2. Effects Of Exercise On The Bodily Systems
2.1 Musculoskeletal System
Like most animals, man is endowed with the ability to move in relationship to whatever situation arises, as well as the ability to move different body parts in relation to each other. As a result, we are capable of maintaining various postures and counterbalancing the effects of gravity. We are also capable of transferring mechanical energy to the outer world by doing work as well as by absorbing various mechanical effects. These skills are the result of the ability of the skeletal muscles to transform chemical energy into mechanical energy during their contractions.
The human body is composed of many different individual muscles, each with its own particular job to do. From a functional point of view, however, the mass of skeletal muscles in the organism may be looked upon as one large organ of movement, which constitutes about 40% of our total body weight. The reason for this consideration is that most activities, especially those connected with more vigorous activities such as athletics, are the result of the integrated activity of nearly all the muscles in the body. Thus, as Hygienic practitioners, you should develop exercise programs that will utilize all the various muscles. A program that includes stretching, contracting and aerobic exercise will fulfill this requirement.
Though the skeletal muscles function like one large organ, the instantaneous distribution of activity is continuously changing as the individual muscles are called into action or released into passivity, depending upon how our central nervous system responds to the particular activity.
Individual muscles, in most cases, act as a functional unit with other similar muscle groups. These groups are usually categorized as flexors, extensors, supinators, etc. In most situations, it is possible to determine one or a few prime movers. These prime movers are then assisted by synergists and are controlled or stopped by antagonists that are placed on the other side of the instantaneous axis of movement. So even though it is anatomically possible to define individual muscles, a functional or physiological subdivision of the individual musculature is not possible.
The skeletal muscle is made up of a series of different sense organs: mechanoreceptors, thermoreceptors, pain receptors, etc. Of these, only the mechanoreceptors have been thoroughly studied. The two major mechanoreceptors are the muscle spindles, situated in the center of the muscle, and the Golgis tendon organs located within the tendons.
All skeletal muscles in the body are made up of numerous muscle fibers. In most muscles these fibers extend the entire length of the muscle. Each muscle fiber is enervated by a mixed nerve (motor and sensory) located in the middle of the fiber. Each muscle fiber contains several hundred to several thousand myofibrils. Each myofibril in turn is made up of myosin and actinfilaments, which are specific protein molecules responsible for contraction. The myofibrils are suspended inside the muscle fiber in a matrix ailed sarcoplasm.
The muscle spindles are suspended in a network of connective tissue that is parallel with the muscle fibers. During exercise, the skeletal muscles are continuously undergoing lengthening and contraction. When a muscle fiber is lengthened, sensory impulses are generated and the muscle spindle becomes excited. On the other hand, when a muscle fiber is contracted, the effects of the muscle spindle become inhibited. This response of the muscle is known as the dynamic stretch reflex.
The dynamic stretch reflex is caused by the potent signals originated from key receptors located in the muscle spindle and transmitted by the primary nerve endings of these same spindles. When a muscle is suddenly stretched, a signal is transmitted through these primary endings to the spinal cord and will continue as long as the degree of stretch increases. The end result is a reflex contraction of the same muscle from which the muscle spindle signals originated. Thus a sudden stretch of a muscle causes reflex contractions of the same muscle returning the length of the muscle back toward its original length.
The muscle spindle is also responsible for the “negative stretch reflex.” When a muscle is shortened (contracted), exactly the opposite effect of the dynamic stretch reflex occurs. Upon contraction, the muscle spindle becomes inhibited, thereby discouraging the further shortening of the muscle. Thus the negative stretch reflex opposes the shortening of the muscle in the same manner that the positive stretch reflex opposes lengthening of the muscle. Therefore, it is the muscle spindle reflex that maintains the relative length of a muscle.
