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