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Effect on fast/slow twitch muscle fibres

There are two types of muscle fibre:

slow-twitch muscle fibres
fast-twitch muscle fibres.

Slow-twitch fibres contract slowly and release energy gradually as required by the body during steady-state activity such as jogging, cycling and endurance swimming. These fibres are efficient in using oxygen to generate energy (ATP), making them resistant to fatigue but unable to produce the power of fast-twitch fibres. When the body is engaged in endurance-type activity, slow-twitch fibres are preferentially recruited for the movement because they are more efficient in meeting the immediate demands of the working muscles.

Fast-twitch fibres contract quickly and release energy rapidly however, they fatigue rapidly due to anaerobic metabolism providing the energy. The body preferentially recruits fast-twitch fibres to perform explosive type activities such as weight-lifting, field athletics and sprint track athletics. Two types of fast-twitch fibres can be explored further through the link at the bottom of this page.

The individual’s ratio of fast-twitch to slow-twitch muscle fibre is genetically determined, making them more suited to certain sports or activities. The ratio of both types of muscle fibre varies in each individual and each muscle. In summary:

Ratio of fast to slow-twitch fibres is thought to be genetically determined.
The metabolic capabilities of both types of fibres can improve through specific strength and endurance training.
Sprinters and weight lifters have a large percentage of fast-twitch fibres.
Marathon runners generally have a higher percentage of slow twitch fibres.
Muscles that primarily maintain posture against gravity (core strength) require more endurance and generally have a higher percentage of slow-twitch fibres.
Muscles that produce powerful, rapid, explosive strength movements tend to have a greater percentage of fast-twitch fibres.

Source: http://www.exrx.net/ExInfo/Muscle.html

Sport specific training will assist in appropriate development and adaptation of each type of muscle fibre. Fast-twitch muscle fibres benefit most by anaerobic training, such as sprint or interval training and resistance training. Slow-twitch muscle fibres benefit most from endurance type activities that engage the aerobic system, such as running, cycling and swimming.

The adaptations that can occur in response to training are shown in the table below.

Aerobic training - in slow-twitch fibres	Anaerobic training - in fast-twitch fibres
hypertrophy of slow-twitch muscle fibres increased capillary supply to muscle fibres, improving gaseous exchange & movement of nutrients and waste products increased number and size of mitochondria (energy factory of cells) enabling more efficient energy production significant increase in myoglobin content (transports oxygen from the cell membrane to mitochondria)	increase and efficiency of ATP/PC supply increased glycolytic enzymes which improve functioning of the cell hypertrophy of red-twitch muscle fibres increased tolerance of lactic acid, allowing performance to be sustained for longer periods muscle contractions can be made more forcefully and quickly as there are a greater volume of fast-twitch fibres

For more information on muscle fibre types click on the following link:
http://www.brianmac.co.uk/muscle.htm

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