Skeletal muscle fibres

We all have a mixture of different skeletal muscle fibres within our bodies. Some of our fibres lend themselves particularly well to endurance activities whilst others and more suited to higher intensity activities. The factor that determines our distribution of fibre type within our bodies is genetics (our parents). Although we can not change the fibre types we are born with we can train them to become better at endurance or better at high intensity activities.

We can categorise muscle fibres depending on their structure and function. We generally refer to fibres as being fast twitch (type I) or slow twitch (type II). However, there is also an intermediate category which can change it’s characteristics depending how it is trained. Which means it can become more like a type I or more like a type II.

Muscle fibres also vary in proportion in different parts of the body depending on their uses. For example the muscles of the neck, back and legs that are involved in posture an standing have a greater distribution of type I fibres. Where as the muscles of the arms that aren’t used for posture but more for lifting and at times high force production have more type II fibres.

Fibre type Slow twitch (type I)

Fast twitch (type IIa)


Fast twitch (type IIb)
  • Small diameter
  • Large myoglobin content
  • Lots of mitochondria
  • Lots of capillaries
  • Red in colour
  • Low force production
  • Resistant to fatigue
  • Aerobic
Take on the characteristics of type I or type IIb fibres depending on how they are trained.
  • Large diameter
  • Small myoglobin content
  • Few mitochondria
  • Few capillaries
  • White (pale) in colour
  • High force production
  • Explosive
  • Fatigue quickly
  • Anaerobic


  • Endurance based activities
  • Long distance running, cycling, swimming
  • High intensity activities
  • Sprinting
  • Jumping
  • Throwing
  • Weight lifting

Now let’s see if you can remember the difference between the different fibre types!

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