Slow and Fast Twitch Muscle Fibres
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of aerobic and anaerobic respiration. You can test your knowledge on these below.
What are the products of anaerobic respiration in animal cells?
Lactate and a small amount of ATP (2 molecules per glucose).
Which organelle is essential for aerobic ATP production?
Mitochondria – the site of the Krebs cycle and oxidative phosphorylation.
Why does anaerobic respiration result in fatigue?
Lactate builds up in muscles, reducing pH and enzyme activity.
Topic Explainer Video
Overview of Muscle Fibre Types
Skeletal muscle contains two main types of fibres adapted for different types of activity:
Slow Twitch Muscle Fibres
- Adapted for aerobic respiration and endurance activities (e.g., long-distance running).
- Contract slowly and provide less powerful contractions, but can contract for longer without fatigue.
Key Features:
- Many mitochondria: For aerobic respiration.
- Rich blood supply (many capillaries): Supplies oxygen and glucose.
- High myoglobin content: A red oxygen-storing pigment – gives fibres a deep red colour- similar to haemoglobin but found in muscle fibres.
- Low glycogen stores: Not needed as much due to rich blood supply and delivery of glucose.
- Thin fibres: Shorter diffusion distance for oxygen.
Fast Twitch Muscle Fibres
- Adapted for anaerobic respiration and short bursts of intense activity (e.g., sprinting, weightlifting).
- Contract rapidly and powerfully, but fatigue quickly.
Key Features:
- Fewer mitochondria: Anaerobic respiration predominates.
- Low myoglobin content: Appear white.
- Less blood supply: Don’t rely on oxygen delivery.
- High glycogen content: Hydrolysed into glucose for anaerobic respiration.
- High levels of enzymes for anaerobic respiration.
- Store phosphocreatine: For rapid ATP regeneration.
Table Summary: Comparison
| Feature | Slow Twitch | Fast Twitch |
| Main ATP Source | Aerobic respiration | Anaerobic respiration + PCr |
| Contraction Speed | Slow | Fast |
| Fatigue Resistance | High | Low |
| Myoglobin Content | High (red fibres) | Low (white fibres) |
| Mitochondria | Many | Fewer |
| Capillary Density | Dense network | Sparse |
| Glycogen Stores | Low to moderate | High |
Key Terms
- Myoglobin: Oxygen-storing pigment in muscles, similar to haemoglobin.
- Phosphocreatine (PCr): High-energy molecule used to regenerate ATP quickly in muscle cells.
- Mitochondria: Organelles that carry out aerobic respiration and produce ATP.
- Lactate: Product of anaerobic respiration in muscles; contributes to fatigue.
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Exam Tip
Always link structure to function clearly in comparisons. For example, fast twitch fibres do not have many mitochondria as they use mainly anaerobic respiration which occurs in the cytoplasm.
No answer provided.
Describe and explain how the structure of slow and fast twitch muscle fibres are adapted to their functions. (6 marks)
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Slow twitch fibres:
- Contain many mitochondria → produce ATP by aerobic respiration.
- Rich capillary network → ensures steady oxygen delivery.
- High myoglobin content → stores oxygen, supports aerobic respiration.
- Contract slowly but do not fatigue quickly → suited to endurance.
-
Fast twitch fibres:
- Contain high levels of glycogen → used in anaerobic respiration.
- Store phosphocreatine → regenerates ATP rapidly for quick contractions.
- Fewer capillaries and low myoglobin → not reliant on oxygen.
- Contract rapidly but fatigue quickly → adapted to short bursts of intense activity.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!