Slow twitch fibres

Also referred to as slow oxidative (S.O.) or Type I.

They are red in colour, contract slowly, are aerobic (supplied with oxygen), are used in endurance based activities because they can contract repeatedly, but they exert less force. They have a low firing threshold and are used for light loads.

The myelin sheath (the insulation around a nerve axon) of the motor neurone stimulating the slow twitch muscle fibre is not as thick as that of the fast twitch unit, and this reduces the amount of insulation, slowing down the nerve impulse.

Slow twitch fibres do not produce as much force as fast twitch fibres, but can easily cope with prolonged bouts of exercise. They are more suited to aerobic work as they contain more mitochondria and myoglobin and have more blood capillaries than fast twitch fibres.

Slow twitch fibres have the enzymes necessary for aerobic respiration and are able to break down fat and carbohydrates to carbon dioxide and water. This is a slower process than releasing energy anaerobically, but it does not produce any fatiguing by-products.