1.5.5

Slow & Fast Twitch Fibres

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Slow and Fast Twitch Fibres

There are two types of skeletal muscle fibres: slow and fast twitch fibres. They differ in their location, structure and properties.

Location

Location

  • Slow twitch fibres -
    • Found in muscles used for posture such as the back and neck.
  • Fast twitch fibres -
    • Found mainly in muscles such as the arms and legs.
Adaption to function

Adaption to function

  • Slow twitch fibres -
    • Adapted for endurance and slow movement over long periods of time.
    • Muscle fibres are long and thin.
    • The muscles fatigue slowly and contract slowly.
  • Fast twitch fibres -
    • Adapted for fast or strong movement over short periods of time.
    • Muscle fibres are short and wide.
    • The muscles fatigue quickly and contract quickly.
Energy source

Energy source

  • Slow twitch fibres -
    • Rely on energy released through aerobic respiration.
  • Fast twitch fibres -
    • Rely on energy released through anaerobic respiration.
Cell structure

Cell structure

  • Slow twitch fibres -
    • Lots of mitochondria to maintain aerobic respiration.
    • Lots of capillaries to supply muscle fibres with oxygen.
    • Low levels of glycogen.
    • Low levels of phosphocreatine.
    • Large stores of myoglobin (pigment that stores oxygen), so appear reddish.
    • Less sarcoplasmic reticulum (contains calcium ions).
Cell structure (cont'd)

Cell structure (cont'd)

  • Fast twitch fibres -
    • Fewer mitochondria.
    • Fewer capillaries.
    • High levels of glycogen.
    • High levels of phosphocreatine.
    • Small stores of myoglobin.
    • More sarcoplasmic reticulum.
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