Fast twitch fibres
Also known as Type 2.
These are white in colour, contract rapidly, are anaerobic in nature (little oxygen supply), are used for strength/speed based activities because they can exert great forces, but they are easily exhausted. Fast twitch fibres are further divided into Type 2a and Type 2b. They have a high firing thresholds and are used for heavy loads.
Type 2a fibres
These are also referred to as fast oxidative glycolytic (F.O.G.) take on certain type 1 characteristics through endurance training. They therefore tend to have a greater resistance to fatigue, and are used in activities which are fairly high in intensity and of relatively short duration.
The motor nerve stimulating the type IIa fibre has a thicker myelin sheath than the slow twitch fibres, so it can therefore contract more quickly and exert more force.
The amount of force produced by these fibres is greater than slow twitch ones because there are more muscle fibres in each motor unit.
They can produce energy both aerobically and anaerobically by breaking down carbohydrates to pyruvic acid, but it is more suited to anaerobic respiration, allowing it to release energy very quickly.
The rapid build up of lactic acid (a by-product of anaerobic respiration) lowers the Ph (increased acidity) and has a negative affect on enzyme action, causing the muscle fibre to fatigue quickly.
Type 2b fibres
These are also referred to as fast twitch glycolytic (F.T.G.). These are used for activities of very high intensity and have a much stronger force of contraction. This is because the motor nerve that carries the impulse is much larger. There are generally more fibres within a fast twitch motor unit, and the muscle fibres themselves are larger and thicker.
The nerve also activates a greater number of muscle fibres allowing each motor unit to produce a far greater force than slow twitch fibres. These fibres are very quick to contract, and can exert a large amount of force.
They rely heavily on anaerobic respiration for releasing energy as they have very few mitochondria. Energy is, therefore, released rapidly, but the muscle fibre is also quick to fatigue.