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Muscle fibers can be classified into three types based on two criteria: the speed of contraction and the method of ATP production. Slow oxidative (SO) fibers contract slowly and produce ATP through aerobic respiration, making them fatigue-resistant. Fast oxidative (FO) fibers, also known as intermediate fibers, exhibit characteristics between fast and slow fibers, with faster contractions and primarily aerobic respiration. However, they can switch to anaerobic respiration, leading to quicker fatigue. Lastly, fast glycolytic (FG) fibers have rapid contractions and rely on anaerobic glycolysis, resulting in faster fatigue compared to the other types. While most skeletal muscles contain a mixture of these fiber types, the predominant type in a muscle depends on its primary function.
| Characteristics | Values |
|---|---|
| Name | Fast oxidative (FO) fibers, also known as intermediate fibers |
| Other names | Type IIa fibers, Fast-twitch fibers |
| How fast do fibers contract? | Relatively fast contractions |
| How do fibers regenerate ATP? | Primarily use aerobic respiration, but may switch to anaerobic respiration (glycolysis) |
| Fatigue | Fatigue more quickly than SO fibers |
| Use | Useful for movements that require more energy than postural control but less energy than an explosive movement, e.g. walking |
| Amount of tension | Produce relatively high amounts of tension |
| Amount of mitochondria | Possess high amounts of mitochondria |
| Colour | Red due to the presence of myoglobin |
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What You'll Learn
Fast oxidative fibres are sometimes called intermediate fibres
Muscle fibres can be classified based on two criteria: how fast the fibres contract relative to others, and how they produce ATP. Using these criteria, there are three main types of skeletal muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
Slow oxidative fibres have structural elements that maximize their ability to generate ATP through aerobic metabolism. These fibres contain many more mitochondria than the glycolytic fibres, as aerobic metabolism, which uses oxygen (O2) in the metabolic pathway, occurs in the mitochondria. This allows slow oxidative fibres to contract for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and thus do not produce a large amount of tension. The fact that SO fibres can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small movements that happen often but do not require large amounts of energy. They do not produce high tension, and thus they are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.
FG fibres primarily use anaerobic glycolysis as their ATP source (meaning carbohydrates are their primary fuel source). They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white colour. FG fibres are used to produce rapid, forceful contractions to make quick, powerful movements. These fibres fatigue quickly, permitting them to only be used for short periods. Most muscles possess a mixture of each fibre type. The predominant fibre type in a muscle is determined by the primary function of the muscle.
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Intermediate fibres are used for movements like walking
Muscle fibres can be classified based on two criteria: how fast the fibres contract relative to others, and how they regenerate adenosine triphosphate (ATP) – the energy source for muscle contraction. Using these criteria, there are three main types of skeletal muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles in the human body contain all three types, although in varying proportions.
FO fibres are sometimes called intermediate fibres because they possess characteristics that are intermediate between SO fibres and FG fibres. They produce ATP relatively quickly and can generate relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly.
FO fibres are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, like sprinting. FO fibres are useful for this type of movement because they produce more tension than SO fibres but are more fatigue-resistant than FG fibres.
ATP can be produced through aerobic pathways, which use oxygen, or anaerobic pathways, which do not. If a fibre primarily produces ATP through aerobic pathways, it is classified as oxidative. More ATP can be produced during each metabolic cycle, making the fibre more resistant to fatigue. Oxidative fibres contain many more mitochondria than glycolytic fibres because aerobic metabolism occurs in the mitochondria.
FG fibres primarily use anaerobic glycolysis as their ATP source and have a large diameter and high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white colour. FG fibres are used to produce rapid, forceful contractions to make quick, powerful movements. These fibres fatigue quickly, permitting them to only be used for short periods.
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Intermediate fibres produce ATP quickly
Muscle fibers can be classified based on two criteria: how fast the fibers contract relative to others, and how the fibers regenerate ATP. There are three main types of skeletal muscle fibers: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
Fast oxidative fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between slow oxidative and fast glycolytic fibers. FO fibers produce ATP relatively quickly, more quickly than SO fibers, and can thus produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly.
FO fibers have fast contractions and primarily use aerobic respiration. However, they may switch to anaerobic respiration (glycolysis), causing them to fatigue more quickly than SO fibers. FO fibers are used for movements, such as walking, that require more energy than postural control but less energy than an explosive movement.
In contrast, SO fibers contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They produce low-power contractions over long periods and are slow to fatigue. The fact that SO fibers can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, and stabilizing bones and joints.
FG fibers primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white color. FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods.
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Intermediate fibres can produce high amounts of tension
Muscle fibers can be classified based on two criteria: how fast some fibers contract relative to others, and how fibers produce ATP. Using these criteria, there are three main types of skeletal muscle fibers: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
FO fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers. They produce ATP relatively quickly, more quickly than SO fibers, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly.
FO fibers have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), they can fatigue more quickly than SO fibers. They are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement.
FG fibers primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white color. FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods.
Most muscles possess a mixture of each fiber type. The predominant fiber type in a muscle is determined by the primary function of the muscle.
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Intermediate fibres are oxidative and don't fatigue quickly
Muscle fibres can be classified based on two criteria: how fast they contract relative to others, and how they regenerate adenosine triphosphate (ATP) – the molecule providing energy for muscle contraction. Using these criteria, there are three main types of skeletal muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
Slow oxidative fibres, also called slow-twitch or Type I, contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They contain many mitochondria and are capable of contracting for long periods due to the large amount of ATP they can produce. However, they have a small diameter and do not produce a large amount of tension.
Fast oxidative fibres, also known as fast-twitch or Type IIa, have fast contractions and primarily use aerobic respiration. They are sometimes called intermediate fibres because they possess characteristics that are intermediate between fast and slow fibres. They produce ATP relatively quickly and can thus generate relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly.
Fast glycolytic fibres, also called Type IIx, have fast contractions and primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria and fatigue quickly, permitting them to only be used for short periods.
In summary, intermediate muscle fibres, or fast oxidative fibres, are oxidative and do not fatigue quickly. They produce ATP through aerobic pathways and contain many mitochondria, making them more resistant to fatigue than glycolytic fibres.
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Frequently asked questions
Intermediate muscle fibers, also known as fast oxidative (FO) fibers, are a type of muscle fiber that falls between slow oxidative fibers and fast glycolytic fibers in terms of characteristics. They produce ATP relatively quickly and can generate relatively high amounts of tension. They are oxidative because they produce ATP aerobically and do not fatigue quickly.
Intermediate muscle fibers primarily use aerobic respiration to produce ATP. However, they can also switch to anaerobic respiration (glycolysis) and may fatigue more quickly than slow oxidative fibers.
Intermediate muscle fibers have fast contractions and are used for movements that require more energy than postural control but less energy than an explosive movement, such as walking. They possess a large number of mitochondria and can produce relatively high amounts of tension.
Intermediate muscle fibers possess characteristics that are intermediate between slow and fast muscle fibers. They produce ATP more quickly than slow oxidative fibers, resulting in higher tension contractions. However, they may fatigue more quickly than slow oxidative fibers due to their ability to switch to anaerobic respiration.
