Physical activity in the prevention of cardiovascular disease.

[Francis K. Physical activity in the prevention of cardiovascular disease. Phys Ther. 1996;76:456-468.

Key Words: Exercise, Heart disease, Physical activity.

Cardiovascular heart disease (CHD) is one of the major public health problems in the United States. Despite dramatic declines in mortality from heart disease and stroke in the past two decades, CHD remains the principal cause of mortality in the United States, with over 7 million Americans affected by the disease.[1,2] In 1990, more than 934,300 Americans died as a result of cardiovascular disease (approximately one of every three deaths), which exceeds the number of deaths in all the wars during this century.[2] Of those having their first heart attack, 30% die instantaneously or during the acute phase. For men, the chances for a first heart attack before the age of 60 years is 1 in 5.[3] The need for preventive strategies is obvious.

Regular physical activity has long been regarded as an important component of a healthy lifestyle. Sedentary living is estimated to relate to about one third of deaths due to CHD,[4,5] and millions of adult Americans exhibit a completely sedentary lifestyle.[6] A logical strategy for reducing the risk of mortality and morbidity and the economic burden associated with CHD, therefore, is to change lifestyles to enhance physical activity. This strategy is particularly attractive in light of recent reports suggesting that health benefits can be gained by performing moderate-intensity physical activities outside of formal exercise programs. Therefore, the focus of this review is on physical activity and the