6.3.5Organisms Respond to Changes in their Environments (A Level only)

Slow and Fast Skeletal Muscles — AQA A Level Biology 7402 Revision Notes & Topical Questions

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📚 AQA A Level Biology 7402 Hub / Topical Revision

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6.3.5 Slow and Fast Skeletal Muscles — AQA A Level Biology 7402 notes: Compare slow-twitch and fast-twitch muscle fibres and relate their structure to the type of activity they support.Skeletal muscle contains two main types of fibre, which differ in their speed of contraction, h…

Slow-twitch (type I) fibres
Fast-twitch (type II) fibres
phosphocreatine
Exam-style question 1
Exam-style question 2
Slow-twitch fibre

Section 1 of 9Two types of skeletal muscle fibre

Skeletal muscle contains two main types of fibre, which differ in their speed of contraction, how they obtain their ATP and how long they can work before they tire. Most muscles contain a mixture, but the proportions vary with the muscle's function and between individuals.

Slow-twitch (type I) fibres contract slowly and can keep going for long periods. They are used for sustained, low-intensity activity such as maintaining posture, endurance running and long-distance swimming. Fast-twitch (type II) fibres contract rapidly and with greater force, but fatigue quickly. They are used for short bursts of powerful activity such as sprinting, jumping and weightlifting.

Section 2 of 9Structural and metabolic differences

Feature	Slow-twitch	Fast-twitch
Speed of contraction	Slow	Fast
Force produced	Lower	Higher
Fatigue	Resistant; fatigue slowly	Fatigue quickly
Main ATP source	Aerobic respiration	Anaerobic glycolysis, with lactate formation, and phosphocreatine
Mitochondria	Many	Few
Myoglobin	High (red colour)	Low (pale)
Blood supply	Dense capillary network	Fewer capillaries
Glycogen store	Lower	Higher
Phosphocreatine store	Smaller	Larger
Fibre diameter	Narrower	Wider, more myofibrils

Section 3 of 9Explaining the adaptations

The features of slow-twitch fibres are all linked to aerobic respiration. A good blood supply delivers oxygen and glucose and removes carbon dioxide; myoglobin stores oxygen and releases it when the oxygen concentration is low; many mitochondria carry out oxidative phosphorylation to produce large amounts of ATP; and since the ATP is made aerobically, there is little lactate to cause fatigue.

Fast-twitch fibres rely on energy made quickly without oxygen.

Phosphocreatine

They store high concentrations of phosphocreatine,

which donates its phosphate to ADP to regenerate ATP very quickly, and they have more glycolytic enzymes so glycolysis is fast.

They contain more myosin and thicker filaments, and the sarcoplasmic reticulum releases and removes Ca²⁺ faster, so the contraction cycle is faster.

The price is that lactate builds up, lowering the pH and inhibiting enzymes, which causes fatigue.

Section 4 of 9Athletes and training

The proportions of fibre types are partly inherited.

Athletes and training

Marathon runners have a high proportion of slow-twitch fibres in the leg muscles, while sprinters have more fast-twitch fibres.

Training can increase the number of mitochondria and capillaries in fibres, and resistance training increases the size of fast-twitch fibres.

In exam questions on performance data, match the fibre type to the activity by using the key terms: duration, intensity, source of ATP and fatigue.

Section 5 of 9Evaluating data

Data may show the percentage of each fibre type in muscle biopsies from different groups, or the time to fatigue.

Evaluating data

Describe the difference with figures, then explain using the structure of the fibre.

Avoid claiming that one type of fibre is better: they are adapted to different tasks.

Also remember that findings from a small sample of athletes may not apply to everyone, and that people differ in age, training status and sex.

Section 6 of 9Mixed muscles and the control of force

Most muscles contain both fibre types, so the nervous system can select the right kind of fibre for a task.

Mixed muscles and the control of force

For a low-intensity activity such as standing, only a few motor units containing slow-twitch fibres are recruited;

as the demand for force rises, more motor units,
and then fast-twitch units, are recruited.

This allows efficient use of energy, as fast-twitch fibres are only used when they are needed.

A large motor unit contains many fibres supplied by one motor neurone and is used for powerful movements,

while a small motor unit allows fine control.

Muscles of the back and the legs that maintain posture have a high proportion of slow-twitch fibres, because they must work for hours without fatigue. Muscles that move the eyes, or muscles of the hand, have smaller motor units. Another difference is that fast-twitch fibres have a faster-acting myosin ATPase and sarcoplasmic reticulum, so the cross-bridge cycle is quicker.

In an exam, a good strategy for a comparison question is to set out a feature of one fibre type and then the contrasting feature of the other in the same sentence, and to finish by linking the difference to the activity. For example: slow-twitch fibres contain many mitochondria for aerobic ATP production during long exercise, whereas fast-twitch fibres contain few mitochondria and rely on phosphocreatine and glycolysis during short sprints.

Section 7 of 9Exam-style questions

Explain why slow-twitch fibres are better adapted for long-distance running than fast-twitch fibres. [4]4 marks
Explain why a sprinter needs fast-twitch fibres. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Slow-twitch fibre	Fibre that contracts slowly and resists fatigue
Fast-twitch fibre	Fibre that contracts quickly with high force but fatigues rapidly
Myoglobin	Oxygen-storing protein in muscle
Phosphocreatine	Store of phosphate used to regenerate ATP rapidly
Anaerobic glycolysis	ATP production from glucose without oxygen, forming lactate
Lactate	Product of anaerobic respiration that lowers pH

Section 9 of 9Exam technique

Saying that fast-twitch fibres have more mitochondria; slow-twitch fibres do.

Writing that slow-twitch fibres respire only anaerobically; they use mainly aerobic respiration.

Saying one fibre type is better; each is adapted to a different activity.

Forgetting the role of phosphocreatine in fast-twitch fibres.

Stating that lactate is produced by aerobic respiration.

Last updated: 18 October 2026