These two fibres are as follows:
1- Fast Twitch (FT) or Type II (also called white fibres because of their microscopic appearance)
2- Slow Twitch (ST) or Type I (also called red fibres, again because of their microscopic appearance)
FT fibres are more anaerobic (does not rely of oxygen), larger, fatigue quicker, and have a faster contraction speed compared to ST fibres. These qualities make FT fibres ideal for actions that involve quick bursts of power and energy, such as jumping and sprinting. Events such as swimming, long-distance running, or cycling that require endurance, depend on the aerobic (relies of oxygen), smaller, slow contracting ST fibres.
Muscles fibres do not always fall into these two categories. There are also fibres that contain characteristics from both. These fibres fall into the Type II category, further dividing them into Type IIa and Type IIb. The more aerobic Type IIa fibre that contains more surrounding capillaries, thus; expressing greater resistance to fatigue than the anaerobic Type IIb fibre.
Through heredity an individual is dealt a composition of muscle fibres. These fibres cannot be altered by training, i.e. an individual cannot use training stimuli to transform one fibre type to another. However, most skeletal muscle contain both FT and ST fibres. The amount varies from one individual to another, as well as one muscle to another.
A conclusion can be made that individual performances occur as a result of varying percentages of muscle fibre types, which make some individuals better suited to specific activities. That does not mean, however; that training will not improve what fibres you already have.
References
Klavora, P. (2004). Foundations Of Exercise Science: Studying Human Movement And Health. Toronto: Sports Books Publisher.
