Exercise
From Encyclopædia
Exercise may be described as the active use of the body to build or maintain strength and endurance and to make the body healthier. This can include anything from running marathons to brisk walking, for anyone from the professional athlete to the sedentary person trying to get in shape. Exercise as a recreational activity is relatively modern development. In the past, many aspects of life in and out of the home involved physical labor. Now, relatively few occupations involve such labor, and physical activity has become associated with recreational exercise.BASIC EXERCISE PHYSIOLOGYThe primary physiologic event in exercise is contraction of skeletal muscle, or so-called voluntary muscle. Muscles perform movements through a series of either concentric shortening or eccentric lengthening. Concentric contractions draw the ends of the muscles closer together, shortening the fibers, while eccentric
muscle contractions move the ends of the active muscles farther apart, elongating the fibers. Concentric contractions lift an object; eccentric contractions lower the object. Eccentric
muscle contractions use more energy compared to concentric exercise. When a muscle
contracts, it compresses the blood vessels in it, but between contractions, blood flow in exercising muscle is increased as much as thirtyfold. Extra
oxygen must be carried to active
cells and
carbon dioxide away from them at high rates, resulting in increased circulatory and respiratory rates.The energy for this activity comes from the two systems for different types of activity. The anaerobic system, which does not require
oxygen, provides energy for short-term activity of moderate intensity. This system uses
glycogen (
glucose) from food as the fuel source. It is the major source of energy for the first minute and a half of exercise. Beyond the second minute of exercise, the
oxygen-using aerobic system, for endurance activities, predominates. The aerobic system requires
glycogen, fats, and proteins for fuel sources. Exercise that depends on stamina and endurance uses the aerobic system and has been found to contribute more to cardiovascular health. Most daily activities are aerobic since they require little power and occur over prolonged periods, but heavy labor is usually both aerobic and anaerobic.The initial energy for muscle action comes from the manufacture of adenosine triphosphate (ATP). When energy is released by the breakdown of ATP, the muscle
cells contract. This system provides energy for short, quick bursts of activity and is the major source of power for the first 20 to 30 seconds of intense exercise. After this the anaerobic and aerobic systems take over.As the heart rate rises, cardiac output increases, and there is an increase in the systemic arterial blood
pressure. Exercise causes an increased blood flow, which will increase
oxygen delivery to muscles for work. If an area of the body is not used during the exercise program, peripheral resistance equalizes the blood
pressure in parts not being used.During exercise the body consumes
oxygen in two different ways, one for the total body and the other for the myocardium, or heart muscles. Total body or ventilatory
oxygen consumption (VO2) is the amount of
oxygen consumed by the body as it performs work. Maximal VO2 is figured by multiplying maximal cardiac output and the maximal difference between the
oxygen in the arteries and veins. Cardiac output is equal to the product of the
stroke volume of the heart and the heart rate; this is used to determine exercise tolerance and to measure cardiac performance.Contracting muscle
cells may increase total heat production 10 to 20 times the normal temperature and thus place severe demands on the mechanisms that regulate
body temperature. As exercise continues over a period of time, the
body temperature rises, causing the temperature-sensitive
cells in the brain to be activated, which inhibits the nervous input of sympathetic outflow to the
skin vessels. This stimulates the nerve fibers to the sweat glands enlarging them so perspiration occurs, lowering the
body temperature.BENEFITS OF EXERCISEExercise has both physical and psychological benefits. Physically, regular exercise helps develop muscle
tone and strength and
Control weight. Besides strengthening the muscles, including the heart, regular exercise is believed to make bones stronger by increasing calcium uptake and has been shown to reduce high blood
pressure and
cholesterol levels. Psychologically, regular exercise is thought to contribute to a feeling of well-being, as well as to help relieve
stress.Regular exercise helps keep younger people in shape, in areas such as weight maintenance, heart rate, lung capacity, and
cholesterol and blood
pressure levels. People who exercise regularly are more likely to continue to exercise throughout their lives. As people age, the benefits of regular exercise grow more important. While genetics plays a role in the effects of aging, exercise has a beneficial effect on the aging body, helping to maintain fitness and slow
Down the physical effects of aging. The body becomes deconditioned with age and, if not properly exercised, can develop significant problems in the muscles, bones, and cardiovascular system. Muscles wither away and lose their
tone, leading to more frequent tearing of muscle tendon units. Bones become weak and brittle, fracturing easily and more often. The cardiovascular system adjusts
Down to the point where any little chore can cause an increase in the pulse rate and blood
pressure and earlier development of
atherosclerosis.High blood
pressure, heart disease, and high
cholesterol work together in the development of cardiovascular disease. Regular exercise is believed to protect against cardiovascular disease, especially myocardial infarction. Of major importance is the consistent finding that being physically active does not increase an individual's chance of cardiovascular disease. Exercise has been shown to lower
cholesterol levels while increasing the level of high
density lipoproteins, the so-called "good"
cholesterol.Physiologic studies of older runners, tennis players, and swi?mers reveal superior fitness among this active group, compared to others in the same age group. Studies show that highly active individuals lose an
average of 4% in cardiovascular fitness, compared to 8% for inactive subjects. Regular exercise retards or postpones the age-related decrease in physiologic capacity by 50%.EXERCISE AND WEIGHT LOSSA good proportion of the U.S. population is obese, and the major cause is lack of physical activity. Most sensible weight loss programs now recommend a combination of diet and exercise to slowly take off extra weight, with an ideal weight loss of two to three
pounds per week.An excess expenditure of 350
carbohydrate calories is required in order to use one
pound of pure fat. The caloric deficit should not exceed 2000 to 2500
carbohydrate calories per day or four
pounds of fat per week and should represent both increased expenditure (through exercise) and reduced caloric intake. Once the desired weight has been reached, an exercise program of 20 minutes per day, three times a week is necessary to maintain the new weight.EXERCISE PROGRAMSBefore undertaking an exercise program, a person should have a physical examination by a doctor, to establish that he or she can participate in this kind of activity without adverse effects. The program should be designed around the person's work capacity, to estimate the intensity of training needed to get this person into good condition. Work capacity can be determined in several ways. The most accurate method is to determine the maximal
oxygen uptake (VO2 max), the greatest rate at which
oxygen is utilized during work. This test requires special and expensive equipment, so work capacity is usually measured by other techniques that are simpler to perform. Exercise tests using the
treadmill or
bicycle ergometer are the most common methods of indirectly measuring work capacity, because of their value for medical clearance.Once physical work capacity has been determined, the amount of exercise needed to get a person into shape can be computed. Generally, an exercise program should approximate 70 to 75% of VO2 max. Physical activity can also be prescribed by training heart rate, which is related to work intensity. The formula for calculating training intensity from heart rate is as follows: Lower conditioning HR = (maximum HR - resting HR) x 60% + resting HR; upper conditioning HR = (maximum HR - resting HR) x 80% + resting HR.The three components of a good exercise program are the warm-up period, the aerobic exercise period, and a cool-
Down period. A warm-up is very important to prevent injuries, because it brings blood to the muscles, readying them for exercise, whether it is walking, running, biking,
swimming, or playing a sport. A warm-up should include stretching and flexibility moves, to prevent tears in the muscles and tendons.An exercise program should slowly work the individual into shape. How much exercise is enough and what type is best for developing and maintaining fitness depends on the starting condition of the individual. In general, the exercise should be done every other day, and should bring the target heart rate to between 60 and 90% of the maximal heart rate reserve. The activity should be performed continuously at the proper intensity for 15 to 60 minutes per exercise period.
