Relaxation meaning muscle relaxation. The importance of muscle relaxation. Possibility and conditions for correcting the physical development of the physique Muscle tension at rest

Relaxation of muscles– this is a decrease in voltage muscle fibers components of the muscle. Each muscle connected by a joint is opposed by another, attached to the same joint, but on its other side and providing movement of some part of the body in the other direction. Such opposing muscles are called antagonists. Almost every major muscle has its own antagonist.

The ability to spontaneously reduce excess tension during muscle activity or relax antagonist muscles has great importance in everyday life, work, sports, because thanks to it, physical and mental stress is relieved or reduced.

IN strength exercises unnecessary tension in antagonistic muscles reduces the amount of force exerted externally. In exercises that require endurance, it leads to unnecessary waste of energy and faster fatigue. But excessive tension especially interferes with high-speed movements: it greatly reduces maximum speed.

For example, if a person does not know how to relax muscles that are not needed to perform a given exercise, the result becomes lower. Excessive constraint can be caused by various psychological factors, such as the presence of spectators, unfamiliar surroundings, subjective and personal reasons, etc. Meanwhile, constant special work aimed at developing relaxed, free movements always leads to a positive result. You should also know that mental tension is always accompanied by muscle tension, but muscle tension can occur without mental tension.

Tension

Muscle tension can manifest itself in the following forms:

Tonic– increased tension in the muscles at rest.

Express– muscles do not have time to relax when performing fast movements.

Coordination– the muscle remains excited in the relaxation phase due to imperfect coordination of movements.

To master relaxation in each of these cases, it is necessary to master special methodological techniques.

overcome tonic tension can be achieved with the help of targeted exercises to increase the elastic properties of muscles, i.e. to relax at rest and in the form of free movements of the limbs and torso (such as free swings and shaking). Sometimes tonic tension temporarily increases as a result of fatigue from a previous load. In such cases, light warm-up (before sweating appears), massage, sauna, swimming or bathing in warm water are useful.

Cope with express tension can be reduced by increasing the rate at which the muscles transition to a state of relaxation after a rapid contraction. But! This speed is usually less than the speed of transition from relaxation to arousal. That is why, as the frequency of movements increases, sooner or later (which is better) a moment comes when the muscle does not have time to completely relax. To increase the speed of muscle relaxation, use exercises that require rapid alternation of relaxation and tension (repeated jumps, throwing and catching medicine balls at a close distance, etc.).

The general coordination tension characteristic of those who are beginning to learn movements and have not engaged in physical exercises can be overcome using special techniques. For example, the usual focus of beginners on immediate results interferes with the fight against coordination tension.

You can also use special relaxation exercises to correctly form your own feeling, perception of the relaxed state of the muscles; teach voluntary relaxation of individual muscle groups. These can be contrasting exercises - for example, from tension immediately to relaxation; combining relaxation of some muscles with tension of others. At the same time, it is necessary to comply general rule: When performing one-time relaxation exercises, combine muscle tension with inhalation and breath-holding, and relaxation with active exhalation.

It is also necessary to follow private recommendations: watch the facial expressions on which tension is most clearly expressed. When performing the exercise, it is recommended to smile and talk, this helps relieve excess tension. To overcome coordination tension, it is sometimes useful to train in a state of significant fatigue, which forces you to concentrate your efforts only at the necessary moments.

class words: scientific, physical qualities, methodology, mental tension, muscle tension, tonic tension, speed tension, exercises, relaxation, static balance, dynamic balance, sense of space.

Rational muscle relaxation and methods for its improvement.

Any movement is essentially the result of a combination of excitation and relaxation in the muscles. Relaxation (of specific muscles, at the right moment) is just as necessary for the successful execution of a movement as arousal. Lack of relaxation, i.e. tension muscle groups, which at perfect execution must be in this moment relaxed, leads to tension and stiffness of movement.

Tension negatively affects the results of implementation (V.L. Fedorov, 1955). In strength exercises, unnecessary antagonistic tension reduces the amount of force exerted externally. In exercises that require endurance, the lack of rational relaxation leads to unnecessary waste of strength and contributes to a faster onset of fatigue. But tension is especially harmful when performing high-speed movements; here it greatly reduces the maximum speed.

It is useful to distinguish mental and muscle tension.

Mental (or affective *) tension can be caused by various psychological factors, mainly of an emotional nature (performance in the presence of spectators, novelty of the situation, etc.). It manifests itself in a kind of psychological constraint (narrowing of the object of attention, difficulty in switching it, forgetting the order of actions, etc.), as well as in muscle tension (A. N. Krestovnikov, 1949; G. M. Morozov, 1958; A. A. Krauklis, 1964; V. L. Marishchuk, 1964, etc.). Ways to overcome mental tension by means and methods of physical education are considered by V. L. Marishchuk (1964), Scott (1960). The following presentation concerns only cases of muscle tension (mental tension is always accompanied by muscle tension; motor stiffness is possible in the absence of significant manifestations of mental tension).

Muscle tension may occur for various reasons; The method of preventing it should also be different**. Let's consider the work of a muscle when performing a cyclic movement. In this case, states of tension and relaxation alternate rhythmically in the muscle (Fig. 44). Note that:

1) at rest (before work) there is still a certain degree of tension in the muscle;

2) the transition of a muscle from one state to another requires a certain time. At the same time, the transition from an excited state to a relaxed state (section "G" in Fig. 44) more time may be spent than on the reverse transition from relaxation to tension (section “b”);

3) during the period of relaxation, muscle tension decreases to approximately the resting value (sometimes even below the resting level).

Tension can manifest itself in three forms (see Figure 44):

1) increased tension in the muscles under resting conditions (so-called “tonic tension”, or hypermyotonia);

2) insufficient relaxation speed, as a result of which, when performing fast movements, the muscle does not have time to relax (“speed tension”);

3) in the relaxation phase, the muscle remains more or less excited due to imperfect motor coordination (“coordination tension”).

Let's consider these cases.

Tonic tension.

As is known from physiology, even in the absence of a visible engine Whenever active, the muscle always exhibits some tension (tone)*. The nature of this tension is twofold.

Firstly, the muscle itself has certain viscoelastic properties, which manifest themselves even in the absence of stimulation coming through the motor nerves (the so-called intrinsic muscle tone). In a living organism, the viscoelastic properties of muscles are under the control of the central nervous system, carried out through trophic nervous influences and humoral influences.

Secondly, the muscle almost always receives stimulation through the motor nerves, which causes them to be weakly excited. The immediate cause of this stimulation is myotatic reflexes (stretch reflexes): a change in the length of the muscle excites the nerve endings in the muscle spindles, which reflexively leads to the appearance of some excitation in the muscles. These reflex “additions” to the muscle’s own tension are called “reflex tone.” If the muscle takes part in maintaining the posture, the reflex tone increases. U different people the amount of tone (i.e., the amount of muscle tension under “rest” conditions) is different. This happens for two reasons.

