Athena Vale
Muscle fibres can be classified into two types: slow-twitch and fast-twitch. Slow-twitch muscle fibres are used for endurance or long-lasting energy, while fast-twitch muscle fibres provide sudden bursts of energy but get tired quickly. The speed of contraction and regeneration of ATP determines the type of muscle fibre. Slow oxidative fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue, whereas fast glycolytic fibres use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly. The natural makeup of muscle fibres influences athletic performance and response to training.
| Characteristics | Values |
|---|---|
| Muscle fiber type | Fast glycolytic (FG) fibers |
| Other names | Fast-twitch fibers, Type II fibers, Type IIx and IIa fibers |
| Appearance | Lighter color due to less blood supply |
| Function | Short, powerful movements |
| Energy source | Anaerobic glycolysis |
| Fatigue rate | Quick to fatigue |
| Examples of activities | Sprinting, powerlifting, Olympic lifting, plyometrics |
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What You'll Learn
Slow-twitch vs. fast-twitch fibres
Muscle fibres can be classified based on two criteria: how fast they contract and how they regenerate adenosine triphosphate (ATP), the body's energy currency.
Slow-twitch muscle fibres, also known as slow oxidative (SO) fibres, contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They produce low-power contractions over long periods and are resistant to fatigue. Slow-twitch fibres are used for endurance activities such as distance running, swimming, cycling, hiking, low-to-moderate intensity dancing, and walking. They are also useful in maintaining posture, producing isometric contractions, and stabilizing bones and joints.
On the other hand, fast-twitch muscle fibres, also known as fast glycolytic (FG) fibres, contract quickly and produce powerful, high-tension contractions. However, they fatigue more quickly. Fast-twitch fibres use anaerobic metabolism to create energy and are responsible for short, powerful movements such as sprinting, powerlifting, and plyometrics. They have a large diameter and high amounts of glycogen, which is used in glycolysis to generate ATP quickly. Because of their reliance on anaerobic metabolism, these fibres do not possess substantial numbers of mitochondria, have a limited capillary supply, and lower amounts of myoglobin, resulting in a white coloration for muscles containing large numbers of these fibres.
It is important to note that some muscle fibres are hybrids, expressing both slow-twitch and fast-twitch types. The ratio of these fibres can vary among individuals, with some people having a higher proportion of one type, which may influence their natural athletic abilities. For example, those with a higher percentage of slow-twitch fibres may be better at endurance sports, while those with more fast-twitch fibres may excel at sprinting or weightlifting. Training can also influence the performance of these fibres, with endurance training improving the endurance of fast-twitch fibres and sprint training enhancing the power generated by slow-twitch fibres.
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Anaerobic glycolysis vs. aerobic metabolism
Muscle fatigue is closely related to the concentration of lactate and the resulting acidosis within muscle tissue. Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen are available. This process occurs during high-intensity exercises, such as lifting heavy objects, or in disease states like sepsis. Anaerobic glycolysis can provide energy for a short duration, typically ranging from 10 seconds to 2 minutes, until it is limited by the buildup of lactate, leading to fatigue.
During anaerobic glycolysis, one molecule of glucose is broken down into two molecules of pyruvate and NADH+. Pyruvate can then be converted into lactate or enter the tricarboxylic acid (TCA) cycle after converting into Acetyl-CoA. The accumulation of lactate in the muscle tissue leads to acidosis, which is a significant contributor to muscle fatigue.
Aerobic metabolism, on the other hand, is a more efficient process that occurs during normal, daily activities and involves the cells making energy through glycolysis, the citric acid cycle, and electron transport/oxidative phosphorylation. This process produces ATP (adenosine triphosphate), which is essential for muscle contraction. While aerobic metabolism also produces lactate, it does so in the presence of oxygen, preventing the accumulation of lactate and acidosis.
The relative contribution of these metabolic pathways depends on the intensity and duration of exercise. For example, high-intensity intervals can turn endurance running, which is typically an aerobic exercise, into an anaerobic activity. Additionally, the lactate threshold, which is the point at which lactic acid builds up, can be influenced by specific training programs, allowing athletes to improve their performance and delay muscle fatigue.
In summary, anaerobic glycolysis and aerobic metabolism are both involved in energy production, but they differ in their efficiency, the duration of energy provision, and their impact on muscle fatigue. Anaerobic glycolysis leads to muscle fatigue faster due to the rapid buildup of lactate and resulting acidosis, while aerobic metabolism is a more sustainable and efficient process that avoids the accumulation of lactate.
