Modes of muscle contractions. Efficiency of muscle contraction
There are several forms and types of muscle contractions.
1. Dynamic form of muscle contraction. With this type of contraction, the length of the muscle changes, but the tension does not change. This form includes two types:
a) Isotonic type or concentration (the muscle shortens, but does not change its tension). For example, walking.
b) Eccentric type. If the load on a muscle is greater than its tension, then the muscle is stretched. For example, when lowering a heavy object.
2 Static form of muscle contraction. This form is observed when maintaining a pose or overcoming the force of gravity.
This form includes one type of muscle contraction - isometric. During isometric contraction, the muscle changes its tension, but does not change its length.
3. Form of auxotonic contraction or mixed.
The division into forms and types of muscle contractions is conditional because all abbreviations are mixed. However, one type predominates.
Modes of muscle contraction.
The nature or mode of muscle contraction depends on the frequency of impulses that come from the motor neuron.
Highlight solitary and tetanic muscle contractions.
If a muscle is acted upon with a single impulse, then single muscle contraction , in which several phases are distinguished:
1. Latent (hidden) period - the time after the action of the stimulus before the onset of contraction.
2. Shortening phase (with isotonic contraction) or tension phase (with isometric contraction).
3. Relaxation phase.
A single muscle contraction is characterized by insignificant fatigue, but the muscle is not able to realize its capabilities.
Tetanic muscle contraction. If a muscle fiber is exposed to two impulses that quickly follow each other, the contractions are superimposed and a strong contraction occurs.
The superposition of two consecutive pulses is called summation.
There are two types of summation:
1. If the second stimulus arrives at the moment when the muscle begins to relax, then the curve has a peak separate from the peak of the first contraction. This type of summation is called incomplete.
2. If the second stimulus arrives at a moment when the muscle contraction has not yet reached the apex, i.e. the muscle has not begun to relax, then both contractions merge into a single whole. This type of summation is called complete.
Prolonged and strong contraction of a muscle, under the influence of the rhythm of impulses, followed by relaxation is called tetanus. In humans, tetanus can be obtained at a frequency of 50-70 pulses/sec.
There are two types of tetanus:
1. Toothed. Occurs at a low pulse frequency (up to 150 pulses/sec).
2. Smooth. Occurs at a high pulse rate (more than 150 pulses/sec).
In this case, optimal and pessimal rhythms of muscle work are distinguished.
So, if the frequency of delivery and the strength of the impulses causes the maximum contractile effect, then this is the optimal rhythm of work. The optimal rhythm of work is formed through the phase of exaltation (i.e. supernormality).
If the frequency of impulses and the strength of the stimulus are too great, this causes a decrease in the force of contraction. This rhythm is called pessimal. This rhythm of muscle work is formed through the phase of absolute refractoriness.
When performing strength exercises in various modes of their operation.
Definition
Isometric muscle work mode
Overcoming mode of muscle work (concentric mode of muscle work)
The muscle works in overcoming mode, if her length decreases. An example is bending the arm at the elbow joint while holding a dumbbell in your hand. Overcoming mode is muscle work. When working in this mode, the force developed by the muscles is greater than the external force (it would be more correct, of course, to say that the moment of force developed by the muscles is greater than the moment of the external force). The muscle seems to “overcome” the external load. In English literature this mode of muscle contraction is called concentric.
Inferior mode of muscle work (eccentric mode of muscle work)
The muscle works in inferior mode, If its length increases. As an example, extend the arm at the elbow joint while holding a dumbbell in your hand. Yielding mode is a type of dynamic mode. When working in this mode, the force developed by the muscle is less than the moment of external force (it would be more correct to say the moment of muscle force is less than the external moment of force). The muscle seems to “give way” to the external force. In English-language literature this mode is called eccentric mode muscle work.
Various modes of muscle work are illustrated in Fig. 1 and Fig. 2.