Firstly, the viscoelastic properties of muscles are not the same.

Secondly, the intensity of reflex tonic “supplements” is different. The second is explained by the fact that the sensitivity of the receptor apparatus of muscle spindles can change under the influence of the influences of the central nervous system, carried out through the so-called gamma motor innervation system. If this sensitivity is great at the moment, then even a small change in the length of the muscle will reflexively lead to significant excitation; on the contrary, with reduced sensitivity, relatively large changes in muscle length will not have any consequences. As is commonly said in such cases, the gamma motor system changes the level of stretch reflexes.

In athletes, especially in representatives of the species

sports that require the manifestation of maximum speed of movements, the resting tone (which is approximately judged by the hardness of the muscle) is usually reduced (A. I. Makarova, 1955; A. V. Daridanova and A. F. Koryakina, 1958; A. B. Gan- Delsman and A.I. Makarova, 1958, etc.). One might think that this has a positive effect, since it reduces the resistance exerted by the antagonist muscles. In fast movements, such resistance is very high, and therefore athletes with reduced muscle tone rest obviously have some advantages here. Tasks special training aimed at reducing tonic tension are:
1) change in the elastic properties of muscles;
2) a decrease in the level of stretch reflexes manifested under resting conditions.

To reduce tonic tension, relaxation exercises are used in the form of free movements of the limbs and torso (such as shaking, free swings, etc.). Such exercises, in addition to their direct purpose, contribute to faster recovery after loading (N.A. Komarov, 1938, a, b). Therefore, they should be widely used in rest intervals between repeated attempts, especially in the case of static stress. Stretching exercises, swimming, and massage are also useful. Long-term static stress is undesirable.

Sometimes there is a temporary increase in tonic tension as a result of fatigue from the previous load. This is usually accompanied by some temporary increase in muscle volume (Huh, 1902, etc.), which is explained by two reasons.
Firstly, an increase in the amount of intramuscular fluid (R. Chagovets, 1938) due to changes in osmotic pressure in muscle cells,
secondly, the formation of vasodilatory metabolites during work, which leads to dilation of capillaries and working hyperemia of active muscles (Gaskell, 1877; Anrep and Van Saalfeld, 1935; Dean and Skinner, 1960). In such cases, a light warm-up (before sweating appears), massage, swimming or bathing is useful. V sufficiently warm water. Especially effective means there is a steam bath here (Karvonen, 1962).

Speed tension.

As already noted, the speed of transition of a muscle from an excited state to a relaxed state is usually lower than the speed of transition from relaxation to excitation (V.L. Fedorov, 1955; V.L. Fedorov and V. Safonov, 1960). Therefore, when As the frequency of movements increases, sooner or later a moment comes when the muscle does not have time to completely relax. In this case, the antagonist muscles become tense at the same time * - this sharply reduces the frequency and speed of movements. “Speed tension,” which is based on the insufficient speed of transition of the muscle from an excited to a relaxed state, is one of the main factors preventing an increase in the maximum speed of movements (V.L. Fedorov, 1960).

To improve the ability to quickly relax, exercises are used that require rapid alternation of tension and relaxation (V.L. Fedorov, I.P. Ratov, 1962). Examples of such exercises include jumping and throwing. Catching and throwing medicine balls, snatching and pushing a barbell (if performed technically correctly), etc. are very useful.

Coordination tension. We can distinguish between specific and general coordination tensions. The first refers to the tension that manifests itself when performing a specific movement (usually only in the initial phase of developing a skill). Methods for overcoming it are discussed in works on teaching methods (V.V. Belinovich, 1958, etc.). General coordination tension manifests itself in a wide range of movements. At the same time, movements are constrained and awkward. These are usually the movements of people who do not exercise. They find it difficult to perform movements easily and freely.

As is known from biomechanics, our motor system has a large number of degrees of freedom. Managing such a multi-link system is a very difficult task. If a person’s coordination capabilities are insufficient, then he often seeks to facilitate the execution of a movement (usually without realizing it) by fixing as many joints as possible. This leads to tension in movements. At the same time, the task of controlling the motor apparatus is greatly facilitated, in particular the control of multi-/N L l NN to MI R ea. active forces that arise during movement (P. A. Bernstein, 1947). However, it is natural that such coordination is not advisable (D. D. Donskoy, 1960).

Proper execution

As a person masters movement, he becomes more and more master of the emerging reactive forces. If earlier he had to fight them by fixing his joints, now he uses these forces to achieve better effect movements. That's why,from the point of view of biomechanics, perfect relaxation in movement is the result of mastering reactive forces (L. V. Chkhaidze).

Depending on the mode of operation of the antagonist muscles, two types of movements are distinguished: fixed and ballistic. With fixed movements, antagonists work throughout the entire movement. With ballistic ones, they are relaxed until the end of the movement, and the agonists are turned on only at the beginning, providing the moving part of the body with a reserve of kinetic energy, after which the movement occurs due to inertia. With fixed movements, stiffness and tension are observed; relaxation occurs in ballistic-type movements (Stetson and Troner, 1936; Hubbard, 1939; Sperry, 1939, etc.).

Freedom and ease of movement are also achieved through the use of the morphological properties of our body as a multi-link lever system. For example, if you need to raise your leg forward from the main stance, then this can be done either with a straight leg (as similar swings are performed in gymnastics), or first raise the leg bent in knee joint, and then straighten it. In the second case, the movement is much easier to perform. Here, firstly, the leverage of gravity is smaller, and, secondly, the load is distributed among a large number of muscle groups. The use of such properties of the motor system makes it possible to achieve ease and “softness” of movements (Fig. 45).

In the process of developing dexterity, general coordination tension is gradually overcome.

The meaning of relaxation. The importance of muscle relaxation (relaxation)

The effectiveness of relaxation special method has been studied and proven, its possibilities are limitless, but in practice it is mainly used in the following areas:

· As a means of relieving muscle “clamps”, accompanied by pain, local fatigue and limitation of movements. The appearance of painful lumps in the muscles of the neck and limbs can be associated with both psychological reasons, that is, chronic stress, and with initially physical reasons, disorders of the peripheral nervous system (osteochondrosis of the spine, musculofascial pain). More often, there are reasons of both types, which overlap each other (the “mutual burden” syndrome).

· As a way to restore the body's energy balance. Good relaxation helps restore the body's energy and gives all muscles and joints proper rest. Excellent physical condition is closely related to improved blood and lymph circulation. All organs, from the brain to the limbs, are enriched with oxygen, which stimulates the metabolic, respiratory, digestive and other functions of the body, and, in addition, the body gains strength to overcome stress.

· As a means of restoring mental balance and emotional response. When talking about relaxation as a psychotechnique for personal growth, it is necessary first of all to keep in mind its use as a subtle tool for creating transformational, altered states of consciousness in combination with the technique of sensory awareness.

