Biochemical Foundations of Human Vital Activity - Volkov, N. I., & Nesen, E. N. 2000

Biochemistry of Sports
Biochemistry of Muscle and Muscle Contraction
Types of Muscles and Muscle Fibers

Types of Muscles

There are Three types of muscles in The Human Body: skeletal, cardiac (myocardium), and smooth. They differ in their morphological, biochemical, and functional features, as well as in their developmental pathways. Microscopic examination reveals striations in skeletal and cardiac muscles; therefore, they are referred to as striated muscles. Smooth muscles lack such striations. Functionally, cardiac Muscle differs from skeletal muscles, occupying an intermediate position between smooth and skeletal muscles. The cardiac muscle contracts rhythmically with sequentially alternating cycles of contraction (systole) and relaxation (diastole) independently of human will, i.e., involuntarily. Its contraction is regulated by Hormones, such as catecholamines.

The contraction of smooth muscles is initiated by nerve impulses and certain hormones, and it is independent of human will, as their tone is not controlled by our consciousness. Smooth muscles include the muscles of Internal Organs, the Digestive System, Blood vessel walls, as well as the Skin and Uterus, ensuring their contraction and relaxation.

Skeletal muscles are attached primarily to bones, which gave rise to their name. Skeletal Muscle contraction is initiated by nerve impulses and is subject to conscious control, i.e., it is voluntary.

To understand the biochemistry of muscular activity, the functioning of skeletal muscles is of primary interest. An individual muscle of the arm or another body part is surrounded by a Connective Tissue sheath and has a complex morphological Structure. Each Muscle consists of a bundle of muscle fibers that contain numerous contractile filaments—myofibrils (Fig. 112).

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Fig. 112 Structural Organization of human skeletal muscles

Muscle Fiber

The muscle fiber is the structural unit of skeletal muscles, representing a large multinucleated Cell, or more precisely, an acellular structure—a syncytium, since during development, a muscle cell is formed by the fusion of numerous embryonic individual Cells called myoblasts. The Cell is surrounded by a Plasma Membrane—the sarcolemma—which is covered by a network of Collagen fibers that provide it with strength and elasticity. The length of individual muscle cells can reach 10 cm (sartorius muscle) and even 50 cm, with a thickness of up to 0.1 mm. Motor nerve endings, as well as numerous Blood Vessels, approach the muscle fiber.

A motor nerve, or motor neuron, has branched axons and can innervate several muscle fibers, which together represent the functional unit of the muscle, called the neuromotor or motor unit (Fig. 113). Such a unit Functions as a single entity, meaning that all muscle fibers included in it contract. An individual muscle consists of many motor units that may not be recruited into muscle contraction simultaneously. The force and speed of muscle contraction depend on the number of motor units participating in the contraction, as well as on the frequency of nerve impulses.

Fig. 113 Diagram of a muscle motor unit

Muscle cells are incapable of division; therefore, damaged muscle fibers cannot be restored simply by doubling. In the event of injury, which is observed during strenuous muscular activity, the self-repair of a muscle fiber originates from a small satellite cell that resides in an inactive state in close contact with mature muscle fibers. When The structure of the muscle fiber is disrupted, this cell becomes activated and begins to proliferate, leading to The formation of a new muscle fiber.

The number of muscle fibers in a muscle can reach several thousand. Different people may have varying numbers of fibers in the same muscles, which affects their strength capabilities and the processes of adaptation to muscular work. The greater the number of fibers in the muscles, the higher the potential for expressing maximum muscle strength.

Types of Muscle Fibers and Their Recruitment into Muscular Activity

Several types of muscle fibers are distinguished in skeletal muscles, differing in their contractile and metabolic properties. The main fiber types include slow-twitch (ST), or red fibers, and fast-twitch (FT), or white fibers (Table 20).

TABLE 20. Morphological, metabolic, and Functional Characteristics of muscle fibers

Slow-twitch and fast-twitch fibers have different rates of excitation, contraction, and fatigue. For instance, the contraction speed of ST fibers is over 110 ms, whereas that of FT fibers is 50 ms.