You should pay attention to the fact that antagonist muscles work in different modes when performing movements. For example, when bending an arm, the flexor muscles shorten (overcoming mode), and the extensor muscles (their antagonists) lengthen (yielding mode).
Changes that occur in muscles directly or immediately after a training session (acute training effect)
Numerous studies have proven that performing physical exercises in an eccentric (inferior mode, when the muscle lengthens) mode causes pain. O greater structural damage to muscle fibers than other modes of muscle contraction. These damages primarily affect the Z-disks of sarcomeres, as well as cytoskeletal proteins.
From a biochemical point of view, eccentric exercises (exercises performed in an eccentric mode) pose a significant impact on the body. O greater stress than exercise performed in other modes: the level of creatine kinase in the blood (an enzyme contained in muscle fibers and released into the blood when they are destroyed) when working in the eccentric mode is significantly higher than the corresponding indicator when working in the concentric (overcoming) and isometric modes.
If you measure muscle strength after performing exercises in the eccentric mode, it turns out that it decreases significantly more than when performing exercises in the concentric mode. What does this mean? This suggests that more muscle fibers are damaged in the eccentric mode.
Changes that occur in muscles after long-term exercise (cumulative training effect)
It has been shown that long-term adaptation of skeletal muscles to exercises performed in an eccentric mode manifests itself in several O greater hypertrophy of skeletal muscles compared to other modes. Eccentric strength training results in increased skeletal muscle strength and stiffness.
When performing strength exercises in an isometric mode, the degree of overlap of muscle and tendon fibers increases, the tendon thickens somewhat and the area of attachment of the tendon to the bone increases. That is why it is recommended to perform exercises in isometric mode at the end of the workout (about 15 minutes). It is believed that this can reduce the number of injuries to the human musculoskeletal system.
If a muscle contracts in a dynamic mode (concentric or eccentric mode), after some time the length of the muscle fibers increases and the length of the tendon decreases. Computer modeling (U. Proske, D.L. Morgan, 2001) confirmed the feasibility of lengthening the muscle part and shortening the tendon part. The authors showed that long-term adaptation to performing eccentric exercises is manifested in an increase in the number of sarcomeres in the myofibrils of muscle fibers and a decrease in the tendon part. This leads to a change in the optimal length of the muscle when active tension develops.
When performing strength exercises in a dynamic mode (concentric or eccentric), the number of nerve fibers innervating the skeletal muscle increases (4-5 times more than in the isometric mode).
Literature
1. Samsonova A.V., Barnikova I.E., Azanchevsky V.V. The influence of strength training performed in various contraction modes on the hypertrophy of human skeletal muscles // Proceedings of the department. biomechanics. Sat. articles /Ed. A.V. Samsonova. V.N. Tomilova. - St. Petersburg, 2010. - P. 115-131.
Engine efficiency or motor vehicles are calculated as the percentage of energy consumed that is converted into work instead of heat. In muscles, the amount of energy that can be converted into work, even under the best conditions, is less than 25% of the total energy delivered to the muscle (chemical energy of nutrients), and the remaining energy is converted into heat. The reason for this low efficiency is due to the fact that approximately half the energy of nutrients is lost during the formation of ATP, and only 40-45% of the energy of ATP itself can later be converted into work.
Maximum efficiency is realized only if the muscle contracts at a moderate speed. When a muscle contracts slowly or without any shortening during contraction, a small amount of supporting heat is released while little or no work is being done, reducing the conversion efficiency to zero. In contrast, if the contraction is too fast, a greater proportion of the energy is used to overcome viscous friction within the muscle itself, and this also reduces the efficiency of the contraction. Typically, maximum efficiency develops when the contraction rate is around 30%.
Many features of muscle contraction can be demonstrated using the example of single muscle contractions. Such contractions are produced by a single electrical stimulation innervating the muscle of the nerve, or by a brief electrical stimulation of the muscle itself, which leads to the development of a single contraction lasting a fraction of a second.