The Golgi tendon is entirely an inhibitory reflex and functions exactly opposite from the dynamic stretch reflex. Located within the muscle tendons (that portion of muscle which attaches to the bone), the Golgi tendon is stimulated by relative muscle tension. This is different from the muscle spindle, which is responsible for detecting relative muscle length. The Golgi tendon detects tension transmitted into the spinal cord causing reflex effects on the specific muscle involved. When muscle tension becomes extreme, as in extreme physical exertion, the inhibitory effect from the Golgi tendon organ can be so great that it causes reaction and is a protective mechanism which prevents tearing of the muscle from its bony attachment.
The following points should not only help in summarising what has just been discussed, but should also provide a conceptual picture of the role of skeletal muscles in exercise.
- Skeletal muscles allow the organism to adapt to whatever situation arises in the environment.
- The muscular system helps to maintain proper physical posture as well as counterbalancing the negative effects of gravity.
- The muscular system is composed of many different individual muscles. Depending upon what their duties are. They are either flexors, extensors, supinators, provators, etc. These muscles function in groups. Depending upon what activity is taking place, one or two muscles are the prime movers. Other muscles support the prime movers as synergists and yet other muscles provide antagonism that balances the prime movers. During most physical activities, the skeletal muscles of the entire body continually function as an integrated unit.
- The skeletal muscles are intimately connected and are directly dependent upon the smooth functioning of the entire nervous system. In order to maintain the nervous system, attention must be given to all the areas of Hygienic living. Poor nutrition, lack of rest, etc. lead to an imbalance in proper regulation of nerve force, leading to a decrease in vital energy and less effective mechanical activity.
- The sense organs of the musculoskeletal system provide the means by which the tone of the muscle is maintained. These organs provide the means to insure the maintenance of muscular stability by providing the necessary information for either contraction or relaxation of the specific muscle.
It is important to look at the muscular system in a wholistic manner. Though it is possible to develop individual muscle groups with specific exercises, this can only cause an imbalance with various other muscle groups that are not simultaneously developed. Therefore, it is important to provide a balanced and consistent exercise which will provide the necessary strength, flexibility and endurance to the entire muscular system. The end result will be a muscular system that is highly developed and functioning in harmony with all the other systems of the body.
The functions of the skeletal system are complex and varied. Besides providing a structural frame that muscles, nerves, organs and all other various tissue can surround and attach to, the skeletal system provides storage for many nutrients and produces blood cells. Bone tissue continually undergoes a process of bone destruction and repair. Large osteoclastic cells are continuously engaged in destruction of bone cells and osteoblasts continuously repair the damage.
According to Wolff’s law, bone develops its greatest strength along the lines of greatest stress and strain. An increase in physical activity stimulates osteoblastic (building) activity, while a decrease favors an increase in osteoclastic (destructive) activity. In this manner the osteoclastic cells are encouraged to remove the useless bony structures and osteoblasts are stimulated to replace them with stronger bone in the region where greater strength is required. The more activity an individual individual engages in, the stronger and more durable the skeletal system becomes.
In our society, as an individual becomes older, his or her level of physical activity dramatically decreases. As a result, the bony structure begins to demineralize, leading to a weakening of the entire structure. As the individual becomes weak and feeble, the bons may begin to spontaneously fracture. Had the individual maintained his or her exercise program, he or she would not have become dependent upon others to provide the movements no longer available to him.
2.2 The Respiratory System
2.2.1 Basic Mechanisms of lung expansion or contraction
The lungs can be expended and contracted by (1) downward and upward movement of the diaphragm to lengthen or shorten the chest cavity and by (2) elevation and depression of the ribs to increase and decrease the diameter of the chest. Inspiration takes place when the diaphragm contracts, pulling the lower boundary of the lung cavity downward, increasing its longitudinal length. Expiration takes place automatically when the diaphragm relaxes, allowing the elastic recoil of the lungs to draw it back upward. During normal inspiration, respiration takes place simply by contraction of the diaphragm. However, the mechanical means by which respiration takes place during exercise is a little more involved.
