Functions of skeletal and smooth muscles. Muscular system
Skeletal muscles are one of the main systems of the human body and represent an active link in the locomotor system.
Skeletal muscles carry out movements of individual parts of the body and movement of a person in space, and also take an active part in the work of internal organs. In total, there are about 600 muscles in the human body.
Classification of skeletal muscles
Skeletal muscle consists of several main types of fibers:
- Slow fibers. They contain a large amount of myoglobin proteins, which bind oxygen and are a kind of “respiratory substance” for muscles, an analogue of hemoglobin for blood. They are called "reds" because they are dark red in color. These fibers are responsible for maintaining posture. Fatigue occurs slowly in them due to myoglobin and the presence of mitochondria, and recovery occurs quickly.
- Fast fibers. Capable of contracting quickly for a long time without fatigue. The lack of fatigue is explained by increased mitochondrial content and ATP production through oxidative phosphorylation. The number of fibers in the neuromotor unit of such a muscle is less than in the previous one.
- Fast fibers with glycotic oxidation. These fibers use glycolysis to produce ATP and have fewer mitochondria. Muscles with such fibers develop and contract much faster, but tire quickly. They lack the protein myoglobin, which is why they are called “white.”
Muscles are made up of motor or neuromotor units. The part of the muscle responsible for fast and precise movements consists of a small number of fibers. The muscles responsible for maintaining posture are more massive and can contain up to several thousand of these fibers.
Main muscle types
Basically, all muscles are divided into 3 types:
- Synergists. Designed for movement in one direction only.
- Antagonists. They can work in different directions.
- Multifunctional muscles. Affects more than one specific joint. They can impart torque to movements.
Arrangement of fibers in muscles
Skeletal muscle fibers can be located in the muscles:
- Parallel to stretching. This happens when a person performs exercises at a fast pace, and the level of load is minimal.
- Perpendicular to stretch. In this case, short contractions are used at maximum load.
Mechanisms regulating the force of muscle contraction
The force of contraction of muscle fibers is regulated by the central nervous system. In this case, two different mechanisms for selecting motor units are used:
- For precise, coordinated and carefully calculated movements during exercise, motor units are used, the number of fibers in which does not exceed 30.
- Strong and rough movements use muscles with a fiber count of 100 or more.
The more muscle force a person applies to perform a particular exercise, the stronger the impulse generated. This increases the number of muscles involved and produces even greater application force.
Functions of human skeletal muscles
Skeletal muscles are part of the human musculoskeletal system. In this case, skeletal muscles are called upon to perform the following functions:
- ensure adoption and retention of a certain body posture
- move the body in space;
- move individual parts of the human body relative to other parts;
- produce heat, providing thermoregulation of the body.
Properties of skeletal muscles
Skeletal muscles have the following physical properties:
- Excitability. This state is expressed in the ability to respond to stimuli using membrane potential and ionic conductance. The causative agents may be motor neuron transmitters or muscle relaxants, which act by blocking the transmission of nerve impulses. Electrical stimulators are also often used in laboratories.
- Conductivity. Allows action to be carried out deep and along the muscle fiber according to the T-system.
- Contractility. Muscles can shorten and also increase tension under conditions of arousal.
- Elasticity. Muscle fibers are capable of developing tension during stretching.
Skeletal muscle tone
Skeletal muscles cannot be in a completely relaxed state and maintain a certain level of tension, which is called tone. Tone is expressed in maintaining muscle elasticity in a calm state. It is preserved thanks to nerve impulses arriving sequentially at large intervals and irritating different fibers.
At the same time, man, as a highly organized being, is able to regulate his tone at will. For example, he can completely relax or tense the muscles, and also choose the level of tension. To do this, he does not need to do any physical work.
Skeletal muscle function
The main task of skeletal muscles is muscle work. It fully complies with the physical law A = FS, which determines the amount of energy that was expended to move a body under certain conditions (using force). It is also possible to work in an isotonic mode, in which muscle contraction occurs without stress on it.
