Motility is one of the most important and characteristic things that can be seen in the members of animal kingdom because they have to move their body just for their daily activities such as locomotion. That is why a skeletal system (including bones and cartilage) has been developed in advanced animals. Likewise the muscles that are connected to the skeletal system play an important role in movements of their limbs and trunk resulting moving.
Furthermore, though the simple unicellular beings have simple activities, advanced multicellular animals have complicated activities such as eye movements, hearing, ingestion of meals, body balance etc. The contraction and relaxation of muscles are important processes in not only body movements but also these activities mentioned above.
Skeletal muscles get mainly involved in contraction and relaxation during exercises because a large amount of energy is needed for the both processes. Not only skeletal muscles, but also Cardiac muscles and Smooth muscles are involved.
There are a few types of muscle contraction. Among them, isotonic and isometric muscle contractions are very important.
Therefore, “how muscles get contracted, what are the differences in isotonic and isometric contraction, what are the physiological changes during exercise etc.” are discussed in this thesis.
2.0 Muscles and their molecular structure
Most cells possess cytoskeletal elements that are capable of lengthening or shortening and therefore the cell has an ability to change its shape. This capacity is important in a variety of cellular functions such as locomotion, phagocytosis, mitosis and extension of processes. Proteins referred to as molecular motors can change the length of a cell much more rapidly by using energy from the hydrolysis of adenosine 5′-triphosphate (ATP). These ATP-dependent systems are based on the interaction of actin and myosin.
In muscle cells, the filaments of actin and myosin and their associated proteins are so abundant that they almost fill the interior of the cell forming the bulk of the muscle. In addition to that, there are Troponin and Tropomyosin filaments as well. Three types of Troponin can be seen. They are Troponin C, I, and T.
They line predominantly in one direction, so that interactions at the molecular level are translated into linear contraction of the whole cell. The ability of these specialized cells to change shape has become their most important property. Assemblies of contractile muscle cells, forming the muscles, are machines for converting chemical energy into mechanical work. The forces generated during the contraction and relaxation of muscle are used to move limbs, inflate the lungs, pump blood, close and open tubes, etc.
Mainly there are three types of muscles, skeletal, cardiac and smooth muscle. Skeletal muscle forms the bulk of the muscular tissue of the body and consists of parallel bundles of long, multinucleate fibers. This type of muscle is capable of powerful contractions because of the regular organization of its contractile proteins.
Cardiac muscle is found only in the heart and in the walls of large veins where they enter the heart consisting of a branched network of individual cells that are linked electrically and mechanically to function as a unit.
Smooth muscle is found in all systems of the body, in the walls of the viscera, including most of the gastrointestinal, respiratory, urinary and reproductive tracts, in the tunica media of blood vessels etc.
In the longitudinal microscopic section of a typical muscle cell, it appears as ribbons and is interrupted at regular intervals by thin transverse lines known as the Z-lines that divide the myofibril into a linear series of repeating contractile units. Those are called sarcomeres. At higher power, sarcomeres are seen to consist of two types of filament, thick Myosin and thin Actin. The arrangement of thick and thin filaments forms a partially overlapping structure within the sarcomere. The thick filaments, together with lengths of thin filaments that overlap and interdigitate with the thick filaments at either end is known as the A-band. The central, paler region of the A-band is not penetrated by the Actin filaments and this region is called the H-band. At the center of H-bands, the Myosin filaments are linked together transversely by M-line. The adjacent portion of two neighboring sarcomeres in which the thin filaments are not overlapped by thick filaments is the I-line. The Actin filaments of adjacent sarcomeres are anchored in the Z-disc, which divides the I-band in to two parts.
Where to Find Muscle Contraction Animation for Kids1.
3.0 Molecular basis of muscle contraction
During a muscle fiber gets contracted, the actual length of the muscle fiber is constant. What happens on contraction is to increase the overlap within the muscle cell.