· As a way to heal the body. All of the above functions of relaxation in their totality lead to the body getting rid of chronic stress and gaining access to new resources for survival and self-healing. In addition, the process of deep muscular and mental relaxation itself has a beneficial effect on the autonomic system. nervous system regulating the activity of internal organs.

Thus, the effect can be very different: from passive relaxation and restoration of mental balance to recovery from a serious illness. The possibilities for relaxation are endless. It all depends on the level of knowledge, preparation and purpose for which it is carried out.

To correctly understand the basis of changes occurring in the body, it is necessary to consider psychophysiological ideas about the mechanisms muscle relaxation and its influence on the functional state of a person.

Muscle relaxation is a decrease in tension in the muscle fibers that make up the muscle. Each muscle connected to a joint is opposed by another, attached to the same joint, but on its other side and providing movement of some part of the body in the opposite direction. In Fig. 1.2 is shown schematically biceps shoulder (biceps), providing flexion of the arm in elbow joint, And triceps shoulder (triceps) - allowing you to extend the arm in the same joint. Such opposing muscles are called antagonists.

Rice. 1.2.

Almost every major muscle has its own antagonist (or antagonists). The ability to voluntarily reduce excess tension during muscle activity or to relax antagonistic muscles is of great importance in everyday life, work and sports, since thanks to it physical and mental stress is removed or reduced. In strength exercises, unnecessary tension on antagonistic muscles reduces the amount of force exerted externally. In exercises that require endurance, it leads to unnecessary waste of energy and faster fatigue. But excessive tension especially interferes with high-speed movements: it greatly reduces the maximum speed. Each of you could observe the following picture in physical education classes: a student is testing in the 100 m run, he “works” very actively with his legs and arms, but his movements are constrained, his step length is short, there is a terrible grimace on his face, and the result is ultimately low. This is a typical example when overly tense antagonist muscles do not allow the runner to perform at his best. high result.

Such tension manifests itself not only due to the inability to relax muscles that are not currently working while running. Excessive constraint can be caused by various psychological factors, for example, the presence of spectators, the novelty of the situation, subjective and personal reasons. Meanwhile, constant special work aimed at developing relaxed, free movements always leads to a positive result. You should also know that mental tension is always accompanied by muscle tension, but muscle tension can occur without mental tension.

Muscle tension can manifest itself in the following forms:

1. Tonic (increased tension in the muscles at rest).

2. High-speed (muscles do not have time to relax when performing fast movements).

3. Coordination (the muscle remains excited in the relaxation phase due to imperfect coordination of movements).

To master relaxation in each of these cases, it is necessary to master special methodological techniques. You can overcome tonic tension with the help of targeted exercises to increase the elastic properties of muscles, i.e. to relax at rest and in the form of free movements of the limbs and torso (such as free swings). Sometimes tonic tension temporarily increases as a result of fatigue from previous exertion. In such cases, light warm-up, massage, sauna, swimming or bathing in warm water are useful. You can cope with speed tension by increasing the rate at which the muscles transition to a state of relaxation after a rapid contraction. This speed is usually less than the speed of transition from relaxation to arousal. That is why, as the frequency of movements increases, sooner or later a moment comes when the muscle does not have time to completely relax. To increase the rate of muscle relaxation, exercises are used that require rapid alternation of tension and relaxation (repeated jumps, throwing and catching medicine balls at a close distance, etc.).

The general coordination tension characteristic of those who are beginning to learn movements or have not engaged in physical exercises can be overcome using special techniques. For example, students’ usual focus on immediate results interferes with the fight against coordination tension. It is necessary to constantly remind that in educational and training sessions the main thing is not the result, but correct technique, relaxed execution of the movement. You can also use special relaxation exercises to correctly form your own feeling, perception of the relaxed state of the muscles; teach voluntary relaxation of individual muscle groups. These can be contrasting exercises - for example, from tension immediately to relaxation; combining relaxation of some muscles with tension of others. In this case, you must follow the general rule: when performing one-time relaxation exercises, combine muscle tension with inhalation and breath-holding, and relaxation with active exhalation. It is also necessary to follow private recommendations: monitor facial expressions, which most clearly reflect tension. When performing the exercise, it is recommended to smile and talk, this helps relieve excess tension. To overcome coordination tension, it is sometimes useful to exercise in a state of significant fatigue, which forces you to concentrate your efforts only at the necessary moments.

FEDERAL AGENCY FOR EDUCATION

State educational institution

higher vocational education

"Volgograd State Technical University"

KAMYSHIN TECHNOLOGICAL INSTITUTE

VOLGOGRAD STATE TECHNICAL UNIVERSITY

Semester work

Discipline: “Physical Education”

Topic: “The importance of muscle relaxation. Possibility and conditions of correction physical development physique, at student age"

Completed by a student

4 courses gr. KBA-071(v)

Bystritskaya O.V.

Checked by: Gritsak

Kamyshin 2010

1. The importance of muscle relaxation

motor and functional readiness

means physical culture and sports

Literature

1.The importance of muscle relaxation

Muscle relaxation is a decrease in tension in the muscle fibers that make up the muscle. Each muscle connected to a joint is opposed by another, attached to the same joint, but on its other side and providing movement of some part of the body in the opposite direction. In Fig. Figure 5.13 schematically shows the biceps brachii muscle (biceps), which allows flexion of the arm at the elbow joint, and the triceps brachii muscle (triceps), which allows the arm to be extended in the same joint. Such opposing muscles are called antagonists.

Almost every major muscle has its own antagonist (or antagonists).

The ability to voluntarily reduce excess tension during muscle activity or to relax antagonistic muscles is of great importance in everyday life, work and sports, since thanks to it physical and mental stress is removed or reduced.

In strength exercises, unnecessary tension on antagonistic muscles reduces the amount of force exerted externally. In exercises that require endurance, it leads to unnecessary waste of energy and faster fatigue. But excessive tension especially interferes with high-speed movements: it greatly reduces the maximum speed.

Each of you could observe the following picture in physical education classes: a student is testing in the 100 m run, he “works” very actively with his legs and arms, but his movements are constrained, his step length is short, there is a terrible grimace on his face, and the result is ultimately low. This is a typical example when overly tense antagonist muscles do not allow the runner to show a higher result possible for him. Such tension manifests itself not only due to the inability to relax muscles that are not currently working while running. Excessive constraint can be caused by various psychological factors, for example, the presence of spectators, the novelty of the situation, subjective and personal reasons. Meanwhile, constant special work aimed at developing relaxed, free movements always leads to a positive result. You should also know that mental tension is always accompanied by muscle tension, but muscle tension can occur without mental tension.

Muscle tension can manifest itself in the following forms:

Tonic (increased tension in the muscles at rest).

High-speed (muscles do not have time to relax when performing fast movements).

