Fast and slow twitch muscle fibres

Respiration, Muscles and the Internal Environment · Muscles and movement · note 3 of 4

Spec 7.10(ii)

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Fast and slow twitch muscle fibresSpec 7.10(ii)

In short

Slow twitch muscle fibres contract slowly and resist fatigue because they respire aerobically, with many mitochondria, much myoglobin and a dense capillary supply. Fast twitch fibres contract quickly and powerfully but tire quickly, relying on anaerobic glycolysis with plentiful glycogen and fewer mitochondria.

Skeletal muscle contains two main types of fibre, and their proportions differ between muscles and between people. Their structure matches their role.

Fast twitch and slow twitch fibres
Feature	Slow twitch	Fast twitch
Speed of contraction	Slow	Fast
Force produced	Lower	Higher
Resistance to fatigue	High: contract for long periods	Low: tire quickly as lactate builds up
Main way of making ATP	Aerobic respiration	Anaerobic glycolysis (producing lactate)
Mitochondria	Many	Fewer
Myoglobin (stores oxygen)	A lot, so the fibres look red	Little, so the fibres look pale / white
Capillary supply	Dense	Fewer
Glycogen store	Lower	Higher
Sarcoplasmic reticulum	Less extensive	More extensive: releases Ca²⁺ quickly
Used for	Posture and endurance, e.g. marathon running	Sprinting and lifting heavy weights

Exam tip:

Link each structural feature to its function. 'Many mitochondria' is not enough; add 'for aerobic respiration to make ATP for a long time'.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How do antagonistic muscles work?

Antagonistic muscles are pairs that have opposite effects at a joint, such as the biceps (flexor) and triceps (extensor). Muscles can only pull. When one contracts and bends or straightens the joint, the other relaxes and is stretched, and then they swap roles to move the joint back.

What is the sliding filament theory?

The sliding filament theory explains muscle contraction. Calcium ions move tropomyosin away from the actin binding sites, so myosin heads bind and pull the actin filaments inwards using energy from ATP. The filaments slide past each other, the sarcomere shortens and the muscle contracts, although the filaments themselves stay the same length.

What are the differences between fast and slow twitch muscle fibres?

Slow twitch fibres contract slowly, resist fatigue, respire aerobically and have many mitochondria, much myoglobin and many capillaries. Fast twitch fibres contract quickly with more force, tire quickly, rely on anaerobic glycolysis and have more glycogen but fewer mitochondria, so they suit sprinting.

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