When the action potential comes into the Transverse system (T-tubules system), it spreads over the membrane of the T-tubules system. As a result of that, the membrane of the T-tubules system gets depolarized. Then, Dihydro Pyridine Receptors (DHPR) / voltage gated Ca+2 channels on the membrane of T-tubules system are activated. As a result of that, intra cellular Ca+2 concentration is increased significantly because of the influx of Ca+2 from extra cellular fluid. This Ca+2 influx triggers the activation of Ryanodine Receptor (RyR) on the membrane of sarcoplasmic reticulum. So that, the release of Ca+2 from sarcoplasmic reticulum to the cytoplasm through the DHPR is occurred. Later influx of Ca+2 is known as Calcium – induced Calcium release 2. That is how the T-tubules carry the action potential in side of the cell.
As a result of the process that mentioned above, the amount of free Ca+2 in the cytoplasm of the muscle cell is increased and these free Ca+2 ions bind with the Troponin-C. After that, the interaction between Troponin-I and Actin becomes weak and then the Tropomyosin can rotate laterally 2.
When the Tropomyosin rotates, Myosin binding site of Actin is exposed. Then, the Myosin head binds with the myosin binding site of the Actin forming the cross-bridges. In the meantime, the ADP molecule tightly bound to the Myosin head is released. As a result of that, conformational changes in the Myosin head can be occurred. That means the Myosin head bends at its neck resulting the length between two Z-lines reduces gradually (Power Stroke). This is the contraction of muscle. The Ganong says that each power stroke shortens the sarcomere about 10nm 2.
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Then, an ATP molecule quickly binds with the site of the Myosin head where the previous ADP molecule has bound. As a result of the binding of an ATP, the detachment of the Myosin head from the Myosin binding site of the Actin can be occurred. Then the ATP molecule bound to the Myosin head gets hydrolyzed producing an ADP molecule on the head. This hydrolyzing of ATP causes for the Myosin head to come its previous position.
This contraction can be occurred as cycles. The Ganong says that each Myosin head cycles about 5 times per second during rapid contraction 2.
During the relaxation of muscles, the membrane of the T-tubules system gets re-polarized. Therefore the concentration of free Ca+2 in the cytoplasm is reduced because Ca+2 enter in to the sarcoplasmic reticulum through the Sarcoplasmic-Endoplasmic Reticulum Ca+2 ATPase pumps (SERCA). These SERCA uses ATP as a source of energy to pump Ca+2 into the sarcoplasmic reticulum. Therefore the amount of Ca+2 bound to the Troponin-C is also reduced. Then the interaction between the Myosin and Actin is ceased. As a result of that, the muscle gets relaxed 2.
Sliding filaments 3.
4.0 Exercise
Exercise is defined by the World Health Organization (WHO), as any bodily movement produced by skeletal muscles that require energy usage 4.
On the other hand, Exercise is physical activity that is planned, structured, and repetitive for the purpose of conditioning any part of the body. Exercise is used to improve health, maintain fitness and is important as a means of physical rehabilitation 5.
When some person is doing an exercise, his or her body can be exposed to one of the highest level of extreme stresses. For example, a person who is suffering from high fever approaching the level of lethality, the metabolism of his body increases to approximately 100% above normal; by comparison, the metabolism of the body increases to 2000% above normal during an exercise such as marathon race.
Although the bodily movements are known as exercise, these exercises can be classified into several groups. Among them isotonic exercise and isometric exercise are important. In addition to that, isokinetic exercises can also be considered.
5.0 Types of exercise
There are lots of types of exercise. But among them, isometric, isotonic and isokinetic exercises are important 5.
Range of motion exercise
The putting of a joint through its full range of normal movements, either actively or passively.
Aerobic exercise
That designed to increase oxygen consumption and improve functioning of the cardiovascular and respiratory systems.
Endurance exercise
One that involves the use of several large groups of muscles and is thus dependent on the delivery of oxygen to the muscles by the cardiovascular system.
Isokinetic exercise
Dynamic muscle activity performed at a constant angular velocity; torque and tension remain constant while muscles shorten or lengthen.
Isometric exercise
Active exercise performed against stable resistance, without change in the length of the muscle.
Isotonic exercise
Active exercise without appreciable change in the force of muscular contraction, with shortening of the muscle.
Kegel exercises
Exercises performed to strengthen the pubococcygeal muscle.
Active exercise
Motion imparted to a part by voluntary contraction and relaxation of its controlling muscles.
