- 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)