Dominic Stone
Muscle fibres are broadly classified into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). While SO and FO fibres are slow to fatigue, FG fibres fatigue quickly. This is because SO and FO fibres use aerobic metabolism to produce ATP, while FG fibres rely on anaerobic metabolism. As a result, FG fibres produce less ATP per cycle and can only be used for short periods.
| Characteristics | Values |
|---|---|
| Muscle fiber type | Fast glycolytic (FG) |
| Other names | Type IIb, Type IIx, Fast-twitch |
| Metabolic pathway | Anaerobic |
| ATP source | Anaerobic glycolysis |
| Myoglobin content | Low |
| Mitochondria content | Low |
| Blood vessel content | Low |
| Muscle colour | White |
| Diameter | Large |
| Contraction speed | Fast |
| Contraction type | Powerful, high-tension |
| Fatigue rate | Quick |
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What You'll Learn
Fast glycolytic (FG) fibres fatigue quickly
Muscle fibres can be classified as slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). While SO fibres are slow to fatigue, FG fibres are known to fatigue quickly.
FG fibres primarily use anaerobic metabolism to produce powerful, high-tension contractions. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly. This allows FG fibres to produce rapid, forceful contractions associated with quick, powerful movements.
However, because FG fibres rely on anaerobic metabolism, they do not possess a substantial number of mitochondria, a limited capillary supply, or significant amounts of myoglobin. As a result, muscles with large numbers of FG fibres appear white. The lack of mitochondria and myoglobin, which stores O2 within the fibres, makes FG fibres less resistant to fatigue.
FG fibres are more commonly found in muscles that require speed rather than endurance. For example, the muscles in the eyelids that help you blink are all FG fibres. In contrast, muscles that need to function for long periods, such as those in the back of the lower legs and the back, are mostly made up of SO fibres.
The percentage of FG fibres in a muscle can be influenced by training. For example, endurance training can increase the endurance level of FG fibres, although it cannot make them as fatigue-resistant as SO fibres.
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FG fibres use anaerobic metabolism
Muscle fibres can be classified based on two criteria: how quickly they contract and how they regenerate adenosine triphosphate (ATP)—the molecule that provides energy for muscle contraction. There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). FG fibres, also known as Type 2B or fast-twitch fibres, primarily use anaerobic metabolism to produce ATP.
FG fibres use anaerobic glycolysis as their primary ATP source. They have a large diameter and contain high amounts of glycogen, which is used in glycolysis to rapidly generate ATP and produce high levels of tension. This allows FG fibres to produce rapid, forceful contractions to facilitate quick and powerful movements.
Because FG fibres do not rely primarily on aerobic metabolism, they have a lower number of mitochondria and lower amounts of myoglobin compared to other fibre types. Mitochondria are essential for aerobic metabolism as they provide the site for metabolic pathways that utilise oxygen. Myoglobin, on the other hand, is an oxygen-carrying molecule similar to haemoglobin in red blood cells, which helps store and supply oxygen to the mitochondria. The lack of substantial amounts of mitochondria and myoglobin in FG fibres contributes to their white colour.
The process of anaerobic glycolysis produces less ATP per cycle compared to aerobic metabolism. As a result, FG fibres fatigue more quickly than other fibre types and can only be used for short periods. Muscle fatigue occurs when the muscle can no longer contract due to an oxygen debt created by muscle use. FG fibres are more susceptible to fatigue because they rely on anaerobic metabolism, which produces smaller amounts of ATP in the absence of oxygen for short periods.
The type of muscle fibres an individual possesses varies depending on their predominant form of physical activity. People who excel at sprinting tend to have a higher percentage of FG fibres, while those who perform well in endurance sports, such as marathon running, have a higher proportion of SO fibres. Training can influence the characteristics of muscle fibres. For example, endurance training can increase the endurance level of FG fibres, making them more resistant to fatigue.
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Slow-twitch fibres are more fatigue-resistant
Muscle fibres are broadly classified into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). While FG fibres are known to fatigue quickly, SO fibres are slow to fatigue.
Slow-twitch fibres, also known as type I fibres, are slow to fatigue. They are used to power muscles during low-intensity activities and are the first to be activated when a muscle contracts. Slow-twitch fibres use an aerobic energy system, meaning they run on oxygen. They have a higher number of mitochondria than fast-twitch fibres, which allows them to generate ATP through aerobic metabolism and produce a more sustained energy output. This enables them to contract and work for long periods without fatiguing.
