Dominic Stone
Skeletal muscle is the most common type of muscle in the human body, comprising 30% to 40% of total body mass. These muscles are made up of bundles of muscle fibres, which are classified into different types, including slow-twitch (type 1) and fast-twitch (type 2). Slow-twitch fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue, while fast-twitch fibres produce rapid, forceful contractions for quick, powerful movements but fatigue quickly. The three subtypes of fast-twitch fibres are further delineated by their metabolism and contraction speeds. RT muscle fibres are those that have undergone resistance training, which can lead to hypertrophy and changes in muscle protein composition.
| Characteristics | Values |
|---|---|
| Types | Slow oxidative (SO), fast oxidative (FO), fast glycolytic (FG) |
| Other Names | Muscle fiber cells, myocytes |
| Composition | Myofibrils, actin, myosin filaments (myofilaments), sarcomeres |
| Appearance | Striated, striped, red and white |
| Diameter | 0.5–3 inches |
| Length | 2–3 cm |
| Function | Contraction, movement, maintaining body posture, controlling body temperature, stabilizing joints |
| Number of Skeletal Muscles in the Human Body | More than 600 |
| Percentage of Body Weight | 30–40% |
| Sex Differences | Males have more skeletal muscle mass than females |
| Training | High-intensity resistance training leads to changes in fiber type |
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What You'll Learn
Skeletal muscle fibres
Skeletal muscles are one of the three types of vertebrate muscle tissue, the others being cardiac and smooth muscle. They are part of the voluntary muscular system and are attached by tendons to bones. Skeletal muscle cells are long and are also known as muscle fibres. Each muscle fibre is surrounded by a type of connective tissue layer called fascia.
The tissue of a skeletal muscle is striated, meaning it has a striped appearance due to the arrangement of the sarcomeres. Skeletal muscle contains multiple fascicles, or bundles of muscle fibres. These muscle fibres are composed of myofibrils, which are made up of actin and myosin filaments called myofilaments. These are repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fibre necessary for muscle contraction.
There are three types of skeletal muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, although in varying proportions. SO fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. FG fibres, on the other hand, produce rapid, forceful contractions to make quick, powerful movements but fatigue quickly and can only be used for short periods.
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Slow-twitch vs fast-twitch
Slow-twitch and fast-twitch muscle fibres, also known as type I and type II muscle fibres, respectively, are the two main types of muscle fibres in skeletal muscles. They are involved in controlling the physical forces within the body and are responsible for our movements.
Slow-twitch muscle fibres are endurance muscle fibres that are used for daily activities such as walking, jogging, and other light to moderate-intensity activities. They rely on blood and oxygen to produce a type of fuel called adenosine triphosphate (ATP) for energy to function. These muscle fibres are fatigue-resistant and primarily facilitate smaller ranges of motion. They contain a lot of blood-carrying myoglobin, giving them a red colour, and are the first to activate when a muscle contracts.
Fast-twitch muscle fibres, on the other hand, are necessary for speed and power. They are used for high-intensity activities such as sprinting, jumping, powerlifting, and sports like soccer. These muscle fibres need fuel quickly and rely on carbohydrates stored in the muscles, rather than oxygen, for ATP energy production. While this type of fuel can be produced rapidly, it is limited in quantity, causing fast-twitch muscles to tire out more quickly. Fast-twitch muscles also provide more muscle mass and definition and have a greater potential to grow with exercise than slow-twitch muscle fibres.
The proportion of slow-twitch to fast-twitch muscle fibres varies from person to person and is primarily determined by genetics and age. Individuals who regularly engage in activities requiring quick movements tend to have a larger proportion of fast-twitch muscle fibres, while endurance runners, for example, may have a higher proportion of slow-twitch muscle fibres. Additionally, muscle fibres can adapt to changing demands by altering their size or type. This plasticity is the basis for physical therapy interventions aimed at improving a patient's force development or endurance.
It is important to note that the classification of muscle fibres into just two groups is an oversimplification. There are actually three broad classifications: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). These classifications further emphasise the differences in energy production and contraction speeds between the two types.
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Muscle fibre types
Skeletal muscle is the most common type of muscle in the body, comprising 30% to 40% of total body mass. It is a voluntary muscle, meaning we can control how and when it moves. Skeletal muscle is made up of bundles of muscle fibres, also known as muscle fibre types. These muscle fibres are classified into types, generally defined by the particular myosin heavy chain isoforms they express. The three types of muscle fibres are slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions.