In addition, an isothermal regime is distinguished, during which the muscle does not shorten under conditions of maximum load. In this case, chemical energy is converted into thermal energy. When working in natural conditions, contractions in a fixed position are called isothermal, and contractions in an active position are called dynamic.
Strength and work do not remain constant and the effectiveness of exercise gradually decreases. This condition is called fatigue. The static mode is the most tedious. When using it, muscle fibers quickly accumulate products arising during the oxidation process (pyruvic and lactic acid). In this case, the resynthesis of ATP, which is responsible for the energy supply of muscle contractions, is disrupted. In addition, the degree of physical fatigue is influenced by the degree of mental stress during work. The higher it is, the less the muscles get tired.
Types of muscles
Currently, the following types of muscles are distinguished:
- unipinnate, in which muscle bundles are attached to one side of the tendon (such as the flexor thumbs);
- bipinnate, in which bundles are attached to both sides of tendons (such as the flexor hallucis longus);
- multipinnate, in which the feathery groups are adjacent to their counterparts (such as the deltoid muscle);
- triangular, in which the bundles are connected from different directions (temporalis muscle).
In addition, muscles have different numbers of heads and can be:
- two-headed;
- three-headed;
- four-headed.
Skeletal muscles perform many other functions. For example, they can provide tissue respiration to the heart in emergency cases using the substance oxymyoglobin (a compound of oxygen and myoglobin). Therefore, the development of skeletal muscles is one of the foundations of an athletic and well-developed human body, as well as its health.
Knowledge of the basics of anatomy and the structure of your own body, together with an understanding of the meaning and structure of training, allows you to increase the effectiveness of sports many times over - after all, any movement, any athletic effort is performed with the help of muscles. In addition, muscle tissue is a significant part of body weight - in men it accounts for 42-47% of dry body mass, in women - 30-35%, and physical activity, especially planned strength training, increases the proportion of muscle tissue, and physical inaction, on the contrary, reduces it.
Types of muscles
There are three types of muscles in the human body:
- skeletal (they are also called striated);
- smooth;
- and myocardium, or heart muscle.
Smooth muscle form the walls of internal organs and blood vessels. Their distinctive feature is that they work independently of a person’s consciousness: it is impossible to stop, for example, peristalsis (contractions) of the intestines by force of will. The movements of such muscles are slow and monotonous, but they work continuously, without rest, throughout their lives.
Skeletal muscles responsible for maintaining the body in balance and performing a variety of movements. Do you feel like you are “just” sitting in a chair and relaxing? In fact, dozens of your skeletal muscles are working during this time. The work of skeletal muscles can be controlled by willpower. Striated muscles are capable of contracting quickly and relaxing just as quickly, but intense activity leads to fatigue relatively quickly.
Heart muscle uniquely combines the qualities of skeletal and smooth muscles. Just like skeletal muscles, the myocardium is capable of working intensively and contracting rapidly. Just like smooth muscles, it is practically tireless and does not depend on a person’s volitional effort.
By the way, strength training not only “sculpts the relief” and increases the strength of our skeletal muscles - it also indirectly improves the quality of smooth muscle and cardiac muscle function. By the way, this will also lead to a “feedback” effect - the strengthened heart muscle, developed through endurance training, works more intensely and efficiently, which is expressed in improved blood supply to the entire body, including skeletal muscles, which thanks to this can endure even greater loads. Trained, developed skeletal muscles form a powerful “corset” that supports internal organs, which plays an important role in normalizing digestive processes. Normal digestion, in turn, means normal nutrition of all organs of the body, and muscles in particular.
Different types of muscles differ in their structure, but we will take a closer look at the structure of skeletal muscle, as it is directly related to the process of strength training.
Let's focus on skeletal muscles
The main structural component of muscle tissue is the myocyte - a muscle cell. One of the distinguishing features of a myocyte is that its length is hundreds of times greater than its cross section, which is why the myocyte is also called a muscle fiber. From 10 to 50 myocytes are connected into a bundle, and the muscle itself is formed from the bundles - in the biceps, for example, up to a million muscle fibers.
