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Muscle fibres are the small fibres that make up the muscles in the human body. There are over 600 muscles in the human body, and they are all made up of muscle fibres that work together to move your body and your organs. Muscle fibres can be found in skeletal, cardiac, and smooth muscles, and they all have different functions. Skeletal muscles, for example, are attached to the skeleton by tendons and control the voluntary movements of your body, such as walking and bending over. The composition of muscle fibres varies depending on their type and function. For example, skeletal muscle fibres are classified into two types: type 1 and type 2, with type 2 further broken down into subtypes. Understanding muscle fibre composition is important as it can impact muscle performance and be used to inform physical therapy interventions.
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What You'll Learn
Skeletal muscle fibres
The diversity in skeletal muscle fibres allows for a wide range of movements. They exhibit plasticity, meaning they can change in size or convert to a different fibre type to adapt to new functions. Skeletal muscle fibres can be further classified based on their contraction speed and metabolic profile, resulting in slow (type I or slow-twitch) and fast (type II or fast-twitch) fibres. Type II fibres can be further subdivided into type IIa (fast oxidative) and type IIx (fast glycolytic). Type IIx fibres have a large diameter and high glycogen content, which facilitates rapid contractions and powerful movements such as weightlifting, sprinting, and jumping.
The force of muscle contraction depends on the stimulus intensity. At low intensities, only a small proportion of the fibres are recruited, while at higher intensities, a greater proportion of nerve fibres are activated. The speed of contraction and the method of ATP production (aerobic vs anaerobic respiration) are two key criteria for classifying skeletal muscle fibres.
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Cardiac muscle fibres
Muscle fibres are classified into three types: skeletal, cardiac, and smooth muscles. Cardiac muscle fibres, also known as myocardium, make up the thick middle layer of the heart. They are one of the three major categories of muscles in the human body, alongside skeletal and smooth muscles.
The individual cardiac muscle cells are joined at their ends by intercalated discs to form long fibres. Intercalated discs are complex adhering structures that enable the rapid transmission of electrical impulses through the network of cardiomyocytes, allowing for coordinated contractions of the myocardium. These discs appear as thin, dark-staining lines dividing adjacent cardiac muscle cells when viewed under light microscopy.
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Smooth muscle fibres
Smooth muscle is one of the three major types of vertebrate muscle tissue, the other two being skeletal and cardiac muscle. It is found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. Smooth muscle is also present in the walls of blood vessels and lymph vessels, as well as in the tracts of the respiratory, urinary, and reproductive systems. It is an involuntary, non-striated muscle, meaning it contracts and relaxes without conscious control and lacks the banded appearance of skeletal muscle. Smooth muscle cells are spindle-shaped, with a wide middle and tapering ends, and a single nucleus. They are much shorter than skeletal muscle fibres, ranging from 30 to 200 micrometers in length.
Smooth muscle has a unique structure at the cellular level. It consists of thick and thin filaments of the proteins actin and myosin that are not arranged into sarcomeres, resulting in a non-striated pattern. The actin and myosin filaments form continuous chains within the smooth muscle cell, anchored at dense bodies. These chains can stretch and attach to adjacent smooth muscle cells, forming a mesh-like network. This structure allows smooth muscle cells to contract uniformly in a spiral corkscrew fashion, generating strong contractions.
Smooth muscle can be categorised into two types: single-unit and multi-unit. The majority of smooth muscle is of the single-unit type. In this type, the entire bundle or sheet of smooth muscle cells contracts as a syncytium, even without input from a motor neuron. Single-unit smooth muscle is commonly found in the walls of internal organs, blood vessels (excluding large elastic arteries), the urinary tract, and the digestive tract. It is controlled by the autonomic nervous system and can generate rhythmic contractions due to the presence of pacemaker cells.
On the other hand, multi-unit smooth muscle is found in specific locations such as the trachea, the iris of the eye, and the lining of large elastic arteries. This type of smooth muscle is neurogenic, meaning its contraction is initiated by the autonomic nervous system. Smooth muscle, in general, plays a crucial role in various bodily functions, including digestion, nutrient absorption, toxin elimination, and the regulation of blood pressure and tissue oxygenation. Its ability to be controlled involuntarily by the nervous system allows for the tight regulation of many of the body's subsystems.
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Slow oxidative (SO) fibres
Muscle fibres are classified into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles in the human body contain all three types, but in varying proportions.
SO fibres produce low-power contractions over long periods and are slow to fatigue. They are useful for maintaining posture, producing isometric contractions, and stabilising bones and joints. They are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.
SO fibres are often found in the legs and thighs of turkeys, which need to be able to work all day without fatiguing as the birds walk around looking for food. In humans, endurance athletes tend to have more SO fibres. High-intensity resistance training and endurance training can also increase the oxidative capacity of SO fibres, improving their endurance and force production.
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Fast oxidative (FO) and fast glycolytic (FG) fibres
Muscle fibres are classified into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions.
Fast oxidative (FO) fibres are sometimes called intermediate fibres because they possess characteristics that are intermediate between fast and slow fibres. They produce ATP relatively quickly, more quickly than SO fibres, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, FO fibres do not possess significant myoglobin, giving them a lighter colour than the red SO fibres. FO fibres are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, 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.
Fast glycolytic (FG) fibres primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white colour. FG fibres are used to produce rapid, forceful contractions to make quick, powerful movements. These fibres fatigue quickly, permitting them to only be used for short periods.
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Frequently asked questions
Muscle fibers are the small fibers that make up your muscles. They are woven together and move your organs and body through stretching and pressing.
Muscle fibers can be classified into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Skeletal muscle fibers are further classified into type 1 and type 2, with type 2 having two subtypes.
Type 1 muscle fibers utilize oxygen to generate energy for movement and have a high density of mitochondria, giving them a darker color. Type 2A fibers can also use oxygen but contain less mitochondria, making them lighter. Type 2B fibers don't use oxygen; instead, they store energy for short bursts of movement and appear white.
The composition of muscle fibers varies across different muscle types and individuals. Endurance athletes tend to have more slow-twitch fibers, while sprinters and power-lifters have more fast-twitch fibers.
