Muscle fibres


Slow twitch muscle fibres

  • They contract slowly and can work for a long period of time without getting tired. This makes them good for endurance activities, e.g long distance running and maintaining posture. 
  • High proportions of slow twitch muscle fibres are found in the muscles you use for posture, such as the muscles in your back and calves.
  • Energy is released slowly by aerobic respiration in slow twitch muscle fibres.
  • They have lots of mitochondria and blood vessels to supply the muscles with oxygen. 
  • The mitochondria are mainly found near the edge of muscle fibres, so that there's a short diffusion pathway for oxygen from the blood vessels to the mitochondria. 
  • Slow twitch muscle fibres are also rich in myoglobin, so they appear reddish.
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Fast twitch muscle fibres

  • They contract very quickly but also get tired quickly.
  • This makes them good for short bursts of speed and power, e.g sprinting and eye movement.
  • High proportions of fast twitch muscle fibres are found in muscles you use for fast movement such as the legs, arms and eyes
  • Energy is releases quickly through anaerobic respiration using glycogen in fast twitch muscle fibres. They also have stores of PCr so that energy can be generated very quickly when needed.
  • Fast twitch muscle fibres have few mitochondria and blood vessels. They don't have much myoglobin either, so they can't store much oxygen - this gives them more of a whitish colour.
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