Coordination (the muscle remains excited in the relaxation phase due to imperfect coordination of movements).

To master relaxation in each of these cases, it is necessary to master special methodological techniques.

You can overcome tonic tension with the help of targeted exercises to increase the elastic properties of muscles, i.e. to relax at rest and in the form of free movements of the limbs and torso (such as free swings, shaking). Sometimes tonic tension temporarily increases as a result of fatigue from previous exertion. In such cases, light warm-up (before sweating appears), massage, sauna, swimming or bathing in warm water are useful.

You can cope with speed tension by increasing the rate at which the muscles transition to a state of relaxation after a rapid contraction. Note: this rate is usually less than the rate of transition from relaxation to arousal. That is why, as the frequency of movements increases, sooner or later (preferably late) a moment comes when the muscle does not have time to completely relax. To increase the rate of muscle relaxation, exercises are used that require rapid alternation of tension and relaxation (repeated jumps, throwing and catching medicine balls at a close distance, etc.). The general coordination tension characteristic of those who are beginning to learn movements or have not engaged in physical exercises can be overcome using special techniques.

For example, students’ usual focus on immediate results interferes with the fight against coordination tension. It is necessary to constantly remind you that in training sessions the main thing is not the result, but the correct technique and relaxed execution of the movement.

exercises - for example, from tension immediately to relaxation; combining relaxation of some muscles with tension of others. In this case, you must follow the general rule: when performing one-time relaxation exercises, combine muscle tension with inhalation and breath-holding, and relaxation with active exhalation.

It is also necessary to follow private recommendations: monitor facial expressions, which most clearly reflect tension. When performing the exercise, it is recommended to smile and talk, this helps relieve excess tension. To overcome coordination tension, it is sometimes useful to exercise in a state of significant fatigue, which forces you to concentrate your efforts only at the necessary moments.

2. The possibility and conditions for correcting the physical development of the physique,

motor and functional readiness

means of physical culture and sports

The possibilities of physical culture in improving health, correcting physique and posture, increasing overall performance, and mental stability are very great, but ambiguous.

Correction of physical development. It is known that the physical development of a person as a process of change and formation of morphological and functional properties depends on heredity, and on living conditions, as well as on physical education from the moment of birth. Of course, not all signs of physical development are equally amenable to correction at student age: the most difficult thing is height (more correctly, body length), much easier is body mass (weight) and certain anthropometric indicators (chest circumference, hips, etc.). ).

Forecasts for increased growth are generally disappointing, since it has been established that human height refers to hereditary characteristics. However, there are also encouraging moments. According to some authors (V.V. Bunak and others), growth in length in men continues until the age of 25, although many researchers believe that this process stops in girls by the age of 17-18, and in boys by the age of 19. For various reasons, including due to lack of physical activity, some people have metabolic disorders and a “failure” occurs in endocrine system body, and the normal age-related increase in body length sometimes slows down, but does not stop. The physiological mechanisms of this influence are complex, but in a somewhat simplified presentation they are as follows.

Under the influence of physical activity, blood supply to all tissues improves, metabolism increases and, most importantly, a biologically active substance is formed in the body - somatotropic hormone (GH). This hormone (somatotropin) affects the increase in bone length and, consequently, human height. The immediate place of influence of the hormone on the bone is its end formation - epiphyseal cartilage, which is gradually replaced by bone substance, i.e. bone growth occurs. Optimal mechanical stimulation of the epiphyses enhances the effect of the hormone. IN last years determined that physical exercise of moderate power and duration of 1.5-2 hours can more than triple the growth hormone in the body.

However, not all physical activities always stimulate growth. Short-term (10-15 minutes), low intensity (pulse no higher than 100-120 beats per minute), excessively heavy weight loads, as well as long-term (hours of running, etc.) loads do not lead to an increase in growth hormone. Moreover, the latter can promote rapid ossification of the epiphysis.

Experience shows that the most beneficial effect on growth stimulation is sport games(basketball, volleyball, badminton, tennis, etc.). They are recommended to be combined with moderate-power loads (swimming, skiing, running) 2-3 times a week for 40-120 minutes. Daily special jumping exercises (jump ropes, repeated jumps), hanging exercises on a bar or gymnastic wall(Fig. 5.14). Hanging exercises, in addition, strengthen the muscle “corset”, counteract the subsidence of the vertebrae and help maintain good posture. Thus, a healthy lifestyle, physical education and sports can improve the functioning of body systems and enhance body growth. This is not a fantasy, there are examples of this: a significant increase in height among individual students, especially in the first two years. Most often this happens to those who are new to sports activities precisely at student age. The author, who has many years of experience working with students, can confirm a case where a student’s height increased during his studies and regular training(in freestyle wrestling) in the first three years of a technical university by 16 (!) cm. Relatively often there is an increase in body length in male and female students of up to 5-6 cm.

Naturally, this “hereditary indicator” largely depends on both environmental conditions and nutrition - “ building material". A good example: It has been statistically established that during years of war, natural disasters, and famine, the growth of children always decreases.

The second encouraging point in the desire to grow up: during the course of a day, the height of an adult can change up to 2-3 cm. By omitting the description of daily changes in muscle tone and the state of intervertebral cartilaginous discs, we can talk about the influence of a person’s posture on his actual and visually perceived growth. In the morning, when the straightening muscles are toned, a person’s height is higher. In the evening, these muscles become especially tired, the curvature of the spinal column increases, the person stoops and becomes shorter. The muscles that flex the spine also act in the same direction. The tone of these muscles increases, which leads to stiffening of the bends, as a result, growth decreases by 2-3 cm or more. But try to straighten up in the evening, straighten your shoulders... And your height will go up. How long? It depends on many factors, but posture doesn't matter last place. In his brochure "Can You Grow Up?" Candidate of Medical Sciences M.Z. Zalessky leads interesting fact. American circus performer F. Willard has demonstrated such an amazing act for many years. He entered the arena - an ordinary man of average height, and then, in front of the stunned public, began to increase his height. In a few minutes, Willard, relaxing some spinal muscles and tensing others, straightened all four anatomical curves of the spinal column as much as possible, due to this his height increased by 20 cm! It is on this principle that the development of human posture is based.

If you yourself walk with a springy gait, your back is straight - do not hunch, if your head is not lowered - it is raised, then your height both actually and visually becomes larger. Let it be only 2-3 cm, but still! But good constant posture is ensured not only by the desire to have it, but also by good condition, constant high tone of certain muscle groups (mainly extensors) of the neck, torso, and legs.

There are special methods for training posture in the military, in classical and folk dances, and in sports practice. In some sports they work on it specifically (sports, artistic, Rhythmic gymnastics, diving), in other sports (swimming, volleyball, track and field decathlon) it is formed naturally during many years of training. But one thing is indisputable - developing good posture is possible even in student years with regular performance of appropriate exercises.