Between the bundles of muscle cells pass the smallest blood vessels - capillaries, and nerve fibers. Bundles of muscle fibers and the muscles themselves are covered with dense membranes of connective tissue, which at their ends become tendons that attach to the bones.
The main substance of a muscle cell is called sarcoplasm. The thinnest muscle filaments are immersed in it - myofibrils, which are the contractile elements of the muscle cell. Each myofibril consists of thousands of elementary particles - sarcomeres, the main feature of which is the ability to contract under the influence of a nerve impulse.
During targeted strength training, both the number of muscle fiber myofibrils and their cross-sectional area increases. First, this process leads to an increase in muscle strength, then to an increase in its thickness. However, the number of muscle fibers themselves remains the same - it is determined by the genetic characteristics of the development of the body and does not change throughout life. From this we can conclude that athletes have different physical prospects - those whose muscles consist of more fibers have a better chance of increasing muscle thickness through strength training than those athletes whose muscles contain fewer fibers.
So, the strength of a skeletal muscle depends on its cross-section - that is, on the thickness and number of myofibrils that form the muscle fiber. However, strength and muscle mass do not increase at the same rate: when muscle mass doubles, muscle strength becomes three times greater, and scientists do not yet have a single explanation for this phenomenon.
Types of skeletal muscle fibers
The fibers that form skeletal muscles are divided into two groups: “slow”, or ST-fibers (slow twitch fibers) and “fast”, FT-fibers (fast twitch fibers). ST fibers contain large amounts of myoglobin protein, which is red in color, which is why they are also called red fibers. These are endurance fibers, but they work at a load within 20-25% of the maximum muscle strength. In turn, FT fibers contain little myoglobin, which is why they are also called “white” fibers. They contract twice as fast as the “red” fibers and are capable of developing 10 times more force.
At loads less than 25% of the maximum muscle strength, the ST fibers work first, and then, when they become depleted, the FT fibers come into play. When they also use up the energy resource, they will become exhausted and the muscles will need rest. If the load is initially large, both types of fibers work simultaneously.
However, you should not mistakenly associate the types of fibers with the speed of movements that a person performs. Which type of fibers is predominantly involved in work at a given moment depends not on the speed of the movement performed, but on the effort that must be expended on this action. This is also due to the fact that different types of muscles that perform different functions have a variable ratio of ST and FT fibers. In particular, the biceps, a muscle that performs predominantly dynamic work, contains more FT fibers than ST. In contrast, the soleus muscle, which experiences primarily static loads, consists primarily of ST fibers.
By the way, like the total number of muscle fibers, the ratio of ST/FT fibers in the muscles of a particular person is genetically determined and remains constant throughout life. This also explains the innate abilities for certain sports: in the most “talented”, outstanding sprinters, the calf muscles consist of 90% “fast” fibers, while in marathon runners, on the contrary, up to 90% of these fibers are slow.
However, despite the fact that the natural number of muscle fibers, as well as the ratio of their fast and slow varieties, cannot be changed, well-planned and persistent training will force the muscles to adapt to the load and will certainly bring results.
Structural and functional unit skeletal muscle is simplast or muscle fiber- a huge cell in the shape of an extended cylinder with pointed edges (the names simplast, muscle fiber, muscle cell should be understood as the same object).
The length of the muscle cell most often corresponds to the length of the whole muscle and reaches 14 cm, and the diameter is equal to several hundredths of a millimeter.
Muscle fiber, like any cell, is surrounded by a membrane - the sarcolemma. On the outside, individual muscle fibers are surrounded by loose connective tissue, which contains blood and lymphatic vessels, as well as nerve fibers.
Groups of muscle fibers form bundles, which, in turn, are combined into a whole muscle, placed in a dense sheath of connective tissue that passes at the ends of the muscle into tendons attached to the bone (Fig. 1).
Rice. 1.
The force caused by shortening the length of the muscle fiber is transmitted through the tendons to the bones of the skeleton and causes them to move.
