To be Active or Not to be Active?
By Len Kravitz, Ph.D. and Robert R. Robergs, Ph.D.

It is commonly acknowledged in the science community and with the American public that physical activity and exercise exert a positive impact on many aspects of health. This preoccupation in improving one's own health has spawned a host of health products, diet books, exercise videos, and fitness clubs. In addition, a more health conscious community has developed markets of professional health services (e.g. exercise physiologists, personal trainers, and health promotion professionals). The United States Department of Health and Human Services has defined our nation's public health agenda for the 1990's with the release of Healthy People 2000 (1990) , the national health promotion and disease prevention objectives. The three broad goals from this report are : 1) to increase the span of healthy life for Americans, 2) to reduce health disparities among Americans, and 3) to achieve access to preventive services for all Americans. Physical activity and fitness are essential components of this plan. Despite the merits of physical activity towards better health, 25% of the population over 18 report no physical activity, with inactivity rates especially high in older age groups, and only 1 person out of 5 (20%) reports physical activity for at least 30 minutes, five or more times a week (USDHHS, 1990) . In a 1985 National Health Interview Survey, only 7.6% of persons in the United States exercised at the level recommended to attain cardiopulmonary benefits (Caspersen, Christenson, & Pollard, 1985) . The aim of this paper is to present an overview on the health benefits of physical activity and exercise in order for the exercise instructor, personal trainer, and health promotion specialist to better educate their clients (and future clients) about the advantages of compliance to an active way of life.

Coronary Heart Disease
The incidence of coronary heart disease (CHD) has declined considerably over the last 25 years. According to the U.S. Department of Health and Human Services (1988), deaths from CHD have dropped 42% between 1964 and 1985. Yet CHD is still the leading cause of death in the United States for both men and women. CHD is caused by a lack of blood supply to the heart muscle, resulting from a degenerative disorder known as atherosclerosis. Atherosclerosis involves a gradual buildup and deposition of fat and plaque on the inner lining of the coronary arteries. The annual costs associated with CHD range from $41.5 to $56 billion (Lenfant, 1992) . The goals for our nation's CHD interventions need to be directed to youth and adults, as there is resounding evidence which demonstrates that atherosclerosis begins in early childhood. As many as 60% of children in the United States exhibit at least one modifiable adult risk factor for coronary heart disease by the age of 12 (Berenson et al., 1980) . Recent research suggests that physical activity in childhood is a determinant of physical activity as an adult (Powell & Sysinger, 1987) . People who are more active develop less CHD than their inactive counterparts. An analysis of the data from over 40 studies (referred to as a meta-analysis) by Berlin and Colditz (1990) indicated that CHD is 1.9 times more likely to develop in sedentary individuals than in physically active persons. This positive influence of exercise is independent of other CHD risk factors such as hypertension, smoking, obesity, diabetes, or family history of CHD. It is interesting to note that a number of studies have also indicated that when CHD does develop in physically active individuals, it occurs at a later age and tends to be less severe (Haskell et al., 1992) .
In 1986 Dr. Ralph Paffenbarger presented the results of his landmark study with 16, 936 Harvard University alumni men who were followed over a period of 16 years. Results of this famous study indicated that men who exercised and expended at least 2,000 calories per week increased their life expectancy by 1 to 2 years (Paffenbarger, Hyde, Wing, & Hsieh, 1986) . In 1989, the Institute of Aerobics Research in Dallas, Texas published another landmark study involving more than 13,000 men and women over an eight-year period that confirmed that physical fitness level reduced all-cause mortality (Blair et al., 1989) . Results of this study, which actually measured each participants fitness level with a maximal treadmill test (Balke protocol), demonstrated that as fitness level increased, mortality decreased. The least fit men died at a rate almost 3.5 times higher than the most fit, while for women the ratio was 4.5 times higher. The study divided participants into five categories based on their fitness levels with the least fit (sedentary) in category one and the most fit (serious exerciser such as runners who logged 30 to 40 miles per week) in category five. A most surprising finding was that the moderately fit levels, those in categories 2 and 3 had almost a third fewer deaths than those in category one. There were additional, but minimal gains, for those persons in categories 4 and 5.

