What is the difference between fast and slow twitch fibers?

Answer 1

Fast twitch muscle fibres are adapted for powerful contractions over a short period of time, a high proportion of these would be present in sprinters for example. They are adapted to their function, they have a higher proportion of myosin, the thicker filaments to produce stronger contractions. They also have more anaerobic enzymes, this is because these fibres predominantly respire anaerobically. They also have stores of phosphocreatine, this chemical acts as a store of phosphate. Because anaerobic respiration produces smaller amounts of ATP than aerobic respiration, phosphocreatine donates a phosphate molecule to ADP forming ATP which can be used in the quick muscular contractions of fast twitch fibres.

Slow twitch fibres are adapted for less powerful contractions over a long period of time, a high proportion of these would be found in endurance athletes, such as marathon runners. They have the following adaptations, they contain a large amount of glycogen, which can be broken down into glucose. Glucose is the respiratory substrate in anaerobic respiration. There's a large supply of mitochondria and large supply of myoglobin. The oxygen stores in myoglobin can be used in the mitochondria as the final electron acceptor in the elctron transport chain. Finally slow muscle fibres have a large supply of blood vessels, supplying the mitochondria with the oxygen that it require. Aerobic respiration is used in slow fibres to prevent the build up of lactic acid, which would prevent athletes from participating in endurance sports.