The motor neurone stimulating this fibre has a thicker myelin sheath than the slow twitch fibre, so it can contract more quickly and exert more force. There are lots of muscle fibres in each motor unit. This fibre type can produce energy both aerobically and anaerobically by the breakdown of carbohydrates to pyruvic acid, but it is more suited to anaerobic respiration, which means that energy can be released very quickly. There is a rapid build up of Lactic acid, which lowers the pH, and has a negative effect on enzyme action, causing the muscle fibre to fatigue too quickly.
Type 3:Fast Glycolytic Fibres.
These muscle 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 few mitochondria. This means that energy is rapidly released but that the muscle tires quickly. The motor neurones supplying this fibre type are large and this increases the contractile speed. The neurone also activates a greater number of muscle fibres meaning that each motor unit can produce much more force than the slow oxidative motor units.
There are many distinguishing characteristics associated with each muscle fibre type:
A neurone forms the basic cellular unit of the nervous system. It is capable of carrying nerve impulses to muscles. A motor neurone controls large numbers of individual fibres, which together form a motor unit. A slow oxidative fibre contains between 10 and 180 fibres per motor neurone. In relation to the other muscle types, this number is small. This is due to a small motor neurone size. They have a slow type of myosin ATPase which causes the slow process of aerobically releasing energy. The low sarcoplasmic reticulum development means that less calcium ions are released. This means that less reactions are able to take place between the troponin and the calcium due to less calcium ions being present. This results in low contractile speed, which means that movement is slower, hence the name slow witch fibre.
In terms of function, a slow oxidative fibre has a high aerobic capacity. This is due to the large number of mitochondria found in this muscle type. Mitochondria are the site at which aerobic respiration takes place in a cell, and provide energy for muscle contraction via ATP. This means that the anaerobic capacity of this fibre is low. The fatigue resistance is very high because they have a high aerobic capacity. This means that lactic acid that may accumulate in the muscle can be oxidised to prevent fatigue. However, the strength of a motor unit is low which means that the force produced is not as high as the force found in fast twitch fibres.
Fast oxidative glycolytic fibres contain between 300 and 800 fibres per motor neurone. This is because the size of the motor neurone is large. The type of myosin ATPase is fast which results in the rapid breakdown and release of energy. The development of the sarcoplasmic reticulum is high, which allows the release of more calcium ions than a slow twitch fibre. This means that the troponin can bind with the calcium to create a fast contractile speed, and high motor unit strength. The aerobic capacity of this type of muscle fibre is moderate because there are mitochondria present, but not as many as found in a slow twitch fibre. However, due to the presence of mitochondria, fast oxidative glycolytic fibres are able to respire aerobically. The anaerobic capacity is high because glycogen stores in the muscle are present. This causes an increased number of glycogen-converting enzymes, which assist in the conversion of pyruvic acid into lactic acid. These enzymes are found in the fibres that control the anaerobic phase of the glucose breakdown, in addition to larger amounts of ATP being stored. The fatigue resistance is moderate because the presence of mitochondria is fewer than in slow twitch fibres. However, due to the depletion of high-energy phosphates and glycogen stores, and accumulation of carbon dioxide and lactic acid, there is an increase in hydrogen ions that create fatigue in the muscles. The strength of each motor unit is high, which means that the force exerted by this type of muscle fibre is high.
Fast glycolytic fibres contain extremely similar characteristics to those found in a fast oxidative glycolytic fibre. They contain between 300 and 800 fibres per motor neurone. It is able to hold so many because the size of the motor neurone is large. The type of myosin ATPase found within the fast glycolytic fibre is very fast and the sarcoplasmic reticulum development is high. In terms of function, the aerobic capapcity of this type of fibre is low, due to the presence of only a few mitochondria. Therefore, the anaerobic capacity is very high, infact, the muscle fibre relies on this type of respiration very heavily. The contractile speed found in a fast glycolytic fibre is fast because the motor neurones are large and are able to exert a large force. The fatigue resistance is low because there are not many mitochondria found in the fibre but energy is rapidly released, so the muscle tires quickly. However, the strength of each motor unit is high. This is because the neurone is able to activate a greater number of muscle fibres, meaning that each motor unit is very strong and can create a high force.
“The distribution of muscle fibre type can help to determine the type of sporting activity an individual is most suited to”.
In terms of my sport, long jump fast twitch muscle fibres dominate. This is because this activity requires large amounts of instant energy, lasting for only a few seconds, with full recovery between repetitions. This is due to the explosive nature of the event. The movements are rapid and require maximum force with precision timing. One reason for this is that an adaption occurs in the stores of ATP and PC within the muscle cell sarcoplasm are increased. This causes more energy to be available more rapidly and increases the maximum possible peak power output of a muscle.