Slow-twitch (type 1) muscle fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. They are used for endurance activities such as long-distance running. Slow-twitch fibres have a large diameter and high amounts of mitochondria, myoglobin, and capillaries, giving them a red colour.
Fast-twitch (type 2) muscle fibres are further divided into three subtypes: type 2A, type 2X, and type 2B. Type 2A and 2X fibres primarily use oxidative metabolism, while type 2B fibres rely on glycolytic metabolism. Fast-twitch fibres are used for powerful, quick movements such as sprinting or lifting heavy weights. They produce energy anaerobically and have a lower capacity for endurance. Fast-twitch fibres have a white colour due to their lower number of mitochondria and reduced blood flow.
It is important to note that muscle fibres can adapt and change in size or type. High-intensity resistance training can lead to changes in fibre type, with initial increases in force production mediated by neural factors rather than visible hypertrophy. Over time, however, muscle fibres will visibly increase in size, leading to muscle hypertrophy and strength gains. Physical therapy interventions can also affect muscle fibre types, improving muscle performance and endurance.
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Muscle plasticity
Skeletal muscle, in particular, has an enormous capacity for plasticity, allowing it to adapt to various stimuli such as endurance exercise, electrical stimulation, denervation, resistance training, microgravity, nutritional interventions, and environmental factors like hypoxia. These adaptive structural events occur in muscle fibres and associated structures, and functional adaptations involve changes in regulatory mechanisms, contractile properties, and metabolic capacities.
The three types of muscle fibres, slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG), can adapt to changing demands by altering their size or fibre type composition. This plasticity forms the basis for physical therapy interventions aimed at improving a patient's force development or endurance. High-intensity resistance training, for instance, leads to changes in fibre type and muscle hypertrophy, resulting in strength gains.
At the molecular level, muscle plasticity involves nuclear reprogramming, alterations in the myosin type, protein turnover, and the cytoplasm-to-myonucleus ratio. Changes in protein expression, either in the amount or type of protein, are key to meeting functional demands. However, while the cellular-scale changes in skeletal muscle plasticity are well-characterised, the molecular-scale changes in protein balance are less understood.
The study of muscle plasticity is particularly useful for examining gene regulatory phenomena in humans, and it has broad implications for clinical applications and disease states. For example, some muscle diseases may be treated by shifting fibre type characteristics from slow to fast or vice versa, depending on the disease.
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Muscle contraction
The contraction process is powered by adenosine triphosphate (ATP), which provides the energy required for muscle contraction. During contraction, ATP binds to an ATP-binding domain on the myosin head, initiating cross-bridge cycling. The myosin heads attach to the actin filaments, forming cross-bridges that facilitate the sliding of filaments during muscle contraction. This sliding filament theory describes the movement of protein filaments within skeletal muscle fibres, resulting in their simultaneous contraction.
Following the attachment of myosin heads to actin, ATP is hydrolysed into adenosine diphosphate (ADP) and phosphate (P). This causes a conformational change in the myosin heads, moving them towards the positive end of the actin. The phosphate is then released, and the ADP-bound myosin binds to a new location on the actin filament. Subsequently, ADP is released, leading the myosin to return to its original position, pulling on the actin filament and causing the sarcomere, and consequently the muscle fibre, to contract.
The three types of muscle fibres—slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG)—also influence contraction characteristics. SO fibres utilise aerobic metabolism to produce low-power contractions over extended periods with slow fatigue. In contrast, FG fibres generate rapid and forceful contractions for quick, powerful movements but fatigue quickly. FO fibres likely exhibit intermediate characteristics.
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Frequently asked questions
Muscle fibers are the long, multinucleated cells that make up skeletal muscles. They are formed from the fusion of developmental myoblasts in a process known as myogenesis.
There are three types of muscle fibers: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions.
SO muscle fibers use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. They have a red color due to the presence of myoglobin and mitochondria.
FO and FG muscle fibers use anaerobic metabolism to produce rapid, powerful movements. FG fibers, in particular, are used for short, forceful contractions and fatigue quickly. Due to their reliance on anaerobic metabolism, they have a white color and are less fatigue-resistant than SO fibers.
Muscle fibers contract (tighten) to generate movement. This contraction is powered by the oxidation of fats and carbohydrates, as well as anaerobic chemical reactions, which produce adenosine triphosphate (ATP) molecules. ATP provides the energy required for muscle fibers to move.