The contractile activity of the muscle is controlled by a large number of motor neurons (Fig. 2) - nerve cells whose bodies lie in the spinal cord, and long branches - axons as part of the motor nerve - approach the muscle. Having entered the muscle, the axon branches into many branches, each of which is connected to a separate fiber.
Rice. 2.
So one motor neuron innervates a whole group of fibers (the so-called neuromotor unit), which works as a single unit.
A muscle consists of many neuromotor units and is capable of working not with its entire mass, but in parts, which allows you to regulate the strength and speed of contraction.
To understand the mechanism of muscle contraction, it is necessary to consider the internal structure of the muscle fiber, which, as you already understand, is very different from an ordinary cell. Let's start with the fact that muscle fiber is multinucleated. This is due to the peculiarities of fiber formation during fetal development. Symplasts (muscle fibers) are formed at the stage of embryonic development of the body from precursor cells - myoblasts.
Myoblasts(unformed muscle cells) intensively divide, merge and form myotubes with a central location of the nuclei. Then the synthesis of myofibrils begins in the myotubes (see below for the contractile structures of the cell), and the formation of the fiber is completed by the migration of nuclei to the periphery. By this time, the muscle fiber nuclei have already lost the ability to divide, and they only have the function of generating information for protein synthesis.
But not all myoblasts follow the path of fusion, some of them are isolated in the form of satellite cells located on the surface of the muscle fiber, namely in the sarcolemma, between the plasmolema and the basement membrane - the components of the sarcolemma. Satellite cells, unlike muscle fibers, do not lose the ability to divide throughout life, which ensures an increase in muscle fiber mass and their renewal. Restoration of muscle fibers in case of muscle damage is possible thanks to satellite cells. When the fiber dies, the satellite cells hidden in its shell are activated, divide and transform into myoblasts.
Myoblasts merge with each other and form new muscle fibers, in which the assembly of myofibrils then begins. That is, during regeneration, the events of embryonic (intrauterine) muscle development are completely repeated.
In addition to multinucleation, a distinctive feature of a muscle fiber is the presence in the cytoplasm (in muscle fibers it is usually called sarcoplasm) of thin fibers - myofibrils (Fig. 1), located along the cell and laid parallel to each other. The number of myofibrils in a fiber reaches two thousand.
Myofibrils are contractile elements of the cell and have the ability to reduce their length when a nerve impulse arrives, thereby tightening the muscle fiber. Under a microscope, it can be seen that the myofibril has transverse striations - alternating dark and light stripes.
When contracting myofibrils the light areas reduce their length and disappear completely when contracted completely. To explain the mechanism of myofibril contraction, about fifty years ago, Hugh Huxley developed the sliding filament model, then it was confirmed in experiments and is now generally accepted.
LITERATURE
- McRobert S. Hands of Titan. – M.: JV "Wider Sport", 1999.
- Ostapenko L. Overtraining. Causes of overtraining during strength training // Ironman, 2000, No. 10-11.
- Solodkov A. S., Sologub E. B. Physiology of sports: Textbook. – SPb: SPbGAFK im. P.F. Lesgafta, 1999.
- Physiology of muscular activity: Textbook for institutes of physical culture / Ed. Kotsa Ya. M. – M.: Physical culture and sport, 1982.
- Human physiology (Textbook for institutes of physical education. 5th edition). / Ed. N.V. Zimkina. – M.: Physical culture and sport, 1975.
- Human physiology: A textbook for students of medical institutes / Ed. Kositsky G.I. - M.: Medicine, 1985.
- Physiological foundations of sports training: Guidelines for sports physiology. – L.: GDOIFK im. P.F. Lesgafta, 1986.
Muscle tissue is recognized as the dominant tissue of the human body, the proportion of which in the total weight of a person is up to 45% in men and up to 30% in women. Musculature includes a variety of muscles. There are more than six hundred types of muscles.
The importance of muscles in the body
Muscles play an extremely important role in any living organism. With their help, the musculoskeletal system is set in motion. Thanks to the work of muscles, a person, like other living organisms, can not only walk, stand, run, make any movement, but also breathe, chew and process food, and even the most important organ - the heart - also consists of muscle tissue.
