Slow twitch fibres are specialised for slower faster( slower, faster ), sustained contraction and can cope with long short( long, short ) periods of exercise. To do this they need to carry out a large amount of aerobic anaerobic( aerobic, anaerobic ) respiration. They have many mitochondria chloroplasts( mitochondria, chloroplasts ) and high concentrations of respiratory enzymes to carry out the aerobic reactions. They also contain large amounts of the dark red pigment myoglobin myofibril myosin( myoglobin, myofibril, myosin ), which gives them their colour. Myoglobin is a protein similar to haemoglobin (the oxygen-carrying pigment found in red white( red, white ) 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 numerous not very many( numerous, not very many ) capillaries to ensure a good oxygen supply.

Fast twitch fibres are specialised to produce rapid slow( rapid, slow ), intense contractions. The ATP used in these contractions is produced almost entirely from anaerobic aerobic( anaerobic, aerobic ) glycolysis. They have very few lots of( very few, lots of ) mitochondria (glycolysis doesn't occur in mitochondria). They also have very little myoglobin, so have few reserves of oxygen and few lots of( few, lots of ) associated capillaries. Their reliance on anaerobic respiration means there is a rapid build-up of lactate oxygen( lactate, oxygen ), 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 an extensive little( an extensive, little ) sarcoplasmic reticulum allowing calcium ions to reach actin in more places. They also have a high glycogen content.