As we’ve already mentioned, normal inspiration takes place principally when the diaphragm contracts, thereby pulling and lengthening the lungs. This creates a lower pressure in the lungs that is automatically filled by the higher atmospheric pressure outside the body. Usually, expiration is an entirely passive process; that is, when the diaphragm relaxes, the elastic structures of the lungs, chest cage, and abdomen force the diaphragm upward.
During exercise, a greater demand is placed upon the mechanical structures of respiration. More muscles come into activity in order to increase the amount of oxygen now needed by the rest of the body. During inspiration, not only does the diaphragm contract, but muscles from the chest, neck and spine also contract and aid in the process. Expiration is no longer passive; it is aided by contractions from the abdominal muscles as well as from the lower rib muscles. From a mechanical point of view, if these various muscles are not strong and capable of functioning efficiently and effectively, the tissues of the body will not be supplied with sufficient amounts of oxygen during exercise.
2.2.2 Diffusing capacity of respiratory membrane
During respiration, oxygen is taken into the lungs from the atmosphere. In the lungs oxygen is exchanged with CO2 (waste product from cell metabolism) from the bloodstream. The oxygen is carried to all the tissues in the body in order to nourish and provide the necessary component for developing energy necessary for cell functioning. The overall ability of the lungs to exchange gases from the blood is expressed in terms of its diffusing capacity. In the average individual, the diffusing capacity for oxygen under resting conditions averages about 21 milliliters/minute. However, during strenuous exercise, the diffusing capacity for oxygen increases to about 65 ml/minute, or three times the diffusing capacity under resting conditions. In order for this to happen, three bodily functions must take place:
- More blood vessels must become available, thus creating more surface area for the gaseous exchange between the lungs and the blood.
- Dilation or expansion of all blood vessels.
- Stretching of the lung surface responsible for gaseous exchange, thus making the walls thinner for gas exchanges and increasing the surface area.
The more an individual exercises, utilizing increased respiration, the more efficient the diffusing capacity will become. During rest or slight activity, only a small portion of the lung is used, resulting in a significant portion becoming dormant. The same holds true for the circulatory system where a significant number of blood vessels are not utilized. This disuse will lead to a decrease in the ability of the system to provide the necessary oxygen for the tissues even during the slightest degree of activity.
During strenuous exercise, the average individual may require as much as 20 times the normal amount of oxygen. Due to the increase in cardiac output, however, the time that blood remains in the area of gas exchange is greatly reduced. As a result, the oxygenation of blood could suffer for two major reasons: the blood remains in contact with the lungs ,for shorter periods of time, and far greater quantities of oxygen are needed to oxygenate the blood. However, the blood is almost always fully oxygenated when it leaves the lungs even during the heaviest of exercise, due to two major factors:
- As we’ve already mentioned, the diffusing capacity for oxygen increases about three fold during exercise.
- The blood stays in the lungs about three times as long as necessary for oxygenation. Therefore, even with the shortened time of exposure during exercise, the blood can still become fully oxygenated.
As you can see, with exercise, the lungs and their structural synergists become stronger, more resilient and much more efficient. More tissues in the body are oxygenated while simultaneously more blood vessels and lung tissue are utilized. The lungs are interdependent upon all the other systems of the body. Only when all these systems are functioning smoothly and efficiently can we achieve the high levels of health and enjoyment that life has to offer.
2.3 Cardiovascular System
Perhaps the single most important factor that we must consider in relation to the cardiovascular system is “cardiac output.” Cardiac output is the quantity of blood pumped from the heart into the aorta each minute. Venous return is the quantity of blood flowing from the veins back to the heart each minute. Although blood can temporarily increase or decrease in central circulation, the total cardiac output must be equal to venous return. The average cardiac output for normal young males is about 5.6 liters/minute. When including all adults and females, the cardiac output is an average of approximately 10% less than that of the normal male. Since cardiac output changes with body size, the output is commonly stated in terms of the cardiac index. The cardiac index is determined by cardiac output per square meter of body surface area. The average cardiac index for adults is about 3.0 liters per minute per square meter.