Isometric and isotonic contraction. Muscle contraction is called isometric if the muscle does not shorten during contraction, and isotonic if the muscle shortens, but its tension remains constant throughout the contraction.
IN isometric muscle system contracts without reducing its length, and in the isotonic system the muscle shortens against a fixed load: the muscle lifts the scales with uneven weight. The isometric system strictly records changes in the strength of the muscle contraction itself, and the parameters of isotonic contraction depend on the load against which the muscle contracts, as well as on the inertia of the load. In this regard, when comparing the functional characteristics of different types of muscles, the isometric system is most often used.
Peculiarities single isometric contractions, recorded from different muscles. The human body contains many muscles of varying sizes - from the very small stapedius muscle in the middle ear, a few millimeters long and about 1 mm in diameter, to the very large quadriceps muscle, 500,000 times larger than the stapedius. In this case, the diameter of the fibers can be small (10 µm) or large (80 µm). Finally, the energetics of muscle contractions vary significantly from one muscle to another. It is therefore not surprising that the mechanical characteristics of contractions of different muscles differ.
The figure shows the registration curves isometric contractions three types of skeletal muscles: the ocular muscle (duration of isometric contraction less than 1/40 sec), the gastrocnemius muscle (duration of contraction about 1/15 sec) and the soleus muscle (duration of contraction approximately 1/3 sec). Interestingly, these contraction durations are tailored to the functions of the corresponding muscles. Eye movements must be extremely fast to maintain the eyes' fixation on an object for clear vision. The gastrocnemius muscle must contract moderately quickly to provide the lower extremity with sufficient speed for running or jumping. And the soleus muscle deals primarily with slow contractions to continuously support the body against gravity for a long time.
Fast and slow muscle fibers. As discussed in previous articles on sports physiology, each muscle in the body is composed of a collection of so-called fast and slow muscle fibers, as well as other fibers with transient properties. Fast-responding muscles consist mainly of fast-twitch fibers and only a small number of slow-twitch fibers. Conversely, slow-responding muscles are composed primarily of slow-twitch fibers. The differences between these two types of fibers are as follows.
Fast fibers: (1) large fibers providing greater contractile force; (2) have a well-developed sarcoplasmic reticulum for the rapid release of calcium ions that initiate contraction; (3) contain a large number of glycolytic enzymes for the rapid release of energy through glycolysis; (4) have a relatively poor blood supply, since oxidative metabolism is of secondary importance; (5) contain few mitochondria also due to the minor importance of oxidative metabolism.
Slow fibers: (1) smaller fibers; (2) are also innervated by smaller nerve fibers; (3) have a well-developed system of blood vessels and capillaries to deliver large amounts of oxygen; (4) contain significantly more mitochondria to support high levels of oxidative metabolism; (5) contain large amounts of myoglobin, an iron-containing protein similar to red blood cell hemoglobin. Myoglobin binds to oxygen and stores it until it is needed (this also significantly increases the rate of oxygen transport into the mitochondria). Myoglobin gives the slow fibers a reddish appearance, which is why they are called red fibers, and due to the deficiency of red myoglobin in the fast fibers, they are called white fibers.
Isometriccontraction Isotonic contraction
It is useful for a person engaged in various physical exercises, and even more so for those who train on their own, to know how the contraction of an entire muscle occurs.
Muscles are able to develop maximum force when they are not contracted or contracted to a small extent. With isometric muscle contraction tenses, but does not shorten. That is, isometric contraction
occurs when the two ends of a muscle are held apart at a fixed distance and stimulation causes tension to develop in the muscle without changing its length. An example of an isometric contraction would be holding a barbell.
During isometric contraction, almost all bridges between actin and myosin fibers are formed immediately, since there is no need to form new connections in new places, since the muscle does not shorten. Therefore, the muscle can develop greater force.
With isotonic muscle contraction shortens without losing tension.
is carried out when one end of the muscle is free for movement, and the muscle shortens, at this time developing a constant force. An example of an isotonic contraction would be lifting a barbell. 