Individual fiber types also differ in their energy-production mechanisms. As seen from Table 20, slow-twitch fibers, which have a slow contraction speed, possess A large number of mitochondria, Enzymes for the Biological Oxidation of CARBOHYDRATES and fats, myoglobin protein for oxygen storage, as well as an extensive capillary network ensuring an adequate supply of oxygen to the muscles, and large glycogen reserves. All of this indicates that aerobic energy-production mechanisms predominate in ST fibers, providing for prolonged endurance work. The motor neuron innervating ST fibers has a small cell body and controls a relatively small number of muscle fibers (10–180).

Fast-twitch muscle fibers are characterized by a large number of myofibrils, high myosin ATPase and glycolytic enzyme activity, and the presence of significant glycogen reserves. They have a poorly developed capillary network and a small amount of the oxygen-binding protein, myoglobin. Consequently, ATP resynthesis in these fiber types is carried out through anaerobic energy-production mechanisms—the creatine phosphate reaction and Glycolysis. The presence of the aforementioned biochemical features ensures a high contraction speed and rapid fatigue in this type of muscle fiber. FT fibers are adapted to high-speed, intensive work of relatively short duration. Their motor Neurons have large cell bodies and highly branched axons, thereby innervating from 300 to 800 muscle fibers.

Among FT fibers, two subtypes are distinguished: FT_a, or type II_a, and FT_b, or type II_b. They differ primarily in their energy-production mechanisms. FT_a fibers exhibit high anaerobic glycolytic and aerobic ATP resynthesis capabilities. They are also referred to as "fast oxidative-glycolytic fibers." They are recruited during high-intensity endurance work, such as running 1000 m or swimming 400 m. FT_b fibers possess exclusively high anaerobic ATP resynthesis capabilities and are therefore engaged mainly in short-term, explosive muscular activity, such as running 100 m or swimming 50 m. The features of individual muscle fiber recruitment into muscular work are illustrated in Fig. 114. The sequence of fiber recruitment into activity is regulated by The Nervous system and depends on the exercise intensity. During low-intensity physical work—about 20–25% of the maximum voluntary contraction force—ST fibers are primarily recruited. At higher intensities—25–40% of the maximum contraction force—type "a" FT fibers are engaged. If the work intensity exceeds 40% of the maximum, type "b" FT fibers are recruited. However, even at maximum intensity, not all available fibers are engaged: in untrained individuals, no more than 55–65% of the available muscle fibers are recruited (see Fig. 114, a), whereas in highly trained strength-sport athletes, 80–90% of motor units can be engaged (see Fig. 114, b).

Fig. 114 Recruitment of muscle fibers during work of varying intensity in untrained individuals (a) and highly qualified athletes (b):

1 — ST fibers; 2 — FT_a fibers; 3 — FT_b fibers; 4 — unused fibers

The recruitment of muscle fibers depends on the intensity of motor neuron stimulation. The minimum stimulation threshold at which a fiber contracts maximally is referred to as the excitation (stimulation) threshold. Slow-twitch (ST) fibers exhibit the lowest excitation threshold (10–15 Hz), whereas the threshold for fast-twitch (FT) fibers is twice as high. All fiber types are fully recruited at high stimulation frequencies of approximately 45–55 Hz. This is a crucial factor to consider when designing strength-conditioning programs for athletes.

On average, the proportion of ST and FT fibers in human skeletal muscles is 55% and 45%, respectively (see Table 14). Among the FT fibers, the majority are FT_a (~30–35%), while FT_b fibers account for a smaller share (~10–15%).

Elite long-distance runners have over 80% ST fibers in their gastrocnemius muscles, compared to only 23% in sprinters. There is a strong correlation between the proportion of FT fibers and muscle speed capabilities. The relative distribution of muscle fiber types is genetically determined and thus highly resistant to change through training. However, targeted training significantly increases their volume. Recent experimental findings suggest that long-term conditioning may induce transitions between fiber types, such as The conversion of FT_a fibers into FT_b or ST fibers.