Muscle Fibre Types

This lesson covers the three types of muscle fibre as required by the Edexcel GCSE PE specification (1PE0 — Topic 1: Applied Anatomy and Physiology). This topic is unique to the Edexcel specification — other exam boards (AQA and OCR) do not cover it at GCSE level. You need to know the characteristics of each fibre type, how they relate to different types of physical activity, and how training can affect them.

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Why Are There Different Muscle Fibre Types?

Not all physical activities require the same type of muscle contraction. A marathon runner needs muscles that can contract repeatedly for hours without fatiguing, while a sprinter needs muscles that can contract explosively for a few seconds. The body has different types of muscle fibres to meet these different demands.

Every muscle in the body contains a mix of all three fibre types, but the proportion varies between individuals and between muscles. This proportion is largely determined by genetics but can be influenced by training.

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The Three Muscle Fibre Types

Feature	Type I (Slow Twitch)	Type IIa (Fast Oxidative Glycolytic)	Type IIx (Fast Twitch)
Other names	Slow oxidative	Fast oxidative glycolytic (FOG)	Fast glycolytic
Contraction speed	Slow	Fast	Very fast
Force produced	Low	Medium–high	Very high
Fatigue resistance	Very high (resistant to fatigue)	Moderate	Low (fatigues quickly)
Aerobic/anaerobic	Aerobic (uses oxygen)	Both aerobic and anaerobic	Anaerobic (without oxygen)
Oxygen use	High	Moderate–high	Low
Energy source	Fats and glucose (with oxygen)	Glucose (with and without oxygen)	Glucose and creatine phosphate (without oxygen)
Mitochondria	Many	Many	Few
Blood supply	Rich (lots of capillaries)	Good	Poor (fewer capillaries)
Colour	Red (due to high myoglobin content)	Red/pink	White/pale
Size	Smaller	Medium	Largest

graph TD
    A["Muscle Fibre Types"] --> B["Type I<br>(Slow Twitch)"]
    A --> C["Type IIa<br>(Fast Oxidative<br>Glycolytic)"]
    A --> D["Type IIx<br>(Fast Twitch)"]
    B --> B1["Slow contraction<br>Low force<br>High endurance<br>Aerobic"]
    C --> C1["Fast contraction<br>Medium-high force<br>Moderate endurance<br>Aerobic + Anaerobic"]
    D --> D1["Very fast contraction<br>Very high force<br>Low endurance<br>Anaerobic"]

    style A fill:#4a90d9,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#c0392b,color:#fff

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Type I — Slow Twitch Fibres

Type I fibres (also called slow oxidative fibres or slow twitch fibres) are designed for sustained, low-intensity activity. They are the "endurance" fibres.

Key Characteristics

• Contract slowly but can sustain contractions for a very long time.
• Highly resistant to fatigue — they can keep working for hours.
• Use the aerobic energy system (require oxygen).
• Have a rich blood supply with many capillaries, ensuring a constant delivery of oxygen.
• Contain many mitochondria (the organelles that produce aerobic energy).
• Contain high levels of myoglobin (an oxygen-storing protein), which gives them a red colour.
• Produce low force per contraction.

Sporting Applications

Sport/Activity	Why Type I Fibres Are Dominant
Marathon running	Sustained low-moderate intensity over 2+ hours
Long-distance cycling	Continuous pedalling at a moderate pace
Long-distance swimming	Sustained rhythmic movement over a long period
Cross-country skiing	Extended, steady-state aerobic exercise
Walking/hiking	Low-intensity, long-duration activity

Exam Tip: When an exam question asks which fibre type is used in a sport, consider the intensity and duration. If the activity is low-to-moderate intensity and lasts a long time, Type I fibres are dominant.

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Type IIa — Fast Oxidative Glycolytic Fibres

Type IIa fibres (also called fast oxidative glycolytic or FOG fibres) are the middle ground between Type I and Type IIx. They can produce fast, powerful contractions AND sustain them for a moderate period.

Key Characteristics

• Contract quickly — faster than Type I but not as fast as Type IIx.
• Produce medium to high force.
• Have moderate resistance to fatigue — they tire more quickly than Type I but can still sustain activity for a reasonable period.
• Can use both aerobic and anaerobic energy pathways — this is their unique feature.
• Have a good blood supply (though less than Type I).
• Contain a moderate number of mitochondria.
• Appear red/pink in colour.

Sporting Applications

Sport/Activity	Why Type IIa Fibres Are Dominant
800 m / 1500 m running	Fast pace sustained for 2-4 minutes
Swimming 200 m	Powerful strokes at high intensity for a moderate duration
Boxing (rounds)	Repeated powerful actions over 3-minute rounds
Hockey/football midfield	Repeated sprints interspersed with jogging
Rowing (2000 m race)	Powerful strokes sustained for 6-7 minutes