What you need to know
- 7.10(ii)Structural and physiologicalDescribes an adaptation of body processes, such as producing concentrated urine. differences between fast and slow twitchMuscle fibres that contract slowly, resist fatigue and respire aerobically. They have many mitochondria and much myoglobin. muscle fibres.
Do not classify a person as a single fibre type. Training effects on fibre type are not required. The contraction mechanism is the same sliding-filament story in both; the metabolism and the speed differ.
By the end, you should be able to…
- Compare mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place., myoglobinA red protein in muscle that stores oxygen. Slow twitch fibres have a lot of it., capillaryA tiny vessel whose wall is a single layer of endothelium, giving a short diffusion distance for exchange with tissues. and fatigue.
- Link slow fibres to aerobic respirationRespiration using oxygen: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. It releases much more ATP than anaerobic respiration. and fast fibres to glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD..
- Avoid a claim that one muscle contains only one type.
Build your understanding
Slow twitch: endurance
slow twitchMuscle fibres that contract slowly, resist fatigue and respire aerobically. They have many mitochondria and much myoglobin. fibres contract more slowly and fatigue more slowly. Structurally they contain many mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place., a rich blood capillaryA tiny vessel whose wall is a single layer of endothelium, giving a short diffusion distance for exchange with tissues. supply, and a high concentration of myoglobinA red protein in muscle that stores oxygen. Slow twitch fibres have a lot of it., which stores oxygen and makes the fibre redder. Physiologically they rely on aerobic respirationRespiration using oxygen: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. It releases much more ATP than anaerobic respiration.. They suit posture and long, moderate work.
Myoglobin is not haemoglobinA globular protein in red blood cells with four polypeptide chains, each with a haem group containing Fe²⁺ that binds one O₂., though both bind oxygen. Myoglobin is inside the muscle fibre. More capillaries mean a shorter diffusion distanceThe distance molecules have to travel. A shorter distance gives a faster rate of diffusion. for oxygen and for removing carbon dioxide.
Fast twitch: a short, powerful pull
fast twitchMuscle fibres that contract quickly and powerfully but tire fast, relying mainly on anaerobic respiration. fibres contract quickly and with more force, then fatigue sooner. They have fewer mitochondria, less myoglobin, so they look paler, and more glycogenThe storage polysaccharide in animals: highly branched chains of α-glucose, so glucose can be released quickly.. Their physiology depends more on glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD., including anaerobic glycolysis, so lactateThe product of anaerobic respiration in animals, formed when pyruvate is reduced by reduced NAD. is produced when oxygen supply does not match the ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. demand. They suit brief, powerful movements.
The filaments are still actinThe protein of the thin filaments in muscle. Myosin heads bind to it. and myosinThe protein of the thick filaments in muscle. Its heads bind to actin and pull it, using ATP.. “Fast” does not mean a different sliding theory. It means a faster cross-bridge cycle and a metabolism that cannot support it aerobically for long.
Mixtures, not labels for people
A named muscle usually contains both types, in proportions that differ between muscles and between individuals. “This athlete is fast twitch” is not a specification statement. You will not be asked to prescribe training from this table.
If a question gives a histochemical description, match the evidence: many mitochondria and myoglobin point to slow fibres; few mitochondria and rapid fatigue point to fast fibres. One clue is weaker than three.
NOT: a person is not one fibre type, and training is not a required conversion story.
- Slow fibres are darker and aerobic
- Fast fibres rely more on glycolysis
- Both use sliding filaments
- Most muscles contain both
Two traps this diagram blocks
- Do not say fast fibres have no mitochondria.
- Do not claim one named muscle in the body is purely one type unless the question says so.
From the diagram alone
Question
Give two structural differences between slow and fast twitch fibres and one physiological difference. [3]
Show a model answer
Slow fibres have more mitochondria and more myoglobin than fast fibres. Slow fibres fatigue more slowly because they rely more on aerobic respiration, whereas fast fibres rely more on glycolysis.
| Feature | Slow twitch | Fast twitch |
|---|---|---|
| Mitochondria | Many | Fewer |
| Myoglobin and colour | More, redder | Less, paler |
| Capillaries | Denser supply | Less dense |
| Main ATP route | Aerobic respiration | Glycolysis, soon anaerobic |
| Fatigue | Slow | Sooner |
Make your reasoning visible
Structure and job
Give one structural difference and one physiologicalDescribes an adaptation of body processes, such as producing concentrated urine. difference if the question says both.
