Slow & Fast Twitch Fibres

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  • Created on: 19-03-13 13:28
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SLOW TWITCH FIBRES
These are specialised for slower, sustained contraction, and can cope with long periods of exercise ­ they need to carry out a large amount of aerobic
respiration.
Key features:
Many mitochondria
High concentrations of respiratory enzymes to carry out the aerobic respiration
They contain a large amount of myoglobin which has a high affinity for oxygen, and only releases it when the concentration of oxygen on the
cell falls very low; therefore acting as an oxygen store within muscle cells.
Associated with numerous capillaries to ensure a good oxygen supply.
FAST TWITCH FIBRES
These are specialised to produce rapid, intense contractions. The ATP used in these contractions is produced almost entirely from anaerobic glycolysis.
Key features:
Few mitochondria (glycolysis doesn't occur in mitochondria)
They have very little myoglobin ­ few reserves of oxygen and few associated capillaries.
Their reliance on anaerobic respiration means there is a rapid build-up of lactate, therefore the fast twitch muscle fibres fatigue easily.
They may have some characteristics of slow twitch fibres.
In mammals, these two types of muscle fibres are not separated - they are both found in all the skeletal muscles. The proportion of each type is
genetically determined, and varies between different people. Marathon runners will have lots of slow twitch fibres whereas sprinters will not. 3

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