How do muscles work?
The functioning of muscles occurs due to their following properties:
- Excitability is a process of activation, manifested in the form of a response to a stimulus (usually an external factor). The property manifests itself in the form of changes in metabolism in the muscle and its membrane.
- Conductivity is a property that means the ability of muscle tissue to transmit a nerve impulse formed as a result of exposure to a stimulus from the muscle organ to the spinal cord and brain, and also in the opposite direction.
- Contractility is the final action of the muscles in response to a stimulating factor, manifested in the form of shortening of the muscle fiber; muscle tone also changes, that is, the degree of their tension. At the same time, the speed of contraction and maximum muscle tension may be different as a result of different influences of the stimulus.
It should be noted that muscle work is possible due to the alternation of the above-described properties, most often in the following order: excitability-conductivity-contractility. If we are talking about voluntary muscle work and the impulse comes from the central nervous system, then the algorithm will have the form conductivity-excitability-contractility.
Muscle structure
Any human muscle consists of a collection of elongated cells acting in the same direction, called a muscle bundle. The bundles, in turn, contain muscle cells up to 20 cm long, also called fibers. The shape of the cells of striated muscles is oblong, while that of smooth muscles is fusiform.
A muscle fiber is an elongated cell bounded by an outer membrane. Under the shell, contractile protein fibers are located parallel to each other: actin (light and thin) and myosin (dark, thick). In the peripheral part of the cell (in striated muscles) there are several nuclei. Smooth muscles have only one nucleus; it is located in the center of the cell.
Classification of muscles according to various criteria
The presence of various characteristics that are different from certain muscles allows them to be conditionally grouped according to a unifying characteristic. Today, anatomy does not have a single classification by which human muscles could be grouped. Types of muscles, however, can be classified according to various criteria, namely:
- By shape and length.
- According to the functions performed.
- In relation to the joints.
- By location in the body.
- By belonging to certain parts of the body.
- According to the location of muscle bundles.
Along with the types of muscles, three main muscle groups are distinguished depending on the physiological characteristics of the structure:
- Cross-striated skeletal muscles.
- Smooth muscles that make up the structure of internal organs and blood vessels.
- Cardiac fibers.
The same muscle can simultaneously belong to several groups and types listed above, since it can contain several cross characteristics at once: shape, function, relation to a part of the body, etc.
Shape and size of muscle bundles
Despite the relatively identical structure of all muscle fibers, they can be of different sizes and shapes. Thus, the classification of muscles according to this criterion identifies:
- Short muscles move small areas of the human musculoskeletal system and, as a rule, are located in the deep layers of the muscles. An example is the intervertebral spinal muscles.
- Long ones, on the contrary, are localized on those parts of the body that perform large amplitudes of movement, for example, limbs (arms, legs).
- Wide ones cover the main body (on the stomach, back, sternum). They can have different directions of muscle fibers, thereby providing a variety of contractile movements.
Various forms of muscles are also found in the human body: round (sphincter), straight, square, diamond-shaped, fusiform, trapezoidal, deltoid, serrated, single- and double-pinnate and other shapes of muscle fibers.
Types of muscles according to functions performed
Human skeletal muscles can perform various functions: flexion, extension, adduction, abduction, rotation. Based on this feature, muscles can be conditionally grouped as follows:
- Extensors.
- Flexors.
- Leading.
- Abductors.
- Rotational.
The first two groups are always on the same part of the body, but in opposite directions in such a way that when the first ones contract, the second ones relax, and vice versa. The flexor and extensor muscles move the limbs and are antagonistic muscles. For example, the biceps brachii muscle flexes the arm, and the triceps brachii extends it. If, as a result of the work of muscles, a part of the body or organ makes a movement towards the body, these muscles are adductor, if in the opposite direction - abductor. Rotators provide circular movements of the neck, lower back, and head, while rotators are divided into two subtypes: pronators, which provide inward movement, and instep supports, which provide outward movement.