Hypertension
Hypertension, which is often unrecognized, is a significant risk factor for cardiovascular and cerebrovascular disease. Hypertension is especially prevalent among Black Americans. If an individual's systolic blood pressure exceeds 160 mm Hg, the risk of CHD is 4 times greater, and if the diastolic reading exceeds 95 mm Hg the CHD risk is increased 6 times (Heyward, 1991) . The degree to which exercise can help a hypertensive is currently unclear, due largely to methodological shortcomings in research designs. However, it is well established that systolic and diastolic blood pressure can be modestly lowered with aerobic exercise in normotensive and hypertensive individuals.

Blood Lipids and Lipoproteins
People who perform regular vigorous endurance exercise have higher high-density lipoproteins (HDLs) in their blood than do sedentary persons (Haskell, 1984) . This is especially true for the HDL2-C subfraction which has been particularly associated with lowering the risk of CHD (Wood, 1987). The HDLs, which are manufactured in the liver, carry cholesterol from peripheral tissues in the body back to the liver where it is used to produce bile salts (which aid in the digestion of fat). HDLs are known as the "good cholesterol" because they are associated with lowering the risk of artery disease by removing some cholesterol from artery walls. Positive, though modest, improvements in HDL concentrations can be realized even after initiating an exercise program. The recent Helsinki Heart Study strongly demonstrated that raising HDL cholesterol levels lowered the risks of heart attack (Manninen et al., 1988) . However, it is presently unclear how much exercise, at what intensity, and for what period of time will optimally elevate HDL levels. Physical activity may also lower blood concentrations of the harmful cholesterol known as low-density lipoprotein cholesterol (LDL-C), and also very low-density lipoprotein cholesterol (VLDL-C).

Exercise also has a powerful lowering effect on blood triglyceride (fat) levels (Haskell, 1984) . The triglyceride level tends to fall immediately after exercise and often remain low for several days. This change may be attributable to an increase in lipoprotein lipase (an enzyme) which facilitates the breakdown and assimilation of triglycerides in the body. Another factor may be that with regular exercise there is a decrease in the production of insulin, which may stimulate a decrease in the stimulus for triglyceride synthesis. Regular exercise helps to prevent the development of insulin resistance by the cells and increased insulin levels that often occur with advancing age. In spite of lower insulin levels, physically active persons have good blood sugar (glucose) tolerance as a result of better insulin sensitivity.

Cardiac Function
There are a number of processes that are closely linked to the efficiency of the cardiorespiratory system that change with physical activity. Often times there is a reduction in resting heart rate that can be attributable to aerobic exercise. The heart's stroke volume (the amount of blood pumped per beat) increases at rest and during exercise as an adaptation to endurance training (McArdle, Katch & Katch, 1991) . The volume of the left ventricular cavity, the pumping chamber of the heart, often adapts to training by increasing in size as well as wall thickness. In addition, physically active people have larger coronary vessels than less fit persons. The most significant cardiovascular function change is seen with an increase in cardiac output, the amount of blood circulated by the heart each minute (cardiac output = heart rate X stroke volume). This change helps to increase a person's physical work capacity. Aerobic training will also improve the ability of the muscles to extract oxygen from the blood, which is referred to as the arteriovenous oxygen difference (Rowell, 1986). This increased capacity of the muscles to extract oxygen is believed to be primarily due to an increase in capillary density (Rowell, 1986). The total effect of all these changes is a stronger, more efficient cardiovascular system.
There are several notable physiological changes that also occur when the cardiorespiratory system is sufficiently overloaded. Among these adaptations include the following:

1. The trained muscle is more efficient at mobilizing and metabolizing fat attributable to an increase of blood flow within the muscle and greater activity of fat-mobilizing and fat-metabolizing enzymes.
2. The mitochondria, the subcellular structures responsible for the production of ATP (the high energy compounds from which our bodies derive energy) have a significantly greater capacity to generate ATP, due to an increase in their size and number.
3. There is an increase in the oxidative capacity of the muscle in conjunction with an increase in glycogen (the form in which glucose is stored in the body) storage, resulting in a more favorable capacity to oxidize carbohydrates.
4. The skeletal muscle fibers develop to their full aerobic potential.