Unlike height, body mass (weight) is subject to significant changes, both in one direction and the other, with regular exercise in certain physical exercises or sports (with a balanced diet).

As is known, normal body weight is closely related to human height. The simplest height-weight indicator is calculated using the formula: height (cm) - 100 = weight (kg). The result shows normal body weight for a person of this height. However, this formula is only suitable for adults with a height of 155-165 cm. With a height of 165-175 cm, you need to subtract 105, with a height of 175-185, subtract 110.

You can also use the weight-height indicator (Ketley index). In this case, dividing body weight (in g) by height (in cm) gives the quotient, which should be about 350-420 for men and 325-410 for women. This indicator indicates excess or lack of body weight.

Directed changes in body weight are quite accessible at student age. The problem is different - you need to change your usual lifestyle. Therefore, the prevention or treatment of obesity is largely a psychological problem. But whether or not you need to significantly change your body weight, you decide for yourself when assessing the proportionality of your body. All that remains is to select the types of sports (exercises) for regular classes, especially since some types contribute to weight loss (all cyclic - running for medium and long distances, ski race etc.), others can help “gain” body weight (weightlifting, athletic gymnastics, weight-lifting and so on.).

In order to correct physique flaws, it is important to identify them, and, in all likelihood, you need to form your own opinion, an idea of the ideal physique. It is about the ideal (although, as we know, we are only destined to strive for the ideal!), and not about passing tastes and fashion. Tastes and fashion changed in different historical eras and were interpreted differently in different countries and regions, change relatively often even now. Yes, the standard female figure in 2980 BC was the Venus of Willendor (archaeological fund), a symbol of fertility. Her measurements: bust - 244 cm, waist - 226, hips - 244 cm. Ideal woman XIX century (1880) - “corsage model” (97-46-97 cm), an expression of femininity in protruding forms; 1950 - sex goddess of her time - Sophia Loren (95-58-95 cm); 1993 - the first beauty of the world, Claudia Schiffer (92-62-91 cm), was named the standard of beauty. Parameter fluctuations are significant.

In addition, even the opinions of modern men and women on the desirable development of individual parts of the body do not coincide. Thus, the American magazine "Village Voice" recently conducted a survey of men and women: what attracts a woman most in a male figure? Assessments were carried out on 11 parameters (muscular chest and shoulders, tall height, toned stomach etc. to the most intimate). The answers from men and women varied sharply. Men think that what women are most attracted to in their figure is a powerful torso and biceps, but women put... sexy buttocks in first place, a slender build in second place, and a toned stomach in third place.

The true anthropometric proportionality of the human body, recognized by both anatomists and biodynamic specialists, is based on the views of the ancient Hellenes, whose cult of the human body was quite high. This was especially clearly reflected in the classical proportions of the works of ancient Greek sculptors. Their development of body proportions was based on units of measure equal to one or another part of the human body. This unit of measure, called the module, is the height of the head. According to Polycletus, the height of the head for a normal human figure should be eight times the height of the body. So, according to the “square of the ancients” (Fig. 5.15), the span of outstretched arms is equal to the height of the body. The length of the thigh fits four times the height of height, etc. Most often, these measurements reflect the average data of the “ideal” (theoretically) figure, which is important for artists and sculptors (although there are opinions from American scientists that the symmetry of the body is related to the health of its owner). There are other subtleties that should be taken into account when determining figure correction.

So, all girls want to be slim. However, many men like chubby fatties (their opinion should also be taken into account). But a plump woman can also be slim. It's all about proportions here. If we take the average indicators of the “ideal” female figure of our time, having a chest, waist and hip volume ratio of 90-60-90, and divide the waist volume by the hip volume, you get an index equal to 0.7. But a plump woman with a waist of 90 and a hip of 120 (weighing about 100 kg) has approximately the same index of female perfection as a woman with the generally accepted ideal of beauty, i.e. about 0.7. It is this index that is perhaps the only thing that unites the massive goddesses of fertility from the paintings of Rubens’ era and modern top models. Moreover, according to a sociological study conducted by psychologists at the University of Michigan, these plump women are in no way inferior to “ideals” in their attractiveness.

We are talking about general body proportions, but many boys and girls normal height and masses often do not satisfy the shape of individual parts of their own body. There is a need for their correction. And it is possible. Possible with selective use special exercises, developing individual muscle groups that change general shape body parts. Nowadays, techniques have already been developed and there are ways to develop almost every muscle. The greatest experience of such work has been accumulated in weightlifting, athletic and artistic gymnastics, in shaping. At the same time, it is important to choose, with the help of teacher-trainers, the optimal training system, which is based on knowledge of anatomy, physiology, and biomechanics of human movements. In addition, relying on the advice of a teacher-trainer, you yourself need to get acquainted and master the basics sports training. And methods of self-monitoring of the effectiveness of such training are available to everyone through simple anthropometric measurements.

Correction of motor and functional readiness of youth of student age is closely related to the peculiarities of the development of human coordination and functional abilities in ontogenesis. Numerous studies have shown that the most favorable period for mastering the technique of sports movements is the age of 14-15 years. But this does not mean that agility cannot be improved as a student. Coordination abilities can be developed through participation in certain sports (this was discussed in section 5.4). But when starting those sports for the first time, the most favorable age period is already over.

Correction of functional readiness is associated with self-assessment of the level of preparedness by each student. Having passed tests of general physical fitness in the first month of his stay at the university, each student can make a self-assessment of his strength development, general endurance and speed-strength qualities. And here the problem of choice arises: to take up the sport with which you can “pull up” your underdeveloped physical quality and at the same time avoid failure to meet test standards, or give preference to the type to which the body and its physical capabilities are “predisposed”.

In the first case, you can overcome difficulties with additional efforts and independent work. Moreover, any test on a student’s physical fitness for a score of 1-2 points is available to everyone who is practically healthy young man. You just need to work hard, sweat, and sometimes be patient! In the second case, we are talking about a sports orientation associated with the achievements of certain sports milestones.

Both approaches are probably valid, but the motivation needs to be determined immediately. In the first case, the main thing is health-improving orientation classes through improving functional readiness, fulfilling educational test standards. At the same time, one should be aware that success by sports standards and classification will obviously be low. In the second case, it is possible to achieve significant sporting results, and under certain conditions, international class results.

Literature

1. Physical culture of a student / Ed. prof. IN AND. Ilyinich. - M., 2009. - P. 300-331.

2. Physical education. Textbook for university students / Ed. V.A. Golovina, V.A. Maslyakova, A.V. Korobkova and others - M.: Higher. school, 2004. - 391 p.

3. Physical education in questions and answers. Textbook/Ed. V.V. Sadovsky, V.Z. Surovitsky. - Penza RASA, 2006.

4. Physical culture / Ed. V.A. Kovalenko. - M.: ASB publishing house, 2000. – 432 p.

Skeletal muscles

Skeletal muscles are the active part of the musculoskeletal system. As a result contractile activity striated muscles are carried out:

movement of the body in space;
movement of body parts relative to each other;
maintaining a pose.

