• Skeletal muscles are made up of two types of muscle fibres
  • Different muscles have different proportions of slow and fast twitch fibres

Slow Twitch Fibres

  • Slow contraction
  • Work for a long time without fatigue
  • Good for endurance activities
  • High proportion found in muscle used for posture (i.e. back/calves)
  • Energy is released slowly through aerobic respiration in slow twitch muscle fibres
  • They have lots of mitochondria and blood vessels to supply the muscles with oxygen
  • Mitochondria are mainly found near to the edge of muscle fibres to there is a short diffusion pathway
  • Rich in myoglobin, a red coloured protein that stores oxygen (which gives muscles their red colour)

Fast Twitch Fibres

  • Quick contraction
  • Fatigues quickly
  • Good for short high intensity bursts
  • High proportions found in muscles used for fast movements (i.e. legs, eyes)
  • Energy is released quickly through anaerobic respiration using glycogen
  • Stores of PCr for quick energy release
  • Few mitochondria or blood vessels as they are not really needed as they produce their energy through glycogen – not oxygen.
  • No myoglobin (which gives them a pale white colour)