Logan Steele
There are three types of muscle fibres: slow oxidative, fast oxidative, and fast glycolytic. Slow oxidative fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. They have a high number of mitochondria and are capable of contracting for longer periods because they can produce a large amount of ATP. Fast oxidative fibres also use aerobic metabolism to produce ATP but produce higher-tension contractions than slow oxidative fibres. Fast glycolytic fibres use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly.
| Characteristics | Values |
|---|---|
| Types | Slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG) |
| Muscle fiber composition | All three types, in varying proportions |
| Contraction speed | Slow, fast, and fast respectively |
| ATP production | Low power, high tension, and powerful high tension respectively |
| Fatigue | Slow to fatigue, can fatigue more quickly, and fatigues quickly respectively |
| Use | Posture, isometric contractions, and bone and joint stabilization; walking and other movements requiring more energy than posture but less than sprinting; and explosive movements like sprinting |
| Mitochondria | High amounts, moderate amounts, and low amounts respectively |
| Capillaries | Extensively supplied, moderately supplied, and less supplied respectively |
| Myoglobin | High amounts, low amounts, and low amounts respectively |
| Colour | Dark red, light red, and white respectively |
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What You'll Learn
Slow oxidative fibres
The structural composition of slow oxidative fibres makes them well-suited for maintaining posture, producing isometric contractions, and stabilizing bones and joints. They are typically found in muscles that need to be active for extended periods, such as the legs and thighs of a turkey, which allow the bird to walk around all day in search of food. These fibres are also known as red muscle due to the presence of myoglobin, a red pigment similar to hemoglobin in red blood cells.
In contrast to fast-twitch fibres, slow oxidative fibres produce low-power contractions and are slow to fatigue. They have a relatively small diameter, resulting in lower tension generation compared to fast-twitch fibres. Slow oxidative fibres are classified based on the presence of slow myosin and their high degree of oxidative phosphorylation. Training that places high metabolic demands on muscles, such as endurance training, can increase the oxidative capacity of slow oxidative fibres by boosting the number of mitochondria and improving capillarization.
Studies have shown that slow oxidative fibres exhibit distinct responses during aging and obesity compared to fast-twitch glycolytic fibres. While muscles dominated by oxidative fibres maintain their proportion during aging and obesity, muscles with a high content of glycolytic fibres experience a significant decrease in the proportion of oxidative fibres. Additionally, slow oxidative fibres demonstrate elevated expression of antioxidant proteins, while glycolytic fibres exhibit increased expression of genes associated with muscle atrophy and inflammation.
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Fast oxidative fibres
Muscle fibres can be classified based on two criteria: how fast the fibres contract relative to others, and how the fibres regenerate ATP. 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 used primarily for movements, such as walking, that require more energy than postural control but less energy than explosive movements, such as 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. They are sometimes called intermediate fibres because they possess characteristics that are intermediate between slow oxidative fibres and fast glycolytic fibres.
FO fibres are similar in speed to FG fibres but contain more mitochondria and oxidative enzymes. They are also larger in diameter, have a greater blood supply, and more endurance than typical fast-twitch fibres. Most of the body's muscles are composed of these intermediate fibres.
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Fast glycolytic fibres
The percentage of fast-twitch fibres in the body varies between individuals and is determined by genetics. People who excel at sprint events tend to have a higher number of fast-twitch muscle fibres. Fast glycolytic fibres can be converted to fast oxidative-glycolytic fibres through endurance training. These intermediate fibres have more mitochondria and a greater blood supply than typical fast-twitch fibres, resulting in greater endurance.
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Muscle fatigue
There are two main causes of muscle fatigue: neural fatigue and metabolic fatigue. Neural fatigue is caused by limitations on a nerve's ability to generate a sustained signal. Metabolic fatigue is caused by a reduced ability of the muscle fiber to contract due to a shortage of, or inability to metabolize, fuel (substrates) within the muscle fiber, resulting in a low ATP reservoir.
ATP (adenosine triphosphate) binds to the myosin head and causes contraction. Creatine phosphate stores energy so that ATP can be rapidly regenerated within the muscle cells from ADP (adenosine diphosphate) and inorganic phosphate ions, allowing for sustained powerful contractions. Glycogen is the intramuscular storage form of glucose, used to generate energy quickly as intramuscular phosphocreatine stores become exhausted, producing lactic acid as a metabolic byproduct.
Substrate shortage is one of the causes of metabolic fatigue. Substrates are depleted during exercise or are unable to be metabolized, resulting in a lack of intracellular energy sources to fuel contractions. The accumulation of metabolites can directly or indirectly produce metabolic fatigue within muscle fibers through interference with the release of calcium (Ca2+) from the sarcoplasmic reticulum or reduction of the sensitivity of contractile molecules actin and myosin to calcium.
Oxidative stress biomarkers (ROS) such as lipid peroxidation, protein peroxidation, and antioxidative capacity are also associated with muscle fatigue. ROS contribute to fatigue through the oxidation of cell proteins, leading to the inhibition of SR Ca2+ release and myofibrillar Ca2+ sensitivity.
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Endurance training
There are three types of muscle fibres: slow oxidative, fast oxidative, and fast glycolytic. Slow oxidative fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. They are able to function for long periods without fatiguing, making them useful in maintaining posture and stabilizing bones and joints.
In a study on mice, orchiectomy (ORX)-induced metabolic alterations were restored by endurance training. The mice in the training group performed 60 minutes of treadmill running 5 days a week for 5 weeks.
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Frequently asked questions
The three types of muscle fibers are slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
Slow oxidative fibers use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. They contain many more mitochondria than glycolytic fibers, and aerobic metabolism occurs in the mitochondria. This allows them to contract for longer periods.
Fast oxidative fibers use aerobic metabolism to produce ATP and higher tension contractions than slow oxidative fibers. They are used primarily for movements that require more energy than postural control but less energy than explosive movements.