Passive exercise
Motion imparted to a part by another person or outside force, or produced by voluntary effort of another segment of the patient’s own body.
Resistanceor Resistive exercise
that performed by the patient against resistance, as from a weight
5.1 Isometric exercise
Exercise performed by the effort against a resistance that encourages and tones the muscle without changing the length of the muscle fibers 6. Therefore, muscle gets contracted without appreciable shortening or change in distance between the origin and insertion of the muscle while the resistance applied to the contraction increases muscle tension without producing movement of the joint.
This occurs when carrying an object in front of you when the weight of the object is pulling your arms down but your muscles are contracting to hold the object at the same level. Another example is when you grip something such as a pen or a needle. During this period, there is no movement in the joints of the hand, but the muscles of the hand are contracting to provide a force sufficient enough to keep a steady hold on the pen or needle.
In addition to that, the amount of the force of a muscle is able to produce during an isometric contraction depends on the length of the muscle at the point of contraction. Each muscle has an optimum length at which the maximum isometric force can be produced.
A series of isometric contractions performed at varying muscle lengths (from -40% (slack) to +40% (stretched). The maximum force is produced at optimum length (Lo). Note that, when the muscle is stretched, the baseline of the force recorded is raised due to passive tension (PT) in the muscle and contributes more to overall force than the active tension (AT) 7.
Isometric muscle contraction is a great form of exercise for people who are suffering from Arthritis, people who are recovering from a joint injury or an injury to cartilage, tendons and ligaments. Because isometric exercises prevent the joint movements, it is no problem even though the movements are limited at the joints of an individual. Isometrics exercise improves overall muscular strength and can also stimulate muscular growth. Over time, individuals can actually get stronger when practicing isometric exercises 10, 11.
5.2 Isotonic exercise
Isotonic muscular contraction is used to strengthen muscles and improve joint mobility in exercises. That means isotonic contractions are those which cause the muscle to change its length when the muscle contracts and causes for the movements of a part of the body. There are two types of isotonic contraction, concentric and eccentric contraction.
5.2.1 Concentric contraction
During concentric exercises, it will cause for the muscles to shorten when the muscles contract. On the other hand, an exercise that causes for a muscle to get decreased its length is known as a concentric exercise.
This type of isotonic contraction is the most common type of muscle contraction that can be occurred in daily and sporting activities.
Ex: – bending the forearm at the elbow joint from straight to fully flexed
The forearm can be flexed, when the Biceps Brachi muscle contracts. That means the length of the Biceps Brachi muscle is reduced during the contraction 8.
In concentric contractions, the force generated by the muscle is always less than the muscle’s maximum (Po). When the load that the muscle is required to lift something decreases, contraction velocity increases. This occurs until the muscle finally reaches its maximum contraction velocity, Vmax. By performing a series of constant velocity shortening contractions, a force-velocity relationship can be determined 7.
5.2.2 Eccentric contraction
The opposite of the concentric contraction is Eccentric contraction 8. That means this type of contraction can be occurred when the muscle elongates during a contraction 9.
This type is less common but usually involves in the control or deceleration of a movement of a limb, being initiated by the concentric contraction.
Ex: – kicking something
The Quadriceps muscle contracts concentrically to straighten the leg at the knee joint while the Hamstrings contract eccentrically to slowdown the motion of the leg and this type of contraction puts a lot of strain through the muscle and is commonly involved in muscle injuries 8.
However it is difficult to compare that the isometric exercises are important than the isotonic exercises or opposite because both types have its own specific purposes. For example, when someone is doing an isometric exercise, it is only strengthening the muscle in the position that the muscle is being held. That is why this type of exercises are more beneficial for the athletes such as a gymnasts who have to support their bodyweight in difficult positions or hold them self in one position for a long time.
But when the isotonic exercises such as weightlifting are considered, it is going to strengthen the muscles through a range of motion. However both types of isometric and isotonic exercises can increase the amount of force generated during muscle contraction 14, 15, 16.
