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Skeletal muscle is made up of bundles of individual muscle fibres called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull, causing the muscle to contract. There are two main types of muscle fibres: slow-twitch (type I) and fast-twitch (type II). The type of muscle fibre a person has can determine their strength, power, speed and endurance.
| Characteristics | Values |
|---|---|
| Types | Slow-twitch (type I) and fast-twitch (type II) |
| Subtypes | Type IIa, Type IIb (or IIx), Type 1, Type 2A, Type 2B |
| Hybrid Types | Combination of Type IIa and Type IIb, or a combination of slow-twitch and fast-twitch |
| Structure | Bundles of individual muscle fibers called myocytes or myofibers |
| Contraction | Caused by interaction of actin and myosin filaments |
| Function | Movement, posture, and stabilization of bones and joints |
| Energy Source | Aerobic metabolism, anaerobic metabolism, or anaerobic glycolysis |
| Fatigue | Fast-twitch fibers fatigue faster than slow-twitch fibers |
| Genetics | Genetically determined mixture of slow and fast-twitch fibers |
| Hormones | Hormone levels affect fiber type and size |
| Training | Training can influence fiber type and performance |
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What You'll Learn
Skeletal muscle fibres are made up of bundles of individual muscle fibres called myocytes
Skeletal muscles are made up of bundles of individual muscle fibres called myocytes. Myocytes, also known as muscle fibres, form the bulk of muscle tissue. They are bound together by perimysium, a sheath of connective tissue, into bundles called fascicles, which are in turn bundled together to form muscle tissue. Each skeletal muscle is an organ that consists of various integrated tissues, including blood vessels, nerve fibres, and connective tissue.
Myocytes contain many myofibrils, which are strands of proteins (actin and myosin) that can grab onto each other and pull, causing the muscle to contract and shorten. Sarcomeres are the smallest functional unit of skeletal muscle and are composed of two protein filaments: actin and myosin. The interaction between these two filaments results in muscle contraction. The number of myosin cross-bridges formed between actin and myosin determines how much force a muscle can produce.
It is generally accepted that muscle fibre types can be classified into two main types: slow-twitch (type I) muscle fibres and fast-twitch (type II) muscle fibres. Fast-twitch fibres can be further categorised into type IIa and type IIb fibres (also known as IIx fibres). Additionally, there are hybrid fibres that express more than one type. These distinctions seem to influence how muscles respond to training and physical activity, and each fibre type is unique in its ability to contract in a certain way.
The type of muscle fibre a person has is influenced by various factors, including genetics, hormone levels, and training. For example, research suggests that endurance athletes tend to have a higher percentage of slow-twitch fibres, while sprinters or power-lifters often have more fast-twitch fibres. Understanding the different muscle fibre types is essential for optimising training programmes and prescribing specific exercises to clients.
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Slow-twitch (type I) muscle fibres
Skeletal muscle is made up of bundles of individual muscle fibres called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull, causing the muscle to contract.
Muscle fibres can be broken down into two main types: slow-twitch (type I) muscle fibres and fast-twitch (type II) muscle fibres. Slow-twitch muscle fibres are responsible for endurance movements and are resistant to fatigue. They are like the tortoise in the story of the tortoise and the hare. They don't produce a lot of power, but they can contract for a long time. Slow-twitch muscle fibres help with daily movements like walking, cleaning, or sitting upright in a chair. They are also used in endurance activities such as long-distance running, swimming, cycling, hiking, low-to-moderate-intensity dancing, and holding a posture.
Type I muscle fibres get most of their energy from aerobic respiration, meaning they need oxygen to function. The oxygen makes the muscle fibres look red, which is why slow-twitch fibres are sometimes called red fibres. They have a much better blood supply and ability to receive oxygen than type II fibres. They also have a high concentration of mitochondria, which is the powerhouse of the cell where aerobic respiration takes place. Because they use oxygen to produce energy, they are more resistant to fatigue.
Slow-twitch 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 these fibres. The level of improvement varies depending on the individual, and training cannot make slow-twitch fibres as powerful as fast-twitch fibres. However, genetics also plays a role in determining the type and percentage of muscle fibres a person has. It is thought that the average person is born with around 60% fast-twitch and 40% slow-twitch fibres, but this can vary, and some individuals may be more suited to high-force or long-duration activities.
