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Muscle Fibre Types
The athlete’s ability to excel at endurance or speed work will depend largely
on their muscles’ ability to produce a certain speed and strength of
contraction, aided by the type of training that they undertake.
Slow twitch fibres (SO) function predominately during submaximal exercise,
whereas fast twitch fibres are recruited as the exercise intensity increases.
Fast oxidative glycolytic fibres (FOG) are recruited during medium intensity
work and fast glycolytic fibres (FG) high intensity work.
Slow twitch fibres – also known as slow oxidative (SO) or type l fibres –
designed for aerobic work. It uses oxygen to produce a small amount of force
over a long time. Marathon runners have a high percentage in their leg
muscles.
Fast twitch fibres – two types known as fast oxidative glycolytic (FOG) or type
lla and fast glycolytic (FG) or type llb – designed for anaerobic work. It
produces a large amount of force in a very short time. Shot putters have a
high percentage in their arm muscles.
Slow glycolytic fibres
 Myelin sheath of the motor neurone stimulating the muscle fibre is thinner
than that of the fast twitch unit.
 Contain more mitochondria and myoglobin.
 They are supplied by more blood capillaries than fast twitch fibres.
 Have the enzymes for aerobic respiration. They are able to break down fat
and carbohydrate to CO2 and H2O which does not produce any fatiguing
by-products.
 They are designed for long bouts of aerobic exercise
Fast oxidative glycolytic
 Thicker myelin sheath than slow twitch fibres.
 They can contract more quickly and exert more force.
 They can release energy very quickly. However, the rapid build-up of lactic
acid lowers the pH and has a negative effect on enzyme action causing
the muscle fibre to fatigue quickly.
Fast glycolytic
 They have a greater CP content than slow twitch fibres.
 They have a small number of mitochondria and low myoglobin content.
 They have a relatively small capillary network.
 They are quick to contract and can exert a large amount of force, but while
energy is rapidly released the muscle fibre is quick to fatigue.