5.3 Isokinetic exercise
Isokinetic contractions are similar to isotonic contractions. But they differ from isotonic contractions due to movements of a constant speed. Isokinetic Dynamo-meter is used to get the measurements of this type of exercise. Examples for this type of exercise are rare although the best example is breast stork in swimming because a resistance to the movement of adduction is provided constantly by the water 8.
6.0 Muscular changes in exercises
Hypertrophy of skeletal muscles can be occurred after each training session due to acute increased in muscle protein synthesis when there is a good supplement of nutrition 12, 13.
7.0 Cardiovascular changes in exercise
The cardiovascular system helps transport materials all over the body assists with thermoregulation.
Regular exercise makes the cardiovascular system more efficient at pumping blood and delivering oxygen and nutrition to the exercise muscles 17, 18, 19, 21. Releases of adrenaline and lactic acid into the blood during an exercise result in an increase of the heart rate (HR).
Some of the different components of the cardiovascular system, such as stroke volume (SV), systolic blood pressure (SBP), and mean arterial pressure and cardiac output (CO) can be increased by exercises. A considerable percentage of the CO goes to the exercising muscles. While the muscles particularly skeletal muscles get received approximately 20% of the total blood flow at rest, the visceral organs such as spleen, liver and intestine receive a large amount of blood. But during exercise, the blood flow to muscles increases to 80-85%.
Two major adjustments on the blood flow can be occurred during exercise to get the metabolic demands of skeletal muscles fulfilled.
Increasing the cardiac output from the heart.
Returning the blood flow from inactive organs and tissues such as spleen to the active muscles.
Sympathetic and parasympathetic nervous systems regulate the activities of the heart. Acetylcholine (Ach), a neurotransmitter released by the parasympathetic nerve endings, can decrease the activities of Sino-Atrial (SA) node and Atrio-Ventricular (AV) node resulting decreased in HR while the Norepinephrine released by the sympathetic nerve endings causes for the increase in HR and the force of contraction of the heart. Although the sympathetic and parasympathetic nervous stimulations are in balance at rest, during exercises, the parasympathetic stimulation decreases while the sympathetic stimulation increases.
Especially several factors such as baroreceptors, chemoreceptors and temperature receptors directly contribute to adjust the function of the heart.
CO is controlled by the EDV (known as preload), average aortic BP (afterload), and the strength of ventricular contraction. During exercises, the EDV is increased because of the increase in venous return, the afterload is reduced and the strength of ventricular contraction is increased due to increase the EDV according to the Frank-Starling’s law resulting increased the CO.
Constriction of veins that drain skeletal muscles can be occurred as a response to the stimulation of sympathetic nervous system during an exercise.
During exercise, the respiratory pump and the muscle pump help to increase venous return resulting increased the blood flow to the heart. Blood flow during exercise is regulated by changing BP and altering the peripheral resistance of the vessels.
During exercise, BP increases so that blood flow through the body increases. Blood flow is also increased during exercise by decreasing the resistance of the vessels in the systemic circulation of active skeletal muscle. Resistance is determined by the following formula 20.
Resistance = (length of tube X viscosity of blood)/radius
Changing the radius of the vessels has the most considerable effect on blood flow. Doubling the radius of a blood vessel decreases resistance by a factor of 16.
Cardiovascular changes during isometric exercise differ from the changes during isotonic exercise because isometric exercise causes to compress the blood vessels in the contracting muscles. It leads to reduce the blood flow in contracting muscle. So that, the total peripheral resistance will increase instead of the total peripheral resistance that normally falls during isotonic exercise, particularly if several large groups of muscles are involved in the exercise.
The sympathetic system is activated with exercise and thus leads to an increase in BP, HR and cardiac output.
The increase in HR and cardiac output is less due to the total peripheral resistance does not decrease. An increase in the diastolic, systolic and mean arterial pressure is more when compared with those seen with isotonic exercises. Because BP is a major determinant of afterload, the left ventricular wall stress, and thus the cardiac workload, is significantly higher during static exercise compared with the cardiac workload achieved during dynamic exercise.
The musculature of the heart will become certain morphologic changes in response to chronic exercise. Such a heart which has been morphologically changed is commonly referred to as an “athletic heart.” Athletic heart syndrome is characterized by hypertrophy of the myocardium.