Only with very fast movements can the force be relatively small.
The dependence of muscle effort on the speed of muscle contraction is explained by the functioning of an individual sarcomere. With fast muscle contraction move very quickly. This suggests that at each moment of time a certain number of bridges between actin and myosin filaments must disintegrate so that they can arise in new places. As a result, a relatively weak force may develop.
In fact, most acronyms involve both elements. 
So now we have an idea of what it is isometric contraction muscles, isotonic contraction muscles, as well as about the contraction of a whole muscle. During an isometric contraction, the muscle tenses but does not shorten. With isometric muscle contraction can develop more force. With isotonic muscle contraction shortens without losing tension. Most abbreviations include both elements.
Taking an overview of skeletal muscles is very helpful. I recommend! Read.
Muscle contraction is a vital function of the body associated with defensive, respiratory, nutritional, sexual, excretory and other physiological processes. All types of voluntary movements - walking, facial expressions, movements of the eyeballs, swallowing, breathing, etc. are carried out by skeletal muscles. Involuntary movements (except for heart contraction) - peristalsis of the stomach and intestines, changes in the tone of blood vessels, maintenance of bladder tone - are caused by contraction of smooth muscles. The work of the heart is ensured by the contraction of the cardiac muscles.
Structural organization of skeletal muscle
Muscle fiber and myofibril (Fig. 1). Skeletal muscle consists of many muscle fibers that have points of attachment to bones and are located parallel to each other. Each muscle fiber (myocyte) includes many subunits - myofibrils, which are built from blocks (sarcomeres) repeating in the longitudinal direction. The sarcomere is the functional unit of the contractile apparatus of skeletal muscle. The myofibrils in the muscle fiber lie in such a way that the location of the sarcomeres in them coincides. This creates a pattern of cross striations.
Sarcomere and filaments. Sarcomeres in the myofibril are separated from each other by Z-plates, which contain the protein beta-actinin. In both directions, thin actin filaments. In the spaces between them there are thicker myosin filaments.
Actin filament externally resembles two strings of beads twisted into a double helix, where each bead is a protein molecule actin. Protein molecules lie in the recesses of actin helices at equal distances from each other. troponin, connected to thread-like protein molecules tropomyosin.
Myosin filaments are formed by repeating protein molecules myosin. Each myosin molecule has a head and tail. The myosin head can bind to an actin molecule, forming a so-called cross bridge.
The cell membrane of the muscle fiber forms invaginations ( transverse tubules), which perform the function of conducting excitation to the membrane of the sarcoplasmic reticulum. Sarcoplasmic reticulum (longitudinal tubules) It is an intracellular network of closed tubes and performs the function of depositing Ca++ ions.
Motor unit. The functional unit of skeletal muscle is motor unit (MU). MU is a set of muscle fibers that are innervated by the processes of one motor neuron. Excitation and contraction of the fibers that make up one motor unit occur simultaneously (when the corresponding motor neuron is excited). Individual motor units can be excited and contracted independently of each other.
Molecular mechanisms of contractionskeletal muscle
According to thread sliding theory, muscle contraction occurs due to the sliding movement of actin and myosin filaments relative to each other. The thread sliding mechanism involves several sequential events.
Myosin heads attach to actin filament binding centers (Fig. 2, A).
The interaction of myosin with actin leads to conformational rearrangements of the myosin molecule. The heads acquire ATPase activity and rotate 120°. Due to the rotation of the heads, the actin and myosin filaments move “one step” relative to each other (Fig. 2, B).
Disconnection of actin and myosin and restoration of the head conformation occurs as a result of the attachment of an ATP molecule to the myosin head and its hydrolysis in the presence of Ca++ (Fig. 2, B).
The cycle “binding – change in conformation – disconnection – restoration of conformation” occurs many times, as a result of which actin and myosin filaments are displaced relative to each other, the Z-disks of sarcomeres come closer and the myofibril is shortened (Fig. 2, D).
