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Muscle fibres are the building blocks of muscles, which are essential for movement and function in the human body. There are three types of muscle tissue in the body, each with its own unique characteristics: skeletal, cardiac, and smooth muscle. Skeletal muscles, which are voluntary muscles controlled by the individual, make up 30-40% of our total body mass and are responsible for a wide range of movements. These muscles consist of flexible muscle fibres that contract to move bones. On the other hand, cardiac and smooth muscles are involuntary, controlled by the autonomic nervous system. Muscle fibres can be further classified into types 1, 2A, and 2B, each with distinct energy generation mechanisms and contraction speeds. These fibres work through depolarization, a change in electric charge, leading to muscle contraction and movement.
| Characteristics | Values |
|---|---|
| Types | Type 1, Type 2 (Type 2A, Type 2B), Fast-twitch (FT), Slow-twitch (ST), Fast oxidative (FO), Fast glycolytic (FG), Slow oxidative (SO) |
| Appearance | Striated or non-striated, Red and white |
| Function | Cause movement in the body, Contraction, Stabilizing bones and joints |
| Location | Between the bones, throughout the body, Found in the heart (cardiac muscle fibers) |
| Composition | Thick and thin filaments, Mitochondria, Myoglobin |
| Size | Range from less than half an inch to just over 3 inches in diameter |
| Speed | Determined by how quickly they act on ATP |
| Fatigue | Fibers that use oxygen fatigue at a slower rate |
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What You'll Learn
Movement
Muscle fibres are responsible for creating movement in the body. There are three types of muscle in the body: skeletal, cardiac and smooth muscle. Skeletal muscles are the most common type of muscle in the body, comprising 30-40% of total body mass. These muscles are attached to bones and allow us to perform a wide range of movements and functions.
Skeletal muscles consist of flexible muscle fibres that contract, or tighten, to enable movement. Each muscle contains thousands of fibres, ranging from less than half an inch to just over 3 inches in diameter. These fibres are surrounded by connective tissue known as the endomysium, perimysium and epimysium. Skeletal muscles are voluntary, meaning we control how and when they work.
The speed at which a muscle contracts is determined by how quickly it acts on ATP, a molecule that releases energy when it breaks down. There are three types of muscle fibre, categorised by how they produce energy for movement: slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). 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 for quick, powerful movements but fatigue quickly.
Type 1 skeletal muscle fibres are considered slow-twitch (ST) fibres, which are good for endurance activities and stabilising bones and joints. Type 2A and 2B fibres are fast-twitch (FT) fibres, which produce shorter, more explosive bursts of energy. Type 2B fibres, in particular, don't use oxygen to generate energy and are used for short bursts of movement.
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Energy
Muscle fibres are responsible for creating energy, which allows for movement in the body. There are several types of muscle fibres, each with unique characteristics and energy-generating processes. Skeletal muscles, for example, are the most common type of muscle in the body, comprising 30% to 40% of total body mass. These muscles consist of flexible muscle fibres that contract, enabling movement by acting on the bones. Each skeletal muscle can contain thousands of fibres, and their contraction is facilitated by an abundant supply of blood vessels and nerves.
The energy creation process in muscle fibres involves the breakdown of adenosine triphosphate (ATP), a molecule that releases energy. The speed of energy release is determined by how quickly the muscle acts on ATP. Fast-twitch (FT) fibres, including Type 2A and Type 2B, break down ATP twice as fast as slow-twitch (ST) fibres, resulting in shorter, more explosive bursts of energy. Type 2B fibres, in particular, utilise anaerobic glycolysis as their primary ATP source, generating ATP quickly to produce high levels of tension.
On the other hand, Type 1 and Type 2A fibres are slower in energy production but have higher endurance. They use oxygen to generate energy through aerobic metabolism, which produces more ATP and allows muscles to work for longer periods. These fibres are suitable for endurance activities like running, cycling, or swimming. The number of slow and fast-twitch fibres in an individual varies and is influenced by genetics and training.
Additionally, muscle fibres have a striped appearance due to the presence of repeating thick and thin filaments. The structure of these filaments plays a role in muscle contraction, with the thin filaments attached back-to-back at the Z-line and the thick filaments centred by the M-line. The connective tissue surrounding the muscle fibres, known as the perimysium, provides support and protection, allowing the fibres to withstand the forces of contraction.
