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Muscle fibres are classified into three types: slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibres use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. Muscle fibre types can have a profound impact on muscle diseases, including certain muscular dystrophies and sarcopenia, the aging-induced loss of muscle mass and strength.
| Characteristics | Values |
|---|---|
| Types | Slow oxidative (SO), fast oxidative (FO), fast glycolytic (FG) |
| Fatigue | Slow oxidative fibres are slow to fatigue |
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What You'll Learn
Muscle fibre types
Muscle fibres are classified into three types: slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. Muscle fibre types are generally defined by the particular myosin heavy chain isoforms that they express, but many other components contribute to a fibre’s physiological characteristics.
Skeletal muscle fibres are classified into slow twitch versus fast twitch. Slow-twitch fibres correlate with insulin responsiveness. Muscle fibre type can have a profound impact on muscle diseases, including certain muscular dystrophies and sarcopenia, the aging-induced loss of muscle mass and strength.
Alpha motor neurons innervate extrafusal muscle fibres and are the primary means of skeletal muscle contraction. Gamma motor neurons innervate muscle spindles and dictate their sensitivity. ATP provides the energy for muscle contraction. Creatine phosphate provides about the first 15 seconds of ATP at the beginning of muscle contraction. Aerobic metabolism utilises oxygen to produce much more ATP, allowing a muscle to work for longer periods.
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Muscle fatigue
Muscle fibres are sensitive to fatigue. Muscle fatigue is a common complaint in clinical practice and can be defined as an exercise-induced decrease in the ability to produce force. It can be caused by many different mechanisms, ranging from the accumulation of metabolites within muscle fibres to the generation of an inadequate motor command in the motor cortex. There is no global mechanism responsible for muscle fatigue, but there are two main causes: the limitations of a nerve’s ability to generate a sustained signal (neural fatigue); and the reduced ability of the muscle fibre to contract (metabolic fatigue).
SO fibres are slow to fatigue.
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Muscle diseases
Muscle fibres are classified into three types: slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibres use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. The type of muscle fibre can have an impact on muscle diseases, including muscular dystrophies and sarcopenia, the aging-induced loss of muscle mass and strength.
Recent studies have suggested that shifting the characteristics of muscle fibres from slow to fast, or fast to slow, may be a potential treatment for some muscle diseases. This could involve altering the myosin heavy chain isoforms that are expressed in the fibres, as well as other physiological components. By understanding the susceptibility and resistance of different muscle fibre types to disease, researchers hope to develop more effective treatments.
The aetiology of obesity has also been linked to muscle fibre type, with slow-twitch fibres correlating with insulin responsiveness. This suggests that muscle fibre type may play a role in metabolic disorders and could be a target for interventions aimed at preventing or treating obesity. Further research is needed to fully understand the complex relationship between muscle fibre type and disease susceptibility.
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Muscle fibre and obesity
There are three types of muscle fibre: slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibres use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. Muscle fibre type can have a profound impact on muscle diseases, including certain muscular dystrophies and sarcopenia, the aging-induced loss of muscle mass and strength.
Recent studies have begun to address which components of muscle fibre types mediate their susceptibility or resistance to muscle disease. It has been found that the slow-twitch fibre proportion in skeletal muscle correlates with insulin responsiveness. This suggests that some muscle diseases may be treated by shifting fibre type characteristics either from slow to fast, or fast to slow phenotypes, depending on the disease.
Obesity is a condition characterised by excess body fat. It is often associated with insulin resistance, which can lead to type 2 diabetes. As such, the relationship between muscle fibre type and insulin responsiveness may be relevant to understanding obesity and its associated health risks.
Further research is needed to fully understand the role of muscle fibre type in obesity and other diseases. However, it is clear that muscle fibre type plays an important role in muscle function and health.
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Muscle fibre and diabetes
Muscle fibres are classified into different types, including slow twitch and fast twitch, slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). The sensitivity of muscle fibres is dictated by gamma motor neurons.
Muscle fibre type can have a profound impact on muscle diseases, including certain muscular dystrophies and sarcopenia, the aging-induced loss of muscle mass and strength. In terms of diabetes, studies suggest that insulin-mediated glucose metabolism is different between muscle fibre types. Type I fibres compared with type II fibres have higher protein levels of the insulin receptor, GLUT4, hexokinase II, glycogen synthase (GS), and pyruvate dehydrogenase-E1α (PDH-E1α) and a lower protein content of Akt2, TBC1 domain family member 4 (TBC1D4), and TBC1D1. Type I fibres have a higher glucose-handling capacity but a similar sensitivity for phosphoregulation by insulin.
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Frequently asked questions
There are three types of muscle fibre: slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG).
SO fibres use aerobic metabolism to produce low power contractions over long periods.
SO fibres are slow to fatigue.
FO fibres use aerobic metabolism to produce high power contractions over short periods.