Myoglobin holds oxygen
It is not an enzyme of glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD..
Avoid this misconception
The claim: fast twitchMuscle fibres that contract quickly and powerfully but tire fast, relying mainly on anaerobic respiration. fibres have more mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place. so they can respire faster.
The correction: slow twitchMuscle fibres that contract slowly, resist fatigue and respire aerobically. They have many mitochondria and much myoglobin. fibres have more mitochondria and more myoglobinA red protein in muscle that stores oxygen. Slow twitch fibres have a lot of it.. Fast twitch fibres rely more on glycolysis and fatigue sooner.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Give two structural differences between slow and fast twitch fibres.
Show suggested answer
Slow fibres have more mitochondria, more myoglobin and a denser capillary supply. Fast fibres have fewer of these and more glycogen. Any two paired differences.
Practice question
Retrieval and explanation · suggested answer
Which type depends more on anaerobic respiration, and what waste does that produce in an animal?
Show suggested answer
Fast twitch. Lactate.
Practice question
Retrieval and explanation · suggested answer
Why is a whole person not “a slow-twitch organism”?
Show suggested answer
Muscles contain a mixture of fibre types. The specification compares fibres, not personalities.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A muscle sample is red, rich in capillaries, and still contracting after a long moderate task. A second sample is paler and stops producing force quickly. Identify the likely predominant fibre in each. What extra evidence would you want before you were sure?
Show a hint
Colour is myoglobin, but one clue can mislead.
Explore the reasoning
The first is likely mostly slow twitch: myoglobin, capillaries and fatigue resistance fit aerobic fibres. The second is likely mostly fast twitch. Mitochondrion density or a direct fatigue measurement would strengthen the call. Colour alone is not enough if the question had given only colour.
Recall this lesson
Lesson recall questions
Question 1 of 5: List three structural features of a mammalian skeletal muscle fibre.
Skeletal muscle fibre types
Practice question
Retrieval and explanation · suggested answer
List three structural features of a mammalian skeletal muscle fibre.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Endurance and injury are different conversations
7B · Different Types of Muscle FibreNHS pages on strains describe injured muscle. They do not sort people into fibre types or promise that exercise will convert one type into the other. Use the…Specification 7.10(ii)Read & use the scienceClose article
Why this matters
NHS pages on strains describe injured muscle. They do not sort people into fibre types or promise that exercise will convert one type into the other. Use the comparison in the lesson for 7.10. Leave training claims out of the required answer. 7.10(ii): structural and physiological differences, not a coaching programme. An injury, symptom or whole-body performance result is not a direct image of a sarcomere or cardiac conduction pathway.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.10(ii) Structural and physiological differences between fast and slow fibres.
Use the science
Why is “fast fibres are the ones that get strained” not a specification conclusion?
Explore a possible response
A strain is an injury category. Fast fibres fatigue sooner and use glycolysis, but the NHS injury page does not map strains onto fibre types. Do not add that link.
Explore the source
Fibre types are a continuum, not a sports horoscope
7B · Different Types of Muscle FibreA research review describes human skeletal muscle fibres along a slow-to-fast continuum, with intermediate types as well as the familiar slow and fast labels. Slow fibres tend to…Specification 7.10(ii)Read & use the scienceClose article
Why this matters
A research review describes human skeletal muscle fibres along a slow-to-fast continuum, with intermediate types as well as the familiar slow and fast labels. Slow fibres tend to be more fatigue resistant; fast fibres can generate force rapidly but often tire sooner. Training can alter properties, yet the evidence does not support an easy social-media test that fixes a person’s future sport from one fibre percentage. The 7.10 comparison is a useful model of structure and physiology, but actual performance also depends on technique, training, oxygen delivery and many other factors. Researchers use muscle samples and protein measurements, not a quiz about favourite workouts.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.10(ii) Structural and physiological differences between fast and slow fibres.
Use the science
Why would a sprint result alone be weak evidence that a runner has a particular muscle-fibre distribution?
Explore a possible response
Sprint performance depends on many variables, including training and technique. A performance time is not a direct measurement of fibre type or its metabolic properties.
Explore the source
- Muscle fibre type transitions with exercise trainingPeer-reviewed review via NIH PubMed Central