Pairing of excitation and contractionin skeletal muscle
In the resting state, thread sliding in the myofibril does not occur, since the binding centers on the actin surface are closed by tropomyosin protein molecules (Fig. 3, A, B). Excitation (depolarization) of the myofibril and muscle contraction itself are associated with the process of electromechanical coupling, which includes a series of sequential events.
As a result of the activation of a neuromuscular synapse on the postsynaptic membrane, an EPSP arises, which generates the development of an action potential in the area surrounding the postsynaptic membrane.
Excitation (action potential) spreads along the myofibril membrane and, through a system of transverse tubules, reaches the sarcoplasmic reticulum. Depolarization of the sarcoplasmic reticulum membrane leads to the opening of Ca++ channels in it, through which Ca++ ions enter the sarcoplasm (Fig. 3, B).
Ca++ ions bind to the protein troponin. Troponin changes its conformation and displaces the tropomyosin protein molecules that covered the actin binding centers (Fig. 3, D).
Myosin heads attach to the opened binding centers, and the contraction process begins (Fig. 3, E).
The development of these processes requires a certain period of time (10–20 ms). The time from the moment of excitation of a muscle fiber (muscle) to the beginning of its contraction is called latent period of contraction.
Skeletal muscle relaxation
Muscle relaxation is caused by the reverse transfer of Ca++ ions through the calcium pump into the channels of the sarcoplasmic reticulum. As Ca++ is removed from the cytoplasm, there are fewer and fewer open binding sites, and eventually the actin and myosin filaments are completely disconnected; muscle relaxation occurs.
Contracture called a persistent, long-term contraction of a muscle that persists after the cessation of the stimulus. Short-term contracture can develop after tetanic contraction as a result of the accumulation of large amounts of Ca++ in the sarcoplasm; long-term (sometimes irreversible) contracture can occur as a result of poisoning and metabolic disorders.
Phases and modes of skeletal muscle contraction
Phases of muscle contraction
When a skeletal muscle is irritated by a single pulse of electric current of suprathreshold strength, a single muscle contraction occurs, in which 3 phases are distinguished (Fig. 4, A):
latent (hidden) period of contraction (about 10 ms), during which the action potential develops and electromechanical coupling processes occur; muscle excitability during a single contraction changes in accordance with the phases of the action potential;
shortening phase (about 50 ms);
relaxation phase (about 50 ms).
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Rice. 4. Characteristics of a single muscle contraction. Origin of serrated and smooth tetanus. B– phases and periods of muscle contraction, Change in muscle length shown in blue, muscle action potential- red, muscle excitability- purple. |
Modes of muscle contraction
Under natural conditions, a single muscle contraction is not observed in the body, since a series of action potentials occur along the motor nerves innervating the muscle. Depending on the frequency of nerve impulses coming to the muscle, the muscle can contract in one of three modes (Fig. 4, B).
Single muscle contractions occur at a low frequency of electrical impulses. If the next impulse enters the muscle after the completion of the relaxation phase, a series of successive single contractions occurs.
At a higher impulse frequency, the next impulse may coincide with the relaxation phase of the previous contraction cycle. The amplitude of contractions will be summed up, and there will be serrated tetanus- prolonged contraction interrupted by periods of incomplete muscle relaxation.
With a further increase in the pulse frequency, each subsequent pulse will act on the muscle during the shortening phase, resulting in smooth tetanus- prolonged contraction, not interrupted by periods of relaxation.
Optimum and pessimum frequency
The amplitude of tetanic contraction depends on the frequency of impulses irritating the muscle. Optimum frequency they call the frequency of irritating impulses at which each subsequent impulse coincides with the phase of increased excitability (Fig. 4, A) and, accordingly, causes tetanus of the greatest amplitude. Pessimum frequency called a higher frequency of stimulation, at which each subsequent current pulse falls into the refractory phase (Fig. 4, A), as a result of which the amplitude of the tetanus decreases significantly.