When a person rises form a reclining to a standing position with the muscles becoming taut, as if preparing for exercise, the cardiac output rises 1-2 liters per minute. Cardiac output usually remains almost proportional to the metabolic body rate; the greater the degree of activity or the muscles and organs, the greater will be the cardiac output. Therefore, the work output during exercise increases in linear proportion to the cardiac output. In bouts of very intense exercise, the cardiac output can rise as high as 30-35 liter per minute in a well-trained athlete.
This is 5-6 times the normal value.
During heavy exercise, tissues can require as much as 20 times the normal amount of oxygen and other nutrients hat are transported via blood. Thus, transporting enough oxygen from the lungs to these tissues may demand a minimal increase in cardiac output of five to six times its normal value! Since this is far greater than the normal, instimulated cardiac output of the heart, several factors which will insure this massive increase of cardiac output during this heavy exercise are called into play. They are as follows:
- Before exercise begins, the autonomic nervous system is stimulated by the thought of exercise. This stimulation increases the permissive level of the heart pumping from 10-20 liters per minute, depending upon exertion caused by the exercise. Simultaneously, extra quantities of blood are pushed from the periphery toward the heart by constricting veins and increasing systemic filling pressure. These factors may increase cardiac output as much as 50% before exercise even begins!
- At the onset of exercise, the blood vessels of the muscles become dilated, due to the signals transmitted from the motor cortex to the sympathetic nervous system. This instantaneously increases the cardiac output. The by-product of this sympathetic activity increases heart activity, mean systemic pressure, and arterial pressure.
- The direct effect of increased metabolism in the muscles causes an increased use of oxygen and other nutrients as well as the release of vasodilating substances. Thus, local vasodilation and local blood flow increase tremendously.
In summary, an intricate setting of background conditions of the cardiovascular system insures the required blood flow to the muscles during heavy exercise. These conditions include increased activity of the heart muscle. The local vasodilation in the muscles occurs as a direct consequence of muscular activity and finally sets the level to which the cardiac output rises. Thus, it is mainly the muscles themselves that determine the amount of increase in cardiac output, up to the limit of the heart’s ability to respond.
The heart, like any other muscle, can be strengthened or weakened, depending upon the amount of activity or exercise it undergoes. According to Starling’s Law, the heart is an automatic pump that is capable of pumping far more than the normal value of 5 liters per minute of blood which returns from peripheral circulation. Thus, the primary factor that determines how much blood will be pumped by the heart is the amount of blood that flows into the heart from systemic circulation, which is greatly enhanced by physical exercise. After a certain point, averaging about 15 liters per minute, cardiac stimulation (such as the stimulation of exercise) is necessary for this increase in the permissive level to which the heart can pump. Exercise greatly increases the effectiveness by which the heart
provides blood and thereby oxygen and other vital nutrients to all areas and tissues of the body.
In juxtaposition with this increase in cardiac efficiency, these vital blood pathways are cleaned out and overall circulation is enhanced. Like any muscle, when there is an increase in activity
and usage, there is an increase in size or musculature enlargement. Heavy athletic training causes the heart to enlarge, sometimes as much as 50%. Coincident with this enlargement is an increase in the permissive pumping level of the heart that may be as great as 20 liters per minute (as opposed to the maximal normal level of 13-15 liters per minute). So, when exercise is integrated into each day of our lives, there is an overall long-term increase of effectiveness of the entire cardiovascular system.
