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Skeletal muscle fibres can be classified in a number of ways, including how fast they contract relative to others, how they regenerate ATP, and their twitch capabilities. Using these criteria, there are three main types of skeletal muscle fibres: slow oxidative (also called slow twitch or Type I), fast oxidative (also called fast twitch or Type IIa), and fast glycolytic (also called fast twitch or Type IIx).
| Characteristics | Values |
|---|---|
| Speed of contraction | Slow, Fast |
| Use of oxygen | Yes, No |
| Mitochondria density | High, Low |
| Colour | Red, White |
| Myoglobin content | High, Low |
| Capillary density | High, Low |
| Twitch capabilities | Slow, Fast |
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What You'll Learn
Slow oxidative (SO) fibres
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 2 is further broken down into subtypes. Slow oxidative (SO) fibres, also known as slow-twitch or type I fibres, are a type of skeletal muscle fibre that contracts relatively slowly and uses aerobic respiration (oxygen and glucose) to produce ATP. They are called 'slow oxidative' because they use oxidative metabolism to generate ATP (adenosine triphosphate). They are also called 'slow-twitch' because they contract relatively slowly compared to other types of muscle fibres.
Slow oxidative fibres have a higher density of energy-generating organelles called mitochondria, which makes them appear darker in colour. They also have greater local capillary density. These characteristics make them well-suited for endurance and slow to fatigue.
Slow oxidative fibres are often compared to fast oxidative (fast-twitch or type IIa) fibres, which have relatively fast contractions and primarily use aerobic respiration to generate ATP. They are also sometimes compared to fast glycolytic (fast-twitch or type IIx) fibres, which have relatively fast contractions and primarily use anaerobic glycolysis.
The classification of muscle fibres as slow or fast twitch is based on their twitch capabilities, which refer to how fast the fibres contract relative to others. This classification largely, but not completely, overlaps with the traditional classification based on colour, which reflects the myoglobin content of the fibres. Type I fibres appear red due to high levels of myoglobin, while type II fibres are white due to relatively low myoglobin levels.
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Fast oxidative fibres
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 2 is further broken down into subtypes. Type 1 fibres are also known as slow oxidative (SO) or slow twitch fibres. They contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Type 1 fibres have a higher density of energy-generating organelles called mitochondria. This makes them dark.
Type 2 fibres are further classified into type 2A and type 2B. Type 2A fibres can also use oxygen to generate energy for movement. However, they contain less mitochondria, making them light. Type 2B fibres don’t use oxygen to generate energy. Instead, they store energy that can be used for short bursts of movement. They contain even less mitochondria than type 2A fibres and appear white.
Muscle fibres can be further classified based on the myosin heavy chain isoforms that they express. Many other components contribute to a fibre’s physiological characteristics. The type of muscle fibre can have a profound impact on muscle diseases, including certain muscular dystrophies and sarcopenia, the aging-induced loss of muscle mass and strength.
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Fast glycolytic fibres
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 1 muscle fibres are also known as slow oxidative (SO) fibres or slow twitch fibres. They contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Type 1 fibres have a higher density of energy-generating organelles called mitochondria, which makes them dark.
Type 2 muscle fibres are further broken down into subtypes. Type 2A fibres can also use oxygen to generate energy for movement, but they contain less mitochondria, making them light. Type 2B fibres don't use oxygen to generate energy. Instead, they store energy that can be used for short bursts of movement. They contain even less mitochondria than type 2A fibres and appear white.
The classification of muscle fibres is important for understanding muscle diseases and their treatment. For example, some muscle diseases may be treated by shifting fibre type characteristics from slow to fast, or fast to slow phenotypes, depending on the disease.
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Slow twitch fibres
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 1 muscle fibres, also known as slow twitch fibres, contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They have a higher density of energy-generating organelles called mitochondria, which makes them dark in colour. They also have greater local capillary density. These fibres are more suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP.
The classification of muscle fibres as slow twitch or fast twitch is based on their twitch capabilities, or how fast the fibres contract relative to others. Slow twitch fibres contract relatively slowly, while fast twitch fibres contract relatively quickly. This classification largely, but not completely, overlaps with the classifications based on colour, ATPase, or MHC (myosin heavy chain).
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Fast twitch fibres
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 2 is further broken down into subtypes. Type 1 fibres are also known as slow oxidative (SO) or slow twitch fibres. They contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Type 2 fibres are further categorised into type 2A and type 2B.
Type 2A fibres are also known as fast oxidative (FO) or fast twitch fibres. They have relatively fast contractions and primarily use aerobic respiration to generate ATP. Type 2A fibres can also use oxygen to generate energy for movement. However, they contain less mitochondria than type 1 fibres, making them light.
Type 2B fibres are also known as fast glycolytic (FG) or fast twitch fibres. They have relatively fast contractions and primarily use anaerobic glycolysis to generate energy. Unlike type 1 and type 2A fibres, type 2B fibres don't use oxygen to generate energy. Instead, they store energy that can be used for short bursts of movement. They contain even less mitochondria than type 2A fibres and appear white.
Fast-twitch fibres are less oxidative than slow-twitch fibres and have relatively low myoglobin content. They are white in colour and rely on glycolytic enzymes. Fast-twitch fibres contract and relax more quickly than slow-twitch fibres, making them useful for activities that require short bursts of energy, such as sprinting or jumping.
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Frequently asked questions
Muscle fibres are classified into two types: type 1 and type 2. Type 2 is further broken down into subtypes.
Type 1 muscle fibres are slow oxidative fibres that contract relatively slowly and use aerobic respiration to produce ATP. Type 2 fibres are fast oxidative fibres that contract relatively quickly and primarily use aerobic respiration to generate ATP.
Type 2 muscle fibres are further classified into type 2A and type 2B. Type 2A fibres can use oxygen to generate energy for movement, but they contain less mitochondria than type 1 fibres. Type 2B fibres don't use oxygen to generate energy and instead store energy for short bursts of movement. They contain even less mitochondria than type 2A fibres.