In relation to the joints
Muscles are attached to the joints by tendons, causing them to move. Depending on the type of attachment and the number of joints on which the muscles act, they can be single-joint or multi-joint. Thus, if the muscle is attached to only one joint, then it is a single-joint muscle, if it is attached to two, it is a two-joint muscle, and if there are more joints, it is a multi-joint muscle (finger flexors/extensors).
As a rule, single-joint muscle bundles are longer than multi-joint ones. They provide a more complete range of motion of the joint relative to its axis, since they spend their contractility on only one joint, while multi-joint muscles distribute their contractility over two joints. The latter types of muscles are shorter and can provide much less mobility while simultaneously moving the joints to which they are attached. Another property of multi-joint muscles is called passive insufficiency. It can be observed when, under the influence of external factors, the muscle is completely stretched, after which it does not continue to move, but, on the contrary, slows down.
Localization of muscles
Muscle bundles can be located in the subcutaneous layer, forming superficial muscle groups, or in deeper layers - these include deep muscle fibers. For example, the muscles of the neck consist of superficial and deep fibers, some of which are responsible for the movements of the cervical spine, while others pull back the skin of the neck, the adjacent area of the skin of the chest, and are also involved in turning and tilting the head. Depending on the location in relation to a particular organ, there may be internal and external muscles (external and internal muscles of the neck, abdomen).
Types of muscles by body part
In relation to body parts, muscles are divided into the following types:
- The muscles of the head are divided into two groups: chewing muscles, responsible for the mechanical grinding of food, and facial muscles - types of muscles thanks to which a person expresses his emotions and mood.
- The muscles of the body are divided into anatomical sections: cervical, pectoral (sternal major, trapezius, sternoclavicular), dorsal (rhomboid, latissimus dorsal, teres major), abdominal (internal and external abdominal, including the abs and diaphragm).
- Muscles of the upper and lower extremities: brachialis (deltoid, triceps, biceps brachialis), elbow flexors and extensors, gastrocnemius (soleus), tibia, foot muscles.
Types of muscles according to the location of muscle bundles
The anatomy of muscles in different species may differ in the location of muscle bundles. In this regard, muscle fibers such as:
- The feathery ones resemble the structure of a bird's feather; in them, bundles of muscles are attached to the tendons on only one side, and diverge on the other. The feathery shape of the arrangement of muscle bundles is characteristic of the so-called strong muscles. The place of their attachment to the periosteum is quite extensive. As a rule, they are short and can develop great strength and endurance, while the muscle tone will not differ greatly.
- Muscles with parallel fascicles are also called dexterous. Compared to feathery ones, they are longer and less hardy, but can perform more delicate work. When contracting, the tension in them increases significantly, which significantly reduces their endurance.
Muscle groups by structural features
Clusters of muscle fibers form entire tissues, the structural features of which determine their conditional division into three groups:
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Lecture 6. ODA. MUSCULAR SYSTEM
1. Structure and functions of skeletal muscles
2. Classification of skeletal muscles
4. Muscles of the human body
Structure and functions of skeletal muscles
Skeletal muscles are an active part of the musculoskeletal system. These muscles are built from striated (striated) muscle fibers. Muscles are attached to the bones of the skeleton and, when they contract (shorten), set bone levers in motion. Muscles maintain the position of the body and its parts in space, move bone levers when walking, running and other movements, perform chewing, swallowing and breathing movements, participate in the articulation of speech and facial expressions, and generate heat.
There are about 600 muscles in the human body, most of which are paired. The mass of skeletal muscles in an adult reaches 30-40% of body weight. In newborns and children, muscles account for up to 20-25% of body weight. In old and senile age, the mass of muscle tissue does not exceed 20-30%.
Each muscle consists of a large number of muscle fibers. Each fiber has a thin shell - endomysium, formed by a small number of connective tissue fibers. The muscle fiber bundles are surrounded by loose fibrous connective tissue, called the internal perimysium, which separates the muscle bundles from each other. On the outside, the muscle also has a thin connective tissue sheath - the external perimysium, closely fused with the internal perimysium by bundles of connective tissue fibers penetrating into the muscle. The connective tissue fibers surrounding the muscle fibers and their bundles, extending beyond the muscle, form a tendon.