Slow-twitch fibres are commonly found in muscles that need to maintain posture and perform isometric contractions, such as the muscles in the back of the lower legs. They are also prevalent in endurance athletes, such as long-distance runners and cyclists, who excel in endurance sports due to their higher proportion of slow-twitch fibres.
The percentage of slow-twitch fibres in muscles can vary significantly between individuals. For example, the quadriceps femoris muscles in the legs can range from under 20% in sprinters to as high as 95% in marathon runners. Training can also influence the endurance level of slow-twitch fibres, with endurance training shown to increase their endurance capacity.
In summary, slow-twitch muscle fibres are more fatigue-resistant due to their reliance on an aerobic energy system, higher mitochondrial count, and ability to generate sustained energy output over extended periods.
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Slow oxidative (SO) fibres are useful for maintaining posture
Slow oxidative (SO) fibres are one of the three types of muscle fibres, the other two being fast oxidative (FO) and fast glycolytic (FG). SO fibres are useful for maintaining posture because they can function for long periods without fatiguing. They contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. This makes them ideal for producing isometric contractions, stabilizing bones and joints, and making small movements that occur frequently but do not require large amounts of energy.
SO fibres have a rich capillary supply, numerous mitochondria, and aerobic respiratory enzymes. They also have a high concentration of myoglobin, a red pigment similar to hemoglobin that improves oxygen delivery to the slow-twitch fibres. This high myoglobin content gives SO fibres their distinctive red colour. The abundance of mitochondria and oxygen-dependent metabolism allow SO fibres to generate large amounts of ATP through aerobic metabolism, making them more resistant to fatigue.
In contrast, FO fibres produce ATP relatively quickly and can generate higher amounts of tension compared to SO fibres. They are used primarily for movements that require more energy than postural control, such as walking, but less energy than explosive movements like sprinting. While FO fibres do not fatigue as quickly as FG fibres, they can switch to anaerobic respiration (glycolysis) and may fatigue more rapidly than SO fibres.
FG fibres, on the other hand, rely on anaerobic metabolism and primarily use glycolysis as their ATP source. They have a large diameter and high glycogen content, which enables rapid ATP generation and powerful contractions. However, their anaerobic nature limits the amount of ATP produced per cycle, resulting in quicker fatigue. FG fibres are used for short-duration, powerful movements.
The proportions of these three fibre types vary across different muscles in the body. Muscles that require endurance, such as those in the lower legs and back, tend to have a higher percentage of SO fibres, while muscles that require speed and powerful movements, like those involved in blinking, have a higher proportion of FG fibres. Training can also influence the characteristics of these fibres, improving their endurance or power output.
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SO fibres have ample fuel supplies
Slow oxidative (SO) fibres are slow to fatigue and can function for long periods. They use aerobic metabolism to produce low-power contractions over long periods. SO fibres have a high oxidative capacity and can generate ATP through aerobic metabolism. This means they run on oxygen, which is provided by a robust supply of blood vessels.
SO fibres are used for maintaining posture, producing isometric contractions, and stabilizing bones and joints. They are also used for low-intensity activities that require a steady and even supply of energy. SO fibres can be found in the muscles in the back of the lower legs and the back, as these muscle groups need to help the body maintain posture for long periods.
SO fibres have more mitochondria than fast glycolytic (FG) fibres. This is because aerobic metabolism, which uses oxygen, occurs in the mitochondria. FG fibres, on the other hand, rely on anaerobic metabolism and do not have a substantial number of mitochondria.
The fuel selection of muscle fibres depends on the intensity of exercise and the availability of fuels. During exercise at maximum intensity, the main fuels are PCr and muscle glycogen, with the highest energy release occurring with type II fibres. When exercise intensity falls below 60% VO2 max, fat becomes the dominant fuel during prolonged exercise, and the recruitment pattern shifts toward type I (SO) fibres.
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Frequently asked questions
Fast glycolytic (FG) fibers fatigue the quickest out of the three types of muscle fibers.
FG fibers primarily use anaerobic metabolism to produce powerful, high-tension contractions. This means they do not possess a substantial number of mitochondria, resulting in a limited amount of energy and quicker fatigue.
The other two types of muscle fibers are slow oxidative (SO) and fast oxidative (FO). SO fibers are slow to fatigue, while FO fibers can fatigue more quickly than SO fibers.
Muscle fibers produce energy through aerobic and anaerobic processes. Aerobic metabolism uses oxygen to produce ATP, the body's energy source, and is used by SO and FO fibers. Anaerobic metabolism does not require oxygen and is used by FG fibers.