Although the hypertrophy in athlete’s heart is morphologically similar to that seen in patients with hypertension, several important differences exist. In contrast to the hypertension-induced hypertrophy, the hypertrophy in the athletic heart is noted in absence of any diastolic dysfunction, with a normal isovolumetric relaxation time, with no decrease in the peak rate of left ventricular filling, and with no decrease in the peak rate of left ventricular cavity enlargement and wall thinning. Because the wall stress in the athlete’s heart is normal, sometimes the hypertrophy seems to be disproportionate to the level of resting BP.
8.0 Respiratory changes during exercise
The purpose of respiration is to provide O2 to the tissues and to remove Carbon Dioxide from the tissues 17, 18, 19. To accomplish this, four major events must be regulated, as follows:
Pulmonary ventilation
Diffusion of O2 and CO2 between the alveoli and the blood
Transport of O2 and CO2 in the blood and body fluids and to and from the cells
Regulation of ventilation and other aspects of respiration
Although the human body is designed to maintain homeostasis, exercise causes these factors mentioned above to change. The formation of CO2, consumption of O2 and the total alveolar ventilation is increased by approximately 20-fold when someone initiates to do an exercise from the state of rest to the maximal intensity of the exercise. The pulmonary ventilation is 100-110 L/min at maximal exercise while there is a linear relationship between ventilation and oxygen consumption. Anyway, the maximal breathing capacity of a person is about 150-170 L/min. That means, during maximal exercise, the maximal breathing capacity is approximately 50% greater than the actual pulmonary ventilation.
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It has been found that the maximum rate of oxygen consumption (VO2max) under the maximal aerobic metabolism is found to increase only 10% due to the effect of training. However, the VO2max of a person who runs in a marathon is about 45% greater than the VO2 of an untrained person. The reasons for this are somewhat genetically determined (larger chest size in relation to body size, stronger respiratory muscles) and due to long-term training as well.
The Oxygen diffusion capacity is known as the measurement of the rate of the O2 diffusion from the alveoli into the blood stream through the walls of capillaries and alveoli. The diffusing capacity of Oxygen is increased because of the exercise and all most all the pulmonary capillaries are perfused at their maximal level due to increase the blood flow through the lungs. Therefore, a considerable surface area is provided for the gases to be exchanged by diffusion. So, it has been found that the trained-athletes have a higher diffusing capacity.
As a result of the active exercises, partial pressure of Oxygen in arterial blood is decreased while that of Carbon Dioxide in the venous blood is increased more than the normal level. Though the both of these are changed, it is not a case because both of these values remain close to the normal values.
Stimulatory impulses from the higher centers of the brain, the impulses from the joint and muscle via proprioceptive stimulatory reflexes cause for the neurological stimulations of the respiratory and vasomotor center of the medulla oblongata which provides almost all the real increase in pulmonary ventilation to keep the blood respiratory gases almost normal. If nervous signals are too strong or weak, chemical factors such as neurotransmitters bring about the final adjustment in respiration that is required to maintain homeostasis during exercises 17, 18, 19.
9.0 Immunological changes in exercise
Although the positive and negative effects can be seen on the immune system with exercises, regular moderate exercise seems to reduce the incidence of infection, while prolonged intense exercise causes a temporary suppression of many parameters of immune function, depending on the intensity and duration of exercise.
The mobilization and activation of white blood cells, the release of inflammatory mediators such as cytokines, the tissue damage and cell infiltration, the production of free radicals, the activation of the complement and the coagulation and fibrinolytic pathways can be seen during physical activities just like an inflammation. The variety of the previous changes depends on the type of exercise intensity and the duration.
Both acute and chronic effects of exercise on the immune system, yet there are still very few studies that have been able to show a direct link between exercise-induced immune depression and increased incidence of confirmed illness in athletes.
Strenious and/or prolonged physical activity leads to muscle and other tissue damage and, thereby, induce an inflammatory response characterized by secretion of pro-inflammatory cytokines, chemokines, and other cellular or hormonal mediators of inflammation. On the other hand, physical activity also induces counter-regulation of inflammation through secretion of immunosuppressant mediators, such as cortisol and anti-inflammatory cytokines 22.
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