Exercise. I INTRODUCTION Exercise, activity that results in contraction of skeletal muscle. The term is usually used in reference to any activity that promotes physical fitness. Although muscle contraction is the common element of all forms of exercise, many other organs and systems are affected, for example, the heart and lungs. Many people also find that regular exercise enhances their sense of mental well-being along with their general physical health. Today there is an increasing emphasis on preventive medicine, or maintaining health, partly as a result of the increasing costs of health care and our greater awareness of the effects of lifestyle on health and longevity. While public interest in exercise and fitness has increased during the past 20 to 30 years, according to the United States National Center for Health Statistics, in 1990 only 41 percent of adults 18 to 64 years of age reported that they exercised regularly, and only 32 percent of those over 65 years of age reported regular exercise or participation in physical sports. Over one-quarter of Americans (three-quarters by some standards) are significantly overweight and are at risk for a wide variety of health problems. II PHYSIOLOGY OF EXERCISE Contraction of skeletal muscles, the muscles under conscious control, is the primary physiological event during exercise. Because skeletal muscles can actively contract, but are not designed to actively lengthen, they are arranged as opposing pairs. As one muscle shortens, another is stretched. An example of such a pair of muscles can be observed in the upper arm, where the biceps and triceps have opposite actions. To flex the arm at the elbow, the biceps contract, while the triceps stretch. To extend the forearm, the triceps contract, while the biceps lengthen. III CELLULAR CHANGES At the molecular level, muscle contraction occurs when large proteins called actin and myosin slide together to shorten muscle fibers. The energy for contraction and relaxation of skeletal muscle is provided by a molecule called adenosine triphosphate (ATP). ATP is a high-energy molecule formed during the breakdown of glucose (a kind of sugar) or fats. Glucose can be stored in muscle as glycogen, and enters exercising muscle from the blood. The metabolism of glycogen or glucose to provide energy for exercise occurs in one of two ways, depending on the presence of available oxygen to the muscle, which in turn depends on the type of exercise being performed. If oxygen is not available (anaerobic activity), glycogen or glucose will be broken down by the anaerobic pathway (glycolysis). If oxygen is available (aerobic activity), it will be metabolized by the aerobic pathway (known as the Citric Acid Cycle). When oxygen is readily available, glucose reacts completely with the oxygen to produce water and carbon dioxide. A portion of the energy released from one molecule of glucose is utilized to produce ATP. A Anaerobic and Aerobic Exercise During anaerobic metabolism, the breakdown of glucose stops at an early point, producing lactic acid and two molecules of ATP. This anaerobic metabolism produces a so-called oxygen debt, which is repaid later when oxygen becomes available. When a skeletal muscle is heavily worked, the acute soreness that results is due partly to a buildup of lactic acid. The presence of lactic acid can also be felt during exercise as a burning sensation in the muscles. Anaerobic exercise involves heavy work by a limited number of muscles, for example during weight lifting. These types of activities are maintained only for short intervals, and the supply of oxygen is insufficient for aerobic metabolism, resulting in a substantial oxygen debt and anaerobic metabolism within those muscles. Another example is sprinting, in which the exercise is high in intensity but short in duration, resulting in substantial oxygen debt. Weight lifting and other types of anaerobic exercise increase strength and muscle mass, but are of limited benefit to cardiovascular health. Unlike anaerobic exercise, aerobic exercise uses oxygen to keep large muscle groups moving continuously at an intensity that can be maintained for at least 20 minutes. Aerobic exercise uses several major muscle groups throughout the body, resulting in greater demands on the cardiovascular and respiratory systems to supply oxygen to the working muscles. Aerobic exercise includes walking, jogging, and swimming, and is the form recommended for reducing the risk of heart disease and increasing endurance. IV HEART, RESPIRATION, AND EXERCISE Although skeletal-muscle contraction is a main feature of exercise, many other systems in the body are activated to support this process. The heart pumps increased volumes of blood to supply oxygen and nutrients and remove carbon dioxide and metabolic wastes; the respiratory system handles an increased workload, exchanging oxygen and carbon dioxide between the blood and the atmosphere. The nervous system and various hormones have important roles as well, integrating the body's response to exercise and regulating the metabolic changes that occur in muscle and other tissues. Another critical role of the cardiovascular system and respiratory system during exercise is to get rid of the heat produced by increased metabolism. During exercise, increased blood flow to the skin results in direct transfer of heat to the environment as well as loss of heat during evaporation of sweat. Substantial heat is also transferred to the atmosphere in exhaled air during breathing. The effects of exercise on the heart and circulation can vary considerably with intensity of exercise and with physical fitness. Assessment of a person's physical fitness often includes measurement of aerobic capacity in the form of maximum oxygen consumption, or VO2 maximum, during aerobic exercise. Oxygen consumption of likesized fit and unfit individuals will be approximately the same at rest or at a given level of exercise, for example, walking on a treadmill. But the more fit person will be able to achieve a greater maximal oxygen consumption due to the training effect that takes place with regular aerobic exercise. As a person engages in regular aerobic exercise, the heart, lungs, and muscles all become more efficient at using oxygen. The heart pumps more blood with each stroke, the lung capacity of each inhalation increases, and the muscle fibers extract more oxygen from the blood. The training effect on the heart is quite obvious when heart rates are compared between longdistance runners and sedentary individuals. The athlete will have a lower heart rate at rest (perhaps as low as 50 beats per minute) and during light jogging, for example, than the nonathlete (who might have a resting rate of 80). During light jogging, the untrained person will experience a large increase in heart rate, while the athlete's heart rate will not rise nearly as much. V BENEFITS OF EXERCISE The benefits of exercise are far-reaching. Clinical and epidemiological studies have demonstrated that regular aerobic exercise reduces the risk of death due to heart disease and stroke, aids in reducing weight, helps prevent diabetes mellitus, strengthens bones, and enhances immune function. The psychological benefits are also broad, and most studies suggest a positive relationship between physical fitness and mental achievement. The relationship between regular aerobic exercise and cardiovascular health and longevity is well established. Regular exercise leads to a reduction in the risk of coronary heart disease, in which fatty deposits (plaque) form in blood vessels supplying the muscular wall of the heart, compromising oxygen delivery to the heart muscle. In addition, with regular exercise the efficiency of the heart during exercise is increased. Many people exercise to lose weight. A calorie is a unit that measures the energy content of foods and the energy expenditure by the body. When the daily calorie intake from food is the same as calories expended from exercise, weight remains the same. The number of calories burned during exercise varies greatly with the type of physical activity, but the key to successful weight reduction is to exercise regularly, without increasing food intake proportionally. For example, walking one hour per day may utilize only 300 calories of energy per day, a small fraction of an individual's daily caloric intake. But over a period of time, if food consumption is simultaneously reduced or remains the same, significant weight loss will result. One sound approach to reducing calories is to eat healthier foods that contain more fiber and less fat, and therefore fewer calories. This type of diet has also been proven healthier for the heart and blood vessels. One area of controversy has been how much exercise is enough to improve general health, reduce the risk of heart disease, and increase longevity. Meaningful studies on this topic are very difficult to perform because they require large populations of subjects and many years of data collection, and because poor health sometimes results in limitations to physical activity. Despite these difficulties, it is clear that regular exercise, along with a generally healthy lifestyle, is beneficial. People who have sedentary lifestyles make up half the population of industrialized societies, and this group has the most to gain by exercising. One recent U.S. National Institutes of Health (NIH) panel suggested that as little as 30 minutes every day of purposeful, moderately strenuous physical activity--for example, rapid walking or lawn mowing--is sufficient to lower the risk of heart disease. There is no conclusive evidence to prove that an especially rigorous exercise routine, such as running many miles per day, as opposed to walking or jogging daily, will add years to a person's life. VI GETTING IN SHAPE Physical fitness is often defined in terms of four measurements: cardiovascular-respiratory function, body composition (the proportion of lean body mass in comparison to fat), flexibility, and muscular endurance and strength. Exercise is characterized in terms of four variables as well: frequency, intensity, duration, and mode. In planning an exercise program, it is important to take into account one's personal fitness objectives and the exercise regimen that will best meet those objectives. Age and existing health conditions should also be considered. Individuals over the age of 40 or who suffer from serious health problems or physical limitations should first consult a physician for recommendations about the best exercise program to adopt. A Fitness Goals If overall fitness or prevention of heart disease is a primary goal, 20 to 30 minutes of moderate-intensity, daily aerobic exercise--such as walking, jogging, swimming, or dance aerobics--should be considered. In general, begin with shorter exercise sessions and gradually work up to 20 to 30 minutes. In addition to reducing risk of heart disease, such an aerobic-exercise program will also help in weight reduction and altering body composition, and in enhancing flexibility. If improving muscle strength is the primary consideration, regular, high-intensity workouts with weights are more appropriate. Studies have shown that even older people can benefit greatly from a weight-lifting workout. In particular, bone density, often a concern in the elderly, is increased, muscle atrophy (decrease) is prevented, and general strength and coordination are improved by this type of exercise. A structured, supervised weight program after consultation with a physician is recommended. Stretching exercises, including yoga, will enhance flexibility. In planning an exercise program, be sure to include stretching exercises and warm-up and cooldown periods to prevent muscle pulls and other injuries. B Exercise Intensity If an aerobic program is adopted, in addition to duration (at least 20 to 30 minutes) and frequency (daily or several times a week), intensity of the exercise should also be considered. The intensity of the aerobic exercise can be determined by evaluating the heart rate attained during exercise. The maximum heart rate (beats per minute) for an individual is approximately 220 minus age. To improve aerobic capacity (VO2 maximum), exercise should be performed at an intensity that produces a heart rate of at least 70 percent of this maximum. For a 20-year-old, for example, the maximum heart rate is 220 - 20, or 200. The heart rate should rise to at least 140 (70 percent of 200). You can determine your heart rate by placing two fingers over the radial artery in the wrist or the carotid artery of the neck. By keeping an occasional record of heart rate responses to a standard exercise (for example, jogging at a rate of 1 mile per ten minutes), it is possible to track your fitness progress; over a period of several weeks, the same exercise will produce a lower heart rate. Similarly, the intensity of exercise (such as the speed of jogging) required to produce a given rise in heart rate will increase. The most important aspect of getting in shape is to make exercise an integral part of one's lifestyle. Exercising to stay as physically fit and healthy as possible should be a lifelong commitment, and is especially important to people who perform little physical work in their day-to-day lives. Contributed By: Adam K. Myers Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