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Muscle contraction
Before a skeletal muscle fibre can contract, it must receive an impulse from a nerve cell. This nerve impulse causes depolarization, which is a change in electric charge. Depolarization leads to a complex chain reaction within the muscle fibres, resulting in a release of energy and subsequent muscle contraction. The speed of muscle contraction depends on how quickly the muscle acts on ATP, a molecule that releases energy when broken down.
Skeletal muscle fibres can be classified into Type 1 and Type 2, with the latter further divided into subtypes. Type 1 fibres utilize oxygen to generate energy for movement and have a higher density of mitochondria, giving them a darker appearance. Type 2A fibres can also use oxygen to generate energy but contain fewer mitochondria, resulting in a lighter colour. Type 2B fibres, on the other hand, do not rely on oxygen and instead store energy for short bursts of movement. They have the least amount of mitochondria, giving them a white appearance.
The different types of muscle fibres are suited for specific activities. Type 1 fibres, also known as slow-twitch or ST fibres, are ideal for endurance activities like running or cycling as they produce energy for long-lasting movements. In contrast, Type 2B fibres, or fast-twitch or FT fibres, are used for rapid and powerful movements like sprinting or weightlifting. They produce shorter bursts of energy but fatigue quickly.
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Muscle fatigue
ATP (adenosine triphosphate) is a molecule that releases energy when broken down. It is essential for muscle contraction and powerful contractions that last between 5-7 seconds. However, during extremely powerful contractions, nervous fatigue can occur, causing the nerve signal to weaken. This is more common in novice strength trainers, as their muscles' ability to generate force is limited by the nerve's ability to sustain a high-frequency signal.
To prevent and treat muscle fatigue, it is important to stay hydrated, maintain a healthy diet, and ensure proper warming up and stretching before and after strenuous activity. In more severe cases, medical attention may be required, and doctors may prescribe anti-inflammatory or antidepressant medications or recommend physical therapy.
It is worth noting that muscle fatigue is distinct from muscle weakness, although weakness may be an initial symptom. As muscle fatigue progresses, individuals may experience a loss of grip strength, difficulty lifting or pushing with their arms or legs, or an inability to maintain isometric positions.
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Muscle appearance
Skeletal muscle fibres are classified as type 1 or type 2, with type 2 further subdivided into types 2A and 2B. Type 1 fibres, also known as slow-twitch or slow oxidative fibres, have a higher density of mitochondria, giving them a darker appearance. They utilise oxygen to generate energy for movement and are suitable for endurance activities. Type 2A fibres, like type 1, can also use oxygen for energy production and are considered fast-twitch fibres. Type 2B fibres, on the other hand, do not rely on oxygen and instead use anaerobic glycolysis for energy, resulting in a white colour due to the lack of substantial mitochondria. These fibres produce rapid and forceful contractions but fatigue quickly.
The appearance of smooth muscles differs from skeletal muscles as they lack the striated pattern. Smooth muscle fibres have an oblong shape and are significantly shorter than skeletal muscle fibres. Cardiac muscles, found only in the heart, exhibit a striated pattern similar to skeletal muscles. Cardiac muscle fibres have their own rhythm, contracting at a constant pace or adjusting as needed due to impulses from pacemaker cells.
The structure of muscle fibres also contributes to their appearance. Each muscle fibre contains repeating thick and thin filaments, with the thin filaments attached at the Z-line or disk. The relative amounts of sarcoplasm, a substance that surrounds the nucleus and fills the spaces between the myofibrils, can vary among fibres. Additionally, small granules are scattered throughout the sarcoplasm, adding to the intricate structure of muscle fibres.
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Frequently asked questions
Muscle fibres are the individual muscle cells that make up the three types of muscle tissue in the body: skeletal, cardiac and smooth muscle.
Muscle fibres work to cause movement in the body. They contract (tighten) to allow muscles to move bones so you can perform different movements.
There are three types of muscle fibre: slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibres use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. FG fibres, on the other hand, don't use oxygen to generate energy and are used for short bursts of movement.
Muscle fibres usually have a striped appearance, except for smooth muscles. Skeletal muscle fibres are red and white and range from less than half an inch to just over 3 inches in diameter.
