Slow and Fast twitch fibres

The darker muscle is slow twitch fibres and the paler muscle is fast twitch fibres.

Slow twitch fibres are specialised for [blank_start]slower[blank_end], sustained contraction and can cope with [blank_start]long[blank_end] periods of exercise. To do this they need to carry out a large amount of [blank_start]aerobic[blank_end] respiration. They have many [blank_start]mitochondria[blank_end] and high concentrations of respiratory enzymes to carry out the aerobic reactions. They also contain large amounts of the dark red pigment [blank_start]myoglobin[blank_end], which gives them their colour. Myoglobin is a protein similar to haemoglobin (the oxygen-carrying pigment found in [blank_start]red[blank_end] blood cells). It also has a high affinity for oxygen and only releases it when the concentration of oxygen in the cell falls very low; it therefore acts as an oxygen store within muscle cells. Slow twitch fibres are associated with [blank_start]numerous[blank_end] capillaries to ensure a good oxygen supply.

Fast twitch fibres are specialised to produce [blank_start]rapid[blank_end], intense contractions. The ATP used in these contractions is produced almost entirely from [blank_start]anaerobic[blank_end] glycolysis. They have [blank_start]very few[blank_end] mitochondria (glycolysis doesn't occur in mitochondria). They also have very little myoglobin, so have few reserves of oxygen and [blank_start]few[blank_end] associated capillaries. Their reliance on anaerobic respiration means there is a rapid build-up of [blank_start]lactate[blank_end], so the fast twitch muscle fibres fatigue easily. With aerobic training, fast twitch muscle fibres can take on some of the characteristics of slow twitch muscle fibres e.g. they may have more mitochondria, allowing them to use aerobic respiration reactions when contracting. Fast twitch fibres also have [blank_start]an extensive[blank_end] sarcoplasmic reticulum allowing calcium ions to reach actin in more places. They also have a high glycogen content.