Bone Mineral Status
The prevention of osteoporosis in aging women has become a serious health issue. It has been estimated that approximately 1.2 million fractures occur each year as a result of this condition, including 227,000 hip fractures and 530,000 vertebral fractures (Johnson & Slemenda, 1987). Although exercise is recommended, along with calcium supplements and estrogen replacement therapy, information on the best form of exercise for preventing or reversing this bone degradation is still insufficient. Weight-bearing movements such as low-impact activities, walking, and jogging (depending on the person's fitness level and preexisting risk to fractures), increase the mechanical stress on the skeletal system, and may reduce or reverse bone mineral loss in aging women (Smith & Gilligan, 1987) .

Smoking
People who are physically active are less likely to smoke than their sedentary counterparts. In every category of smokers (i.e. non-smokers, former smokers, light, moderate, and heavy smokers) those who are physically active have a lower risk of developing CHD than sedentary people in the same category (Paffenbarger, 1987) .

Body Composition and Weight Control
Approximately 25% of the adult American population is obese. Obesity is a prominent problem in our society and is a contributing risk factor in cardiovascular disease, hypertension, and diabetes. Exercise offers both short-term and long-term benefits to obesity. An immediate effect and benefit of exercise is caloric expenditure. Studies on the increased metabolic demands of the body after an exercise bout have shown 1 to 25% increases in caloric expenditure that can last from 1 to 24 hours (Van Zant, 1992) . Numerous factors such as the type and intensity of exercise, subjects' fitness level, and dietary intake prior to measurement contribute to the range of increase metabolic rate. However, recent data suggest that the bulk of the increased energy expenditure after an exercise bout occurs within the first hour of recovery (Brehm & Gutin, 1986; Gore & Withers, 1990) . A consensus of recent investigations seem to agree that one of the major benefits of exercise, as it relates to weight loss, is in the positive impact it has on maintaining lean body mass while encouraging the loss of fat body weight (Hawk, 1989; Work, 1990) .

Blood Glucose Regulation
Regular physical activity can be beneficial in helping to manage diabetes, particularly Type II, the non-insulin dependent or adult-onset diabetes. There is a considerable amount of deliberation regarding the best prescription of exercise within the total treatment program for diabetics. It does appear that a balance of regular physical activity, satisfactory medications, and adherence to a proper diet, will help diabetics keep their blood sugar under control.

Musculoskeletal Disorders
Physical activity can reduce the potential of various musculoskeletal disorders such as osteoarthritis, bone fractures, connective tissue tears, and low-back syndrome. It has been demonstrated that regular exercise, challenging the musculoskeletal system, can increase bone mineral content (Smith & Gilligan, 1987) . Physical activity programs designed to develop muscular strength and flexibility of the musculoskelatal system will improve structural weaknesses that can contribute to these disorders.

Stress Management and Mental Health
Studies have shown that exercise brings about both short and long term psychological enhancement and mental well-being (Morgan & Goldston, 1987) . A new position statement by the International Society of Sport Psychology (1992) has summarized a number of psychological benefits of physical activity. These include: improvement in self-confidence and awareness, relief of tension, positive changes in mood, relief of feelings such as depression and anxiety, increased mental well-being, influence on pre-mentrual tension, increased alertness and clear thinking, increased energy, the development of positive coping strategies in daily activities, reduction in various stress indices, and the increased enjoyment of exercise and social contacts. The impact of these psychological benefits of physical activity can be realized by persons of all ages and for both sexes.

Conclusion
The knowledge and science documenting the positive changes of physical activity with respect to disease risk factors, several major chronic diseases, and total health is dramatically clear. The last few years have seen a definite, and appropriate, shift in emphasis from only vigorous exercise to that of light to moderate levels of physical activity for improved health. People should be encouraged to engage in activities that are personally pleasing and satisfying, since this may increase their physical activity adherence. Now, more than ever, the involvement of exercise instructors, personal trainers, and health promotion specialists with their clients, students and communities can play a significant role in our achieving the goals for a healthy nation 2000.
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