We have two types of muscle in our body with each having different properties, functions and distributions in the human body.
Fast twitch muscle fibres (also known as type II muscle fibres) are able to contract quickly but they also tire quickly. They are specialised for this purpose in a number of ways:
They use mainly anaerobic respiration. While this allows ATP to be generated rapidly (anaerobic respiration is far quicker than aerobic respiration), it is an inefficient process yielding only 2 ATP per glucose and causes muscles to fatigue quickly due to build up of lactic acid .
They have low capillary density. Fast twitch muscle is less reliant on aerobic respiration and is therefore also less reliant on the oxygen provided by the blood supply.
They have low myoglobin concentrations. This means fast twitch muscle fibres have a reduced oxygen storing capacity. However this is not an issue as they rely primarily on anaerobic respiration. Low myoglobin concentrations make fast twitch fibres appear paler (e.g. chicken breast).
They have a lower density of mitochondria. They do not require as many mitochondria per cell as they are less reliant on anaerobic respiration. Instead they express higher levels of cytoplasmic glycolytic enzymes to allow fast glycolysis in anaerobic respiration.
They have large glycogen stores. Glycogen can be broken down to quickly generate glucose for glycolysis.
Function: seen in higher proportion in muscles which do fast, high precision movements (e.g. eye movements).
Slow twitch muscle fibres (also known as type I muscle fibres) contract more slowly but tire less easily. They are also specialised in the following ways:
They use mainly aerobic respiration. This allows large amounts of ATP to be generated (36-38ATP per glucose) but more slowly. Contraction is slower but the muscle is less likely to fatigue.
They have high capillary density. This provides a large and continuous supply of oxygen and glucose.
They have high concentrations of myoglobin within the muscle fibres. This gives the muscle fibres a high oxygen storage capacity which is important in maintaining a continuous oxygen supply for aerobic respiration. High concentrations of myoglobin make muscle fibres appear more red (e.g. chicken leg).
They have a higher density of mitchondria within cells as they rely on aerobic respiration.
They have smaller glycogen stores. The high capillary density brings in a steady supply of both glucose and oxygen which means that glycogenolysis is not required.
Function: seen in muscle groups where sustained contraction is required such as those involved in posture (e.g. back muscles).
Distribution: all muscles contain a mixture of fast and slow twitch fibres but the proportions of each type differ depending on the function of the muscle. For example, eye muscles are composed of roughly 85% fast twitch fibres and 15% slow twitch fibres as they are specialised for fast, high precision movements. The soleus muscle in the calf, however, is composed of around 20% fast twitch fibres and 80% slow twitch fibres as it is tonically active in many everyday processes such as standing up and walking.