Skeletal muscle work
The strength of skeletal muscle contraction is determined by 2 factors:
- the number of units involved in the reduction;
frequency of contraction of muscle fibers.
The work of skeletal muscle is accomplished through a coordinated change in tone (tension) and length of the muscle during contraction.
Types of skeletal muscle work:
dynamic overcoming work occurs when a muscle, contracting, moves the body or its parts in space;
static (holding) work performed if, due to muscle contraction, parts of the body are maintained in a certain position;
dynamic yielding operation occurs when a muscle functions but is stretched because the force it makes is not enough to move or hold parts of the body.
During work, the muscle can contract:
isotonic– the muscle shortens under constant tension (external load); isotonic contraction is reproduced only in experiment;
isometrics– muscle tension increases, but its length does not change; the muscle contracts isometrically when performing static work;
auxotonic– muscle tension changes as it shortens; auxotonic contraction is performed during dynamic overcoming work.
Average load rule– a muscle can perform maximum work under moderate loads.
Fatigue– a physiological state of a muscle that develops after prolonged work and is manifested by a decrease in the amplitude of contractions, an extension of the latent period of contraction and the relaxation phase. The causes of fatigue are: depletion of ATP reserves, accumulation of metabolic products in the muscle. Muscle fatigue during rhythmic work is less than synapse fatigue. Therefore, when the body performs muscular work, fatigue initially develops at the level of the synapses of the central nervous system and neuromuscular synapses.
Structural organization and reductionsmooth muscles
Structural organization. Smooth muscle consists of single spindle-shaped cells ( myocytes), which are located in the muscle more or less chaotically. Contractile filaments are arranged irregularly, as a result of which there is no transverse striation of the muscle.
The mechanism of contraction is similar to that of skeletal muscle, but the rate of filament sliding and the rate of ATP hydrolysis are 100–1000 times lower than in skeletal muscle.
The mechanism of coupling of excitation and contraction. When the cell is excited, Ca++ enters the cytoplasm of the myocyte not only from the sarcoplasmic reticulum, but also from the intercellular space. Ca++ ions, with the participation of the calmodulin protein, activate the enzyme (myosin kinase), which transfers the phosphate group from ATP to myosin. Phosphorylated myosin heads acquire the ability to attach to actin filaments.
Contraction and relaxation of smooth muscles. The rate of removal of Ca++ ions from the sarcoplasm is much less than in skeletal muscle, as a result of which relaxation occurs very slowly. Smooth muscles perform long tonic contractions and slow rhythmic movements. Due to the low intensity of ATP hydrolysis, smooth muscles are optimally adapted for long-term contraction, which does not lead to fatigue and high energy consumption.
Physiological properties of muscles
The general physiological properties of skeletal and smooth muscles are excitability And contractility. Comparative characteristics of skeletal and smooth muscles are given in table. 6.1. The physiological properties and characteristics of the cardiac muscle are discussed in the section “Physiological mechanisms of homeostasis”.
Table 7.1.Comparative characteristics of skeletal and smooth muscles
Property |
Skeletal muscles |
Smooth muscle |
Depolarization rate |
slow |
|
Refractory period |
short |
long |
Nature of contraction |
fast phasic |
slow tonic |
Energy costs |
||
Plastic |
||
Automatic |
||
Conductivity |
||
Innervation |
motor neurons of the somatic NS |
postganglionic neurons of the autonomic nervous system |
Performed movements |
arbitrary |
involuntary |
Chemical sensitivity |
||
Ability to divide and differentiate |
Plastic smooth muscles is manifested in the fact that they can maintain constant tone both in a shortened and in an extended state.
Conductivity smooth muscle tissue is manifested in the fact that excitation spreads from one myocyte to another through specialized electrically conductive contacts (nexuses).
Property automation smooth muscle is manifested in the fact that it can contract without the participation of the nervous system, due to the fact that some myocytes are able to spontaneously generate rhythmically repeating action potentials.