In summary, exercise greatly increases the demand of blood flow to the muscles and tissues. To insure this required increase of blood flow, there must be an increase in arterial pressure and an overall, increase in the activity of the heart muscle. As a direct consequence of muscular activity, there is local vasodilation of the muscle tissue involved that sets the final level of the rise in cardiac output. This rise is vital in the insurance of muscular efficiency. The heart acts like any other muscle in that it can be strengthened and enlarged with heavy exercise or weakened by neglect. As Hygienic practitioners, it is important to realize the relevance of exercise in providing blood, oxygen and other vital nutrients, to all areas of the body and enhancing overall cardiovascular efficiency in circulation. Only with maximal cardiovascular efficiency can we maintain our strength, endurance and clarity of mind.
2.4 Other Systems
Generally speaking, during heavy exercise there is a constriction of blood flow to certain organ systems that are not as immediately involved in the physiology of exercise as are the musculoskeletal, cardiovascular and respiratory systems. The organs of the gastrointestinal tract and the kidneys in particular are affected by the detour of blood flow and energy during exercise.
Severe exercise appears to have at least two principal effects upon the kidneys: diminished urine flow, and diminished renal (kidney) blood flow. When the blood volume becomes too great, the cardiac output and arterial pressure increase. This has a profound effect on the kidneys, causing loss of fluid from the body and blood volume to return to normal. Conversely, if blood volume falls below normal with a decrease in cardiac output and arterial pressure, the kidneys retain fluid and the progressive accumulation of fluid intake rebuilds up to the normal blood volume. During severe exercise, the rise in body temperature causes increased sweat and respiratory loss of water, which intensify kidney changes. The sweat excreted during heavy exercise contains 300-600 mg. of urea per liter (a concentrated waste product that is diluted by the kidneys to prevent poisoning of the system), thus to a considerable extent compensating for the decrease of excretion of urea through the kidneys during exercise.
During heavy exercise, there is a great reduction in renal blood flow, and a slight alteration in the filtration rate of the kidneys. The reduction of renal blood flow is progressive for at least 30 minutes after the start of exercise and is directly related to the severity of the exercise. (Chapman, 1948 a.b.).
This drop in renal blood flow can be explained by the diversion of blood to the working muscles and the brain. Recovery of renal plasma flow is considerably slower than is the recovery of pulse rate or blood pressure (Chapman et al. 1948). The resting kidney has a large inbuilt safety margin so that the renal blood flow can be drastically altered without significantly altering the functioning of kidney filtering.
2.4.2 Gastrointestinal Tract
It appears that strenuous exercise inhibits both secretory and motor functions of the stomach. Studies done by Campbell, 1928 came to an early conclusion that exercise of moderate intensity (such as running 1-2 miles slowly) inhibited both secretion of gastric juice and the rate of gastric emptying of its contents. Lighter exercise (such as walking) did not change the rate of gastric excretion and actually appeared to enhance the rate of emptying into the stomach. The amount of exercise required to inhibit gastric function varied with the physical fitness of the individual. To generalize their findings, “exercise which produced no discomfort helped digestion, and exercise which produced discomfort delayed it.” These observations were extended later in the century, concluding that all types of exercise after a meal prolonged the final emptying time of the stomach more than the same activity preceding the meal. When flouroscopically examined immediately after exertion, the stomach appeared either totally inactive or had only feeble peristaltic movements. Recovery, however, was prompt and emptying was greatly accelerated during the second hour after exercise.
In summary, there is evidence that heavy exercise has certain effects on kidney functioning. The sweat excreted during heavy exercise contains a high concentration of urea, thus compensating for the decrease of urea excretion via the kidneys during exercise. When there is an increase of sweat and respiratory loss of water, there is an intense change in the kidneys. Also, during extreme exercise, there is a drop in renal blood flow due to the diversion of blood to the brain and working muscles.
It also appears that strenuous exercise inhibits both secretory and motor functions of the stomach, although the amount of exercise required to inhibit gastric function is dependent upon the physical fitness of the individual. Though exercise tends to temporarily inactivate stomach function during exertion, there is a prompt recovery and acceleration of function in the post exercise hour.