In addition, one of the results of muscle contraction is the production of heat.

In humans, as in all vertebrates, skeletal muscle fibers have four important properties:

excitability- the ability to respond to a stimulus by changes in ionic permeability and membrane potential;
conductivity - the ability to conduct an action potential along the entire fiber;
contractility- the ability to contract or change tension when excited;
elasticity - the ability to develop tensile tension.

IN natural conditions excitation and contraction of muscles are caused by nerve impulses entering the muscle fibers from the nerve centers. To cause excitation in an experiment, electrical stimulation is used.

Direct stimulation of the muscle itself is called direct stimulation; irritation of a motor nerve leading to contraction of a muscle innervated by this nerve (excitation of neuromotor units) is an indirect irritation. Due to the fact that the excitability of muscle tissue is lower than nervous tissue, the application of irritating current electrodes directly to the muscle does not yet provide direct irritation: the current, spreading through the muscle tissue, acts primarily on the endings of the motor nerves located in it and excites them, which leads to contraction muscles.

Types of abbreviation

Isotonic regime- a contraction in which the muscle shortens without creating tension. Such a reduction is possible when a tendon is cut or ruptured or in an experiment on an isolated (removed from the body) muscle.

Isometric mode- a contraction in which muscle tension increases, but the length practically does not decrease. This reduction is observed when trying to lift an overwhelming load.

Auxotonic mode - a contraction in which the length of a muscle changes as its tension increases. This mode of reductions is observed during a person’s labor activity. If the tension of a muscle increases as it shortens, then this contraction is called concentric, and in the case of an increase in muscle tension when lengthening it (for example, when slowly lowering a load) - eccentric contraction.

Types of muscle contractions

There are two types muscle contractions: single and tetanic.

When a muscle is irritated by a single stimulus, a single muscle contraction occurs, in which the following three phases are distinguished:

latent period phase - begins from the beginning of the stimulus until the beginning of shortening;
contraction phase (shortening phase) - from the beginning of contraction to the maximum value;
relaxation phase - from maximum contraction to initial length.

Single muscle contraction observed when a short series of nerve impulses from motor neurons arrives at the muscle. It can be induced by applying a very short (about 1 ms) electrical stimulus to the muscle. Muscle contraction begins within a time interval of up to 10 ms from the beginning of the stimulus, which is called the latent period (Fig. 1). Then shortening (duration about 30-50 ms) and relaxation (50-60 ms) develop. The entire cycle of a single muscle contraction takes an average of 0.1 s.

Duration of a single contraction different muscles can vary greatly and depends on the functional state of the muscle. The rate of contraction and especially relaxation slows down as muscle fatigue develops. Fast muscles that have a short-term single contraction include the external muscles of the eyeball, eyelids, middle ear, etc.

When comparing the dynamics of the generation of an action potential on the muscle fiber membrane and its single contraction, it is clear that the action potential always occurs earlier and only then does shortening begin to develop, which continues after the end of membrane repolarization. Let us remember that the duration of the depolarization phase of the muscle fiber action potential is 3-5 ms. During this period of time, the fiber membrane is in a state of absolute refractoriness, followed by restoration of its excitability. Since the duration of shortening is about 50 ms, it is obvious that even during shortening, the muscle fiber membrane should restore excitability and will be able to respond to a new impact with a contraction against the background of an incomplete one. Consequently, against the background of developing contraction in muscle fibers, new cycles of excitation and subsequent cumulative contractions can be caused on their membrane. This cumulative reduction is called tetanic(tetanus). It can be observed in single fiber and whole muscle. However, the mechanism of tetanic contraction in natural conditions in a whole muscle has its own peculiarities.

Rice. 1. Time relationships of single cycles of fiber excitation and contraction skeletal muscle: a - ratio of action potential, release of Ca 2+ into the sarcoplasm and contraction: 1 - latent period; 2 - shortening; 3 - relaxation; b - ratio of action potential, excitability and contraction

Tetanus called a muscle contraction that occurs as a result of the summation of contractions of its motor units caused by the receipt of many nerve impulses from motor neurons innervating them this muscle. The summation of forces developed during contraction of fibers of multiple motor units helps to increase the force of tetanic muscle contraction and affects the duration of contraction.

Distinguish serrated And smooth tetanus. To observe dentate tetanus in an experiment, the muscle is stimulated with electric current pulses at such a frequency that each subsequent stimulus is applied after the shortening phase, but before the end of relaxation. Smooth tetanic contraction develops with more frequent stimulation when subsequent stimuli are applied during the development of muscle shortening. For example, if the muscle shortening phase is 50 ms, the relaxation phase is 60 ms, then to obtain serrated tetanus it is necessary to irritate this muscle with a frequency of 9-19 Hz, to obtain smooth tetanus - with a frequency of at least 20 Hz.

For demonstration various types Tetanus usually uses graphical registration on a kymograph of contractions of the isolated gastrocnemius muscle of the frog. An example of such a kymogram is shown in Fig. 2.

If we compare the amplitudes and forces developed during different modes of muscle contraction, they are minimal with a single contraction, increase with serrated tetanus and become maximum with a smooth tetanic contraction. One of the reasons for this increase in the amplitude and force of contraction is that the increase in the frequency of AP generation on the muscle fiber membrane is accompanied by an increase in the output and accumulation of Ca 2+ ions in the sarcoplasm of muscle fibers, which contributes to greater efficiency of interaction between contractile proteins.

Rice. 2. Dependence of the contraction amplitude on the frequency of stimulation (the strength and duration of the stimuli are unchanged)

With a gradual increase in the frequency of stimulation, the strength and amplitude of muscle contraction increases only to a certain limit - the optimum response. The frequency of stimulation that causes the greatest muscle response is called optimal. A further increase in the frequency of stimulation is accompanied by a decrease in the amplitude and force of contraction. This phenomenon is called response pessimum, and stimulation frequencies exceeding the optimal value are called pessimal. The phenomena of optimum and pessimum were discovered by N.E. Vvedensky.

Under natural conditions, the frequency and mode of sending nerve impulses by motor neurons to the muscle ensure the asynchronous involvement in the contraction process of a larger or smaller (depending on the number of active motor neurons) number of motor units of the muscle and the summation of their contractions. The contraction of an integral muscle in the body is close to smooth-teganic in nature.

To characterize the functional activity of muscles, their tone and contraction are assessed. Muscle tone is a state of prolonged continuous tension caused by alternating asynchronous contraction of its motor units. In this case, visible shortening of the muscle may be absent due to the fact that not all motor units are involved in the contraction process, but only those motor units whose properties the best way adapted to maintaining muscle tone and the strength of their asynchronous contraction is not enough to shorten the muscle. Contractions of such units during the transition from relaxation to tension or when changing the degree of tension are called tonic. Short-term contractions accompanied by changes in muscle strength and length are called physical.

