Muscle Fibre Types

Muscle Fibre Types

Fibres that rely on and use oxygen as fuel are called aerobic or slow-twitch (ST) fibres. Fibres that do not use oxygen as a fuel are anaerobic and are known as fast-twitch (FT) fibres. ST and FT fibres usually exist in equal amounts within the body and strength training is not thought to have much of an affect on this 50-50 relationship to a great extent, although strength training does affect the size of the muscle fibre.

Some characteristics of the two different muscle fibres:

Fast Twitch (FT) Slow Twitch (ST)
White in Colour Red in Colour
Type II Fibre Type I Fibre
Fast Fatiguing Slow Fatiguing
Large Nerve Innervation 300-500+ Muscle Fibres Smaller Nerve Innervation 10-180 Muscle Fibres
Short Forceful Contractions Long & Continuous Contractions
Speed & Power Dominant Endurance Dominant
Used During High-Intensity Work Only Used During Low and High Intensity Work

A motor nerves job is to carry a message from the brain to another part of the body. Depending on the amount of muscle fibres that connect to a single motor nerve, this determines the type of muscle fibre it is. The diagram below is a motor nerve/neuron and its connection to a muscle.

Image result for motor neurone

FT fibres typically have large nerve cells with 300-500+ muscle fibres connecting to one motor nerve. The FT muscle fibre contraction is forceful and more powerful. Successful athletes in speed and power-based sports genetically have a higher amount of FT fibres.

ST fibres usually have smaller nerve cells and connects with about 10-180 muscle fibres. Individuals with more ST fibres are more successful in endurance sports as they can perform work of lower intensity for a longer time.

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FT fibres are recruited from the force that the muscle must generate, instead of the speed of the contraction. This is why athletes in speed-related sports such as sprinting, and football must increase power. The high-power movements performed by these athletes activate the FT fibres, making them capable of performing explosive and fast actions.


Recruitment of muscle fibres depends on the load placed on the muscle. Moderate and low-intensity activity recruits ST fibres and as the load increases, more FT fibres are recruited during a muscle contraction. Research studies have shown that the arms have a higher percentage of FT fibres than the legs. (Fox et al., 1989)

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The amount of FT fibres within a muscle plays an important role in strength sports. Muscles with a higher percentage of FT fibres are capable of quicker, more powerful contractions. Changing the quantity of FT and ST fibres within a muscle by training is important for strength gains, but this possibility has been argued for quite a while. Recent studies say, that a shift from ST to FT may be possible as a result of high-intensity training done consistently over a long period of time. (Abernethy et al., 1990; Jacobs et al., 1987).

The graph below shows a general profile of FT & ST fibre percentages for some sports. Notice the differences between sprinters and marathon runners, which shows that success in some sports is at least partly determined by muscle fibre make-up.

Peak power, velocity (speed) of a movement is highly dependent on FT fibres. People who genetically have higher percentage of FT fibres make great sprinters and jumpers, and with this natural talent they should be channelled into speed-power dominant sports. Trying to make them a distance runner would be a waste of time and talent. There are no clear differences in muscle fibre distribution between female and male athletes.

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