Each muscle branches out into a large number of blood vessels, through which blood brings nutrients and oxygen to the muscle fibers and carries away metabolic products. The source of energy for muscle fibers is glycogen. During its breakdown, adenosine triphosphoric acid (ATP) is produced, which is used for muscle contraction. The nerves entering the muscle contain sensory and motor fibers.
Skeletal muscles have properties such as excitability, conductivity and contractility. Muscles are capable of being excited under the influence of nerve impulses and coming into a working (active) state. In this case, excitation quickly spreads (conducts) from nerve endings (effectors) to contractile structures - muscle fibers. As a result, the muscle contracts, shortens, and sets bone levers in motion.
Muscles have a contractile part (abdomen), built from striated muscle fibers, and tendon ends (tendons), which are attached to the bones of the skeleton. In some muscles, the tendons are woven into the skin (facial muscles), attached to the eyeball or to neighboring muscles (perineal muscles). Tendons are formed from formed dense fibrous connective tissue and are characterized by great strength. The muscles located on the limbs have narrow and long tendons. Many ribbon-shaped muscles have wide tendons called aponeuroses.
Classification of skeletal muscles
Currently, muscles are classified based on their shape, structure, location and function.
Muscle Shape. The most common muscles are the fusiform and ribbon-shaped (Fig. 30). The fusiform muscles are located primarily on the limbs, where they act on long bony levers. The ribbon-shaped muscles have different widths; they usually participate in the formation of the walls of the torso, abdominal, and thoracic cavities. Fusiform muscles can have two bellies, separated by an intermediate tendon (digastric muscle), two, three and four initial parts - heads (biceps, triceps, quadriceps muscles). There are muscles that are long and short, straight and oblique, round and square.
Muscle structure. Muscles can have a feathery structure, when muscle bundles are attached to the tendon on one, two or several sides. These are unipennate, bipennate, and many pennate muscles. Pennate muscles are built from a large number of short muscle bundles and have significant strength. These are strong muscles. However, they are only able to contract to a small length. At the same time, muscles with parallel arrangement of long muscle bundles are not very strong, but they are capable of shortening up to 50% of their length. These are dexterous muscles, they are present where movements are performed on a large scale.
According to the function performed and the effect on the joints, muscles are divided into flexors and extensors, adductors and abductors, compressors (sphincters) and dilators. Muscles are distinguished by their location in the human body: superficial and deep, lateral and medial, anterior and posterior.
3. Auxiliary apparatus of muscles
Muscles perform their functions with the help of auxiliary devices, which include fascia, fibrous and osteo-fibrous canals, synovial bursae, and blocks.
Fascia- These are connective tissue covers of muscles. They separate muscles into muscle partitions and eliminate friction between muscles.
Channels (fibrous and osteofibrous) are present in those places where the tendons spread over several joints (on the hand, foot). Channels serve to hold tendons in a certain position during muscle contraction.
Synovial vaginas formed by a synovial membrane (membrane), one plate of which lines the walls of the canal, and the other surrounds the tendon and fuses with it. Both plates grow together at their ends, form a closed narrow cavity, which contains a small amount of fluid (synovium) and wets the synovial plates sliding against each other.
Synovial (mucous) bursae perform a function similar to synovial vaginas. The bursae are enclosed sacs filled with synovial fluid or mucus, located where a tendon passes over a bony protrusion or through the tendon of another muscle.
In blocks called the bony protrusions (condyles, epicondyles) through which the muscle tendon is thrown. As a result, the angle of attachment of the tendon to the bone increases. At the same time, the force of action of the muscle on the bone increases.
Muscle work and strength
Muscles act on bone levers, causing them to move or holding parts of the body in a certain position. Each movement usually involves several muscles. Muscles acting in one direction are called synergists; muscles acting in different directions are called antagonists.
The muscles act on the bones of the skeleton with a certain force and perform work - dynamic or static. During dynamic work, bone levers change their position and move in space. During static work, the muscles tense, but their length does not change, the body (or parts of it) is held in a certain stationary position. This contraction of muscles without changing their length is called isometric contraction. A muscle contraction accompanied by a change in its length is called an isotonic contraction.