Mechanism of muscle contraction

A muscle fiber is a multinucleated structure surrounded by a membrane and containing a specialized contractile apparatus -myofibrils(Fig. 3). In addition, the most important components of muscle fiber are mitochondria, systems of longitudinal tubules - the sarcoplasmic reticulum and a system of transverse tubules - T-system.

Rice. 3. The structure of muscle fiber

Functional unit of the contractile apparatus muscle cell is sarcomere, The myofibril consists of sarcomeres. Sarcomeres are separated from each other by Z-plates (Fig. 4). Sarcomeres in the myofibril are arranged sequentially, so contractions of the capcomeres cause contraction of the myofibril and overall shortening of the muscle fiber.

Rice. 4. Scheme of the sarcomere structure

Studying the structure of muscle fibers in a light microscope revealed their transverse striations, which are due to the special organization of the contractile proteins of protofibrils - actin And myosin. Actin filaments are represented by a double filament twisted into a double helix with a pitch of about 36.5 nm. These filaments are 1 µm long and 6-8 nm in diameter, the number of which reaches about 2000, and are attached at one end to the Z-plate. Filamentous protein molecules are located in the longitudinal grooves of the actin helix tropomyosin. With a step of 40 nm, a molecule of another protein is attached to the tropomyosin molecule - troponin.

Troponin and tropomyosin play (see Fig. 3) an important role in the mechanisms of interaction between actin and myosin. In the middle of the sarcomere, between the actin filaments, there are thick myosin filaments about 1.6 µm long. In a polarizing microscope, this area is visible as a strip of dark color (due to birefringence) - anisotropic A-disc. A lighter stripe is visible in its center H. At rest, there are no actin filaments. On both sides A- the disk is visible light isotropic stripes - I-discs formed by actin filaments.

At rest, the actin and myosin filaments overlap each other slightly so that the total length of the sarcomere is about 2.5 μm. With electron microscopy in the center H-stripes detected M-line - structure that holds myosin filaments.

Electron microscopy shows that on the sides of the myosin filament there are protrusions called cross bridges. According to modern concepts, the transverse bridge consists of a head and a neck. The head acquires pronounced ATPase activity upon binding to actin. The neck has elastic properties and is a hinged joint, so the head of the cross bridge can rotate around its axis.

The use of modern technology has made it possible to establish that applying electrical stimulation to an area Z-plate leads to reduction of the sarcomere, while the size of the disc zone A does not change, but the size of the stripes N And I decreases. These observations indicated that the length of myosin filaments does not change. Similar results were obtained when the muscle was stretched—the intrinsic length of actin and myosin filaments did not change. As a result of the experiments, it turned out that the area of mutual overlap of actin and myosin filaments changed. These facts allowed X. and A. Huxley to propose the theory of thread sliding to explain the mechanism of muscle contraction. According to this theory, during contraction, the size of the sarcomere decreases due to the active movement of thin actin filaments relative to thick myosin filaments.

Rice. 5. A - diagram of the organization of the sarcoplasmic reticulum, transverse tubules and myofibrils. B - diagram of the anatomical structure of the transverse tubules and sarcoplasmic reticulum in an individual skeletal muscle fiber. B - the role of the sarcoplasmic reticulum in the mechanism of skeletal muscle contraction

During the process of muscle fiber contraction, the following transformations occur in it:

electrochemical conversion:

PD generation;
distribution of PD through the T-system;
electrical stimulation of the contact zone of the T-system and the sarcoplasmic reticulum, activation of enzymes, formation of inositol triphosphate, increase in the intracellular concentration of Ca 2+ ions;

chemomechanical transformation:

interaction of Ca 2+ ions with troponin, change in the configuration of tropomyosin, release of active centers on actin filaments;
interaction of the myosin head with actin, rotation of the head and development of elastic traction;
sliding of actin and myosin filaments relative to each other, reduction in sarcomere size, development of tension or shortening of the muscle fiber.

The transfer of excitation from the motor neuron to the muscle fiber occurs using the mediator acetylcholine (ACh). The interaction of ACh with the endplate cholinergic receptor leads to activation of ACh-sensitive channels and the appearance of an endplate potential, which can reach 60 mV. In this case, the area of the end plate becomes a source of irritating current for the muscle fiber membrane and in areas of the cell membrane adjacent to the end plate, an PD occurs, which spreads in both directions at a speed of approximately 3-5 m/s at a temperature of 36 °C. Thus, the generation of PD is the first stage muscle contraction.

Second stage is the propagation of PD into the muscle fiber through the transverse system of tubules, which serves as a link between the surface membrane and the contractile apparatus of the muscle fiber. The G-system is in close contact with the terminal cisterns of the sarcoplasmic reticulum of two neighboring sarcomeres. Electrical stimulation of the contact site leads to the activation of enzymes located at the contact site and the formation of inositol triphosphate. Inositol triphosphate activates the calcium channels of the membranes of the terminal cisterns, which leads to the release of Ca 2+ ions from the cisterns and an increase in the intracellular concentration of Ca 2+ "from 10 -7 to 10 -5. The set of processes leading to an increase in the intracellular concentration of Ca 2+ is the essence third stage muscle contraction. Thus, at the first stages, the electrical signal of the AP is converted into a chemical one - an increase in the intracellular concentration of Ca 2+, i.e. electrochemical conversion(Fig. 6).

When the intracellular concentration of Ca 2+ ions increases, they bind to troponin, which changes the configuration of tropomyosin. The latter will mix into the groove between the actin filaments; in this case, areas on the actin filaments open with which myosin cross bridges can interact. This displacement of tropomyosin is due to a change in the formation of the troponin protein molecule upon binding of Ca 2+. Consequently, the participation of Ca 2+ ions in the mechanism of interaction between actin and myosin is mediated through troponin and tropomyosin. Thus, fourth stage electromechanical coupling is the interaction of calcium with troponin and the displacement of tropomyosin.

On fifth stage electromechanical coupling occurs when the head of the myosin cross bridge attaches to the actin bridge—to the first of several sequentially located stable centers. In this case, the myosin head rotates around its axis, since it has several active centers that sequentially interact with the corresponding centers on the actin filament. Rotation of the head leads to an increase in the elastic traction of the neck of the cross bridge and an increase in tension. At each specific moment during the development of contraction, one part of the heads of the cross bridges is in connection with the actin filament, the other is free, i.e. there is a sequence of their interaction with the actin filament. This ensures a smooth reduction process. At the fourth and fifth stages, a chemomechanical transformation occurs.

Rice. 6. Electromechanical processes in muscle

The sequential reaction of connection and separation of the heads of the cross bridges with the actin filament leads to the sliding of thin and thick filaments relative to each other and a decrease in the size of the sarcomere and the total length of the muscle, which is sixth stage. The totality of the described processes constitutes the essence of the theory of thread sliding (Fig. 7).