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Fast-twitch (type II) muscle fibres
Muscle fibres are the individual muscle cells that make up a muscle. They are bundled together by fascia, a type of connective tissue, to form skeletal muscles. Each muscle fibre contains many contractile units called myofibrils, which are strands of proteins (actin and myosin) that can grab onto each other and pull, causing muscle contraction.
Type II muscle fibres can be further divided into two subtypes: type IIa and type IIx (formerly known as type IIb). Type IIa fibres are considered intermediate fast-twitch fibres, as they can be used for longer periods due to their slower fatigue rate. They are commonly used in sprinting and powerlifting. Type IIx fibres, on the other hand, provide greater force but fatigue much faster, making them inefficient. These fibres are essential for sudden, explosive movements and are often utilised in power-based sports.
The ratio of type IIa to type IIx fibres is influenced by genetics and the types of activities an individual engages in. For example, endurance training tends to increase the proportion of type IIa fibres, while power-based exercises favour the development of type IIx fibres. It is important to note that both types of fast-twitch fibres can be trained and strengthened through appropriate exercises.
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Smooth muscle fibres are involuntary
Muscle fibres are the individual components that make up a muscle. Skeletal muscle, for example, is made up of bundles of muscle fibres called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab onto each other and pull, resulting in muscle contraction.
There are three types of muscle tissue: cardiac, smooth, and skeletal. Smooth muscle fibres are involuntary, meaning that they contract and relax without conscious control. They are controlled by the autonomic nervous system, which uses hormones, neurotransmitters, and other receptors to regulate their activity. Smooth muscle is present throughout the body and serves a variety of functions. It is found in the stomach and intestines, where it aids in digestion and nutrient absorption. It is also present in the urinary system, where it helps to remove waste and toxins, as well as regulate electrolyte balance. Additionally, smooth muscle plays a crucial role in the regulation of blood pressure and tissue oxygenation by contracting and relaxing the arteries and veins.
Unlike skeletal muscles, which are under voluntary control, smooth muscle fibres cannot be consciously controlled. This is because they are involved in regulating many of the body's subsystems that require constant and spontaneous adjustments, such as blood pressure during exercise. Smooth muscle also differs from skeletal muscle in appearance. While skeletal muscles have a striped or striated appearance due to the arrangement of their protein filaments, smooth muscles do not exhibit this pattern and appear more uniform, giving them their name.
Smooth muscle has an oblong or spindle shape and is thousands of times shorter than skeletal muscle fibres. At a cellular level, smooth muscle contains thick and thin filaments of actin and myosin that do not arrange into sarcomeres, resulting in a non-striated pattern. This gives smooth muscle greater elastic properties compared to striated muscle, which is important in certain organ systems like the urinary bladder, where maintaining contractile tone is essential.
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Muscle fibres can adapt and change size or type
Muscle fibres are contractile units called myofibrils that run the length of each muscle fibre. Skeletal muscle is made up of bundles of individual muscle fibres called myocytes. Each myofibril contains two protein filaments, actin and myosin, that can grab onto each other and pull, causing the muscle to contract.
The type of muscle fibre can also change due to training. Endurance training can increase the oxidative capacity of muscle fibres, leading to greater endurance. High-intensity resistance training can also lead to changes in fibre type and size, with initial gains being neural rather than visible hypertrophy. Additionally, some research suggests that muscle fibres may switch types due to training, with one study finding that an endurance athlete had 55% more slow-twitch fibres than their sedentary twin. However, other studies dispute this, arguing that training only affects the expression of phenotypes, which is influenced by both genotype and external stimuli.
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Frequently asked questions
Muscle fibres are the individual components that make up a muscle. Each fibre is composed of many contractile units called myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull, resulting in muscle contraction.
Muscle fibres can be broken down into three groups: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). SO fibres are slow-twitching fibres that use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. FO and FG fibres are fast-twitching fibres that can produce quick, powerful bursts of speed. FG fibres, in particular, do not primarily use aerobic metabolism and fatigue quickly, so they are only used for short periods.
Muscle fibres produce force through the binding of actin and myosin filaments. Myosin has tiny projections that extend towards the actin filaments, and the point where they bind is called a cross-bridge. The number of myosin cross-bridges formed between actin and myosin determines how much force a muscle can produce.
Yes, muscle fibres can adapt to changing demands by changing size or fibre type composition. For example, high-intensity resistance training can lead to changes in fibre type, and there is some evidence that human skeletal muscle may switch from "fast" to "slow" fibres due to training.