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Muscle colour
Slow oxidative (SO) fibres are red in colour due to the high presence of myoglobin, an oxygen-carrying molecule similar to haemoglobin in red blood cells. SO fibres have a rich capillary supply, numerous mitochondria and aerobic respiratory enzymes, and a high concentration of myoglobin. They 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.
Fast oxidative (FO) fibres are sometimes called intermediate fibres because they possess characteristics that are intermediate between fast and slow fibres. They are oxidative because they produce ATP aerobically, and possess high amounts of mitochondria, but they do not possess significant myoglobin, giving them a lighter colour than the red SO fibres. FO fibres produce ATP relatively quickly, more quickly than SO fibres, and thus can produce relatively high amounts of tension. They do not fatigue quickly.
Fast glycolytic (FG) fibres are white in colour due to their lack of mitochondria and myoglobin. They have a large diameter and possess large volumes of glycogen, which is used in glycolysis to generate ATP quickly. FG fibres use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly.
The speed of contraction is dependent on how quickly myosin’s ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres hydrolyze ATP approximately twice as rapidly as slow fibres, resulting in much quicker cross-bridge cycling.
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Muscle contractions
Slow oxidative fibres use aerobic metabolism to produce ATP, which results in low-power contractions over long periods with slow fatigue. They have a rich capillary supply, numerous mitochondria, and high concentrations of myoglobin, giving them a redder or darker appearance. These fibres are useful for maintaining posture, producing isometric contractions, and stabilizing bones and joints.
Fast oxidative fibres, also known as intermediate fibres, possess a combination of characteristics from slow oxidative and fast glycolytic fibres. They produce ATP relatively quickly through aerobic metabolism and can generate relatively high amounts of tension. Due to their oxidative nature, they do not fatigue quickly. These fibres are used for movements that require more energy than postural control but less energy than explosive movements, such as walking.
Fast glycolytic fibres use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly. They have a large diameter and high amounts of glycogen, which is used to generate ATP rapidly. These fibres are used for short, powerful movements, such as powerlifting or sprinting.
The speed of muscle contractions is influenced by the type of muscle fibres predominant in the individual. Slow-twitch fibres contract slowly but fatigue slowly, making them ideal for endurance activities such as distance running, cycling, or swimming. On the other hand, fast-twitch fibres contract quickly but fatigue rapidly, excelling in activities that require bursts of power and strength, such as weightlifting or sprinting.
The type of muscle fibres an individual possesses can influence their athletic performance and natural predispositions. Training can also modify muscle fibres to some extent, improving their endurance or power generation capabilities.
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Training and fatigue resistance
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 that provides energy for muscle contraction.
Slow oxidative (SO) fibres 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. On the other hand, fast glycolytic (FG) fibres use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly.
The different muscle fibres can be influenced by training. For example, sprint training can improve the power generated by slow-twitch fibres, and endurance training can increase the endurance level of fast-twitch fibres. However, training cannot make slow-twitch fibres as powerful as fast-twitch fibres, nor can it make fast-twitch fibres as fatigue-resistant as slow-twitch fibres.
Interval training, which consists of a number of exercise bouts alternated with short rest intervals of slower-paced activity, is thought to improve the fatigue resistance of active muscles by exposing them to sustained, high-intensity exercise.
Strength training has also been shown to improve fatigue resistance and self-rated health in workers with chronic pain. In one study, time to fatigue increased by 97% following strength training.
Resistance training with progressive loads is the most effective program for managing the symptoms of fatigue in the workplace.
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Frequently asked questions
Fast-twitch muscle fibers are the muscle cells responsible for short, powerful movements. They contract quickly but tire rapidly.
Slow-twitch muscle fibers are all about endurance or long-lasting energy. They contract slowly but also tire slowly.
Fast-twitch muscle fibers fatigue faster.
Fast-twitch muscle fibers use anaerobic metabolism to create energy. They receive less blood flow and oxygen and can only produce force for short periods of time. They also have a limited capillary supply and significant amounts of myoglobin, which results in a white coloration. On the other hand, slow-twitch muscle fibers have a bigger blood supply and are powered by oxygen from the blood, making them more resistant to fatigue.
