very few sports are purely one or the other — most are aerobic overall with sharp anaerobic bursts.
| Activity | Intensity | Duration | System used |
|---|---|---|---|
| Early laps of a cross-country race | Low | Long | Aerobic |
| A long-distance competitive swim | Moderate but sustained | Long | Aerobic, with anaerobic bursts for turns and the finish |
| 100 m sprint | Very high | Seconds | Anaerobic |
| One set of heavy lifts in weight training | Very high | Seconds | Anaerobic |
| A whole game of association football | Mixed | Long, with short sprints | Aerobic overall, anaerobic for sprints, jumps and tackles |
Recovery and EPOC (oxygen debt)
You do not stop breathing hard the moment you stop running. That extra breathing is the body repaying an oxygen debt, and it has a syllabus name: EPOC, which stands for Excess Post-exercise Oxygen Consumption. Its older name is oxygen debt.
EPOC is caused by anaerobic exercise producing lactic acid. The lactic acid has to be removed, and removing it requires oxygen — so the breathing rate stays high after exercise to take that extra oxygen in and clear the lactic acid. The heart rate stays raised too, to carry the oxygen to where it is needed.
- Caused by: anaerobic exercise producing lactic acid.
- Response: a high breathing rate after exercise to take in the oxygen needed to remove the lactic acid.
- The harder and longer the anaerobic effort, the more lactic acid is produced and the longer recovery takes.
| Factor affecting recovery time | Effect on recovery |
|---|---|
| Intensity of the exercise | A higher intensity produces more lactic acid, so more oxygen is needed to remove it and recovery takes longer |
| Duration of the exercise | The longer the anaerobic effort went on, the more lactic acid has built up, so recovery takes longer |
| Level of fitness | A trained performer tolerates lactic acid better and has a stronger heart and lungs, so they recover faster |
| Age | Younger performers generally recover more quickly than older performers |
| Diet and hydration | A performer who is well hydrated and eats sensibly recovers faster than one who is dehydrated |
| Warm-up and cool-down | A proper cool-down keeps the circulation going and helps remove lactic acid, shortening recovery time |
Short-term effects of exercise
Short-term effects are what happens during and just after one bout of exercise. Cambridge lists five, and they are worth easy marks as long as you say what is happening and why.
| Short-term effect | What is happening and why |
|---|---|
| Heart rate increases | The heart beats faster to deliver more oxygenated blood to the working muscles and to remove carbon dioxide more quickly |
| Breathing rate increases | Breathing faster and deeper takes in more oxygen and removes more carbon dioxide, supplying aerobic respiration |
| Red skin / heat control / sweating | Working muscles produce heat, more blood flows near the skin's surface and sweat glands produce sweat, which cools the body as it evaporates |
| Fatigue (feeling tired) | The muscles run short of energy and lactic acid builds up, so performance drops and the performer feels tired |
| Nausea / feeling light-headed | Blood is redirected from the digestive system to the working muscles, and body temperature rises, which can cause nausea or light-headedness |
Long-term effects of exercise
Long-term effects are the adaptations that appear after weeks and months of regular training. Cambridge names four.
The first three are all about the heart, and they are linked: a bigger, stronger heart (hypertrophy) ejects more blood per beat (stroke volume), so it needs fewer beats at rest (bradycardia).
| Long-term effect | What changes | Benefit to the performer |
|---|---|---|
| Heart size (hypertrophy) | The cardiac muscle thickens and the chambers enlarge, so the heart becomes bigger and stronger | The heart can pump a greater volume of blood with each beat |
| Resting pulse rate (bradycardia) | The resting pulse rate falls below the normal rate — this is called bradycardia | Fewer beats are needed to circulate the same volume of blood, so the heart works more efficiently at rest |
| Stroke volume | The volume of blood ejected from the heart in one beat increases | More oxygenated blood is delivered to the working muscles per beat, so more oxygen is available for aerobic respiration |
| Ability to tolerate lactic acid | The muscles become better able to tolerate higher concentrations of lactic acid | The performer can keep working anaerobically for longer before fatigue sets in, and recovers faster |
Turning knowledge into marks
A one-mark state question needs the fact only: "anaerobic respiration produces lactic acid."
An explain question needs the fact plus a consequence: "anaerobic respiration produces lactic acid, which builds up in the muscles and causes fatigue, so the performer can only keep working at that intensity for a short time."
An apply question needs the sport built into the answer: "in a 100 m sprint, the race is short and intense, so the muscles work anaerobically and lactic acid builds up; the sprinter is still breathing heavily at the finish because the oxygen taken in has to remove that lactic acid (EPOC)."
Key terms you must be able to define
- Aerobic respiration
- Respiration that uses oxygen to release energy from glucose: glucose + oxygen → carbon dioxide + water.
- Anaerobic respiration
- Respiration that releases energy from glucose without oxygen: glucose → lactic acid.
- Lactic acid
- The waste product of anaerobic respiration; it builds up in the muscles and causes fatigue.
- EPOC (Excess Post-exercise Oxygen Consumption)
- Also known as oxygen debt. It is caused by anaerobic exercise producing lactic acid, and requires a high breathing rate after exercise to remove the lactic acid.
- Recovery
- The period after exercise in which the body returns to its resting state and the lactic acid is removed; the length of this period varies with the intensity and duration of the exercise and the performer's fitness.
- Fatigue
- Feeling tired — a short-term effect of exercise caused by the muscles running short of energy and lactic acid building up.
- Hypertrophy
- The enlargement of the heart muscle as a long-term effect of training, increasing heart size.
- Bradycardia
- A resting pulse rate below the normal rate; the trained heart is stronger and pumps more blood per beat, so fewer beats are needed.
- Stroke volume
- The volume of blood ejected from the heart in one beat. It increases as a long-term effect of exercise.
Applied to your sport
Cambridge requires you to apply this knowledge to physical activities. Use these as models, then write your own for the sport you play.
A 100 m sprinter works anaerobically: the race is short and intense, so glucose is broken down without oxygen and lactic acid builds up. That is why the sprinter is still breathing hard well after crossing the line — EPOC in action.
A cross-country runner works aerobically for most of the race: the duration is long and the intensity is low to moderate, so oxygen can be delivered to the working muscles and glucose is broken down into carbon dioxide and water.
A midfielder is aerobic for most of the 90 minutes, but an anaerobic burst to close down an opponent produces lactic acid, so the player is still catching their breath while the lactic acid is removed.
A swimmer who trains regularly develops a larger heart (hypertrophy) and a lower resting pulse rate (bradycardia), so stroke volume increases and more oxygenated blood reaches the muscles per beat.
A performer completing heavy sets works anaerobically. As their training continues they can tolerate higher levels of lactic acid, so they recover between sets more quickly.
Flashcards
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Exam-style questions
8 questions, 21 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Exam technique for this topic
- Write both equations in full words. Oxygen belongs only in the aerobic equation — it is the single most commonly dropped mark in this topic.
- For link duration and intensity questions, always give both halves: name the system AND say whether the activity is long and low intensity or short and intense.
- Short-term effects happen during and just after one bout of exercise; long-term effects appear after weeks and months of training. Check which one the question is asking for before you write.
- Count the marks. "Describe the short-term effects of exercise" for 5 marks wants five separate effects, not one effect described five ways.
- Only use activities from the syllabus list in your examples — 100 m sprint, cross-country running, swimming and football all appear on it.
Why students lose marks here
Where this fits in IGCSE PE 0413
This page is one topic of Cambridge IGCSE PE 0413 / 0995. IGCSE PE 0413 is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE PE 0413 syllabus page.