It was initially believed that Ca 2+ ions served as a cofactor for the ATPase activity of myosin. Further research refuted this assumption. In resting muscle, actin and myosin have virtually no ATPase activity. The attachment of the myosin head to actin causes the head to acquire ATPase activity.

Rice. 7. Illustration of the theory of sliding threads:

A. a - muscle at rest: A. 6 - muscle during contraction: B. a. b - sequential interaction of the active centers of the myosin head with centers on the active filament

Hydrolysis of ATP in the ATPase center of the myosin head is accompanied by a change in the conformation of the latter and its transfer to a new, high-energy state. Reattachment of the myosin head to a new center on the actin filament again leads to rotation of the head, which is provided by the energy stored in it. In each cycle of connection and separation of the myosin head with actin, one ATP molecule is cleaved per bridge. The speed of rotation is determined by the rate of ATP breakdown. It is clear that fast phasic fibers consume significantly more ATP per unit time and retain less chemical energy during tonic exercise than slow fibers. Thus, in the process of chemomechanical transformation, ATP provides the separation of the myosin head and the actin filament and provides energy for further interaction of the myosin head with another part of the actin filament. These reactions are possible at calcium concentrations above 10 -6 M.

The described mechanisms of muscle fiber shortening suggest that relaxation first requires a decrease in the concentration of Ca 2+ ions. It has been experimentally proven that the sarcoplasmic reticulum has a special mechanism - a calcium pump, which actively returns calcium to the tanks. The calcium pump is activated by inorganic phosphate, which is formed during the hydrolysis of ATP. and the energy supply for the calcium pump is also due to the energy generated during the hydrolysis of ATP. Thus, ATP is the second most important factor, absolutely necessary for the relaxation process. For some time after death, the muscles remain soft due to the cessation of the tonic influence of motor neurons. Then the ATP concentration decreases below a critical level and the possibility of separation of the myosin head from the actin filament disappears. The phenomenon of rigor mortis occurs with pronounced rigidity of skeletal muscles.

Functional significance of ATP during skeletal muscle contraction

Hydrolysis of ATP by myosin, as a result of which the cross bridges receive energy for the development of pulling force
Binding of ATP to myosin, leading to the detachment of cross bridges attached to actin, which creates the possibility of repeating the cycle of their activity
Hydrolysis of ATP (under the action of Ca 2+ -ATPase) for the active transport of Ca 2+ ions into the lateral cisterns of the sarcoplasmic reticulum, reducing the level of cytoplasmic calcium to the initial level

Summation of contractions and tetanus

If in an experiment two strong single stimulations act on a single muscle fiber or an entire muscle in rapid succession, the resulting contractions will have a greater amplitude than the maximum contraction during a single stimulation. The contractile effects caused by the first and second irritations seem to add up. This phenomenon is called summation of contractions (Fig. 8). It is observed with both direct and indirect muscle irritation.

For summation to occur, it is necessary that the interval between irritations have a certain duration: it must be longer than the refractory period, otherwise there will be no response to the second irritation, and shorter than the entire duration of the contractile response, so that the second irritation affects the muscle before it has time to relax after first irritation. In this case, two options are possible: if the second stimulation arrives when the muscle has already begun to relax, then on the myographic curve the apex of this contraction will be separated from the apex of the first by retraction (Figure 8, G-D); if the second stimulation acts when the first has not yet reached its peak, then the second contraction completely merges with the first, forming a single summed peak (Figure 8, A-B).

Consider the summation in calf muscle frogs. The duration of the ascending phase of its contraction is approximately 0.05 s. Therefore, to reproduce the first type of summation of contractions (incomplete summation) on this muscle, it is necessary that the interval between the first and second stimulation be more than 0.05 s, and to obtain the second type of summation (the so-called complete summation) - less than 0.05 s.

Rice. 8. Summation of muscle contractions 8 response to two stimuli. Timestamp 20ms

With both complete and incomplete summation of contractions, action potentials are not summed up.

Tetanus muscle

If an individual muscle fiber or the entire muscle is subject to rhythmic stimulation with such a frequency that their effects are summed up, a strong and prolonged contraction of the muscle occurs, called tetanic contraction, or tetanus.

Its amplitude can be several times greater than the maximum single contraction. With a relatively low frequency of irritation, it is observed serrated tetanus, at high frequency - smooth tetanus(Fig. 9). With tetanus, the contractile responses of the muscle are summed up, but its electrical reactions - action potentials - are not summed up (Fig. 10) and their frequency corresponds to the frequency of the rhythmic stimulation that caused tetanus.

After the cessation of tetanic irritation, the fibers completely relax, their original length is restored only after some time. This phenomenon is called post-tetanic, or residual, contracture.

The faster the muscle fibers contract and relax, the more frequent the stimulation must be to cause tetanus.

Muscle fatigue

Fatigue is a temporary decrease in the performance of a cell, organ or entire organism that occurs as a result of work and disappears after rest.

Rice. 9. Tetanus of isolated muscle fiber (according to F.N. Serkov):

a — serrated tetanus at a stimulation frequency of 18 Hz; 6 - smooth tetanus at a stimulation frequency of 35 Hz; M - myogram; P — irritation mark; B - time stamp 1 s

Rice. 10. Simultaneous recording of contraction (a) and electrical activity (6) of cat skeletal muscle during tetanic nerve stimulation

If you irritate an isolated muscle for a long time with rhythmic electrical stimuli, to which a small load is suspended, then the amplitude of its contractions gradually decreases to zero. The contraction record recorded in this case is called the fatigue curve.

Decreased performance isolated muscle with prolonged irritation due to two main reasons:

During contraction, metabolic products (phosphoric, lactic acid, etc.) accumulate in the muscle, which have a depressing effect on the performance of muscle fibers. Some of these products, as well as potassium ions, diffuse from the fibers out into the pericellular space and have a depressing effect on the ability of the excitable membrane to generate action potentials. If an isolated muscle placed in a small volume of Ringer's fluid is irritated for a long time and brought to the point of complete fatigue, then it is enough just to change the solution washing it to restore muscle contractions;
gradual depletion of energy reserves in the muscle. With prolonged work of an isolated muscle, glycogen reserves sharply decrease, as a result of which the process is disrupted. ATP resynthesis and creatine phosphate, necessary for contraction.

THEM. Sechenov (1903) showed that the restoration of the performance of tired muscles of a person’s arm after prolonged work lifting a load is accelerated if the work is done with the other hand during the rest period. Temporary restoration of the working capacity of the muscles of a tired arm can be achieved with other types of motor activity, for example, when working muscles lower limbs. In contrast to simple rest, such rest was called by I.M. Sechenov active. He considered these facts as proof that fatigue develops primarily in the nerve centers.