Taking into account the place of application of muscle force to the bone lever and their other characteristics, in biomechanics, levers of the first order and levers of the second order are distinguished (Fig. 32). With a lever of the first kind, the point of application of muscle force and the point of resistance (body weight, load mass) are located on opposite sides of the fulcrum (from the joint). An example of a lever of the first kind is the head, which rests on the atlas (fulcrum). The weight of the head (its front part) is located on one side of the axis of the atlanto-occipital joint, and the place where the force of the occipital muscles is applied to the occipital bone is on the other side of the axis. Balance of the head is achieved under the condition that the torque of the applied force (the product of the force of the occipital muscles and the length of the shoulder, equal to the distance from the fulcrum to the place of application of the force) corresponds to the torque of gravity of the front of the head (the product of gravity and the length of the shoulder, equal to the distance from point of support to the point of application of gravity).
With a second-class lever, both the point of application of muscle force and the point of resistance (gravity) are located on one side of the fulcrum (axis of the joint). In biomechanics, there are two types of levers of the second kind. In the first type of lever of the second type, the shoulder of application of muscle force is longer than the shoulder of resistance. For example, a human foot. The shoulder for applying the force of the triceps surae muscle (the distance from the heel tubercle to the fulcrum - the heads of the metatarsal bones) is longer than the shoulder for applying the force of gravity of the body (from the axis of the ankle joint to the fulcrum). In this lever there is a gain in the applied muscle force (the lever is longer) and a loss in the speed of movement of the body's gravity (the lever is shorter). In the second type of lever of the second kind, the shoulder of application of muscle force will be shorter than the shoulder of resistance (application of gravity). The shoulder from the elbow joint to the insertion of the biceps tendon is shorter than the distance from this joint to the hand where the force of gravity is applied. In this case, there is a gain in the range of movement of the hand (long arm) and a loss in the force acting on the bone lever (short arm of the application of force).
Muscle force determined by the mass (weight) of the load that this muscle can lift to a certain height at its maximum contraction. This force is usually called the lifting force of the muscle. The lifting force of a muscle depends on the number and thickness of its muscle fibers. In humans, muscle strength is 5-10 kg per square meter. cm physiological diameter of the muscle. For the morphofunctional characteristics of muscles, there is the concept of their anatomical and physiological cross sections (Fig. 33). The physiological cross-section of a muscle is the sum of the cross-section (areas) of all muscle fibers of a given muscle. The anatomical diameter of a muscle is the size (area) of its cross section at its widest point. For muscles with longitudinally located fibers (ribbon-shaped, fusiform muscles), the anatomical and physiological diameters will be the same. When a large number of short muscle bundles are obliquely oriented, as is the case in pennate muscles, the physiological diameter will be greater than the anatomical one.
The rotational force of a muscle depends not only on its physiological or anatomical diameter, or lifting force, but also on the angle of attachment of the muscle to the bone. The greater the angle at which a muscle attaches to a bone, the greater the effect it can have on that bone. Blocks are used to increase the angle of muscle attachment to the bone.
Muscles of the human body
Depending on their location in the body and for ease of study, the muscles of the head, neck, and torso are distinguished; muscles of the upper and lower extremities.
Muscles located in different areas of the human body not only perform different functions, but also have their own structural features. On the limbs, with their long bony levers adapted for moving, grasping and holding various objects, the muscles are, as a rule, fusiform in shape, with a longitudinal or oblique arrangement of muscle fibers, narrow and long tendons. In the torso area, in the formation of its walls, ribbon-shaped muscles with wide flat tendons participate. Such wide tendons are called aponeuroses. In the head region, the chewing muscles begin with one end on the fixed bones of the base of the skull, and with the other end they are attached to the only movable part of the skull - the lower jaw. The facial muscles begin on the bones of the skull and attach to the skin. When facial muscles contract, the relief of facial skin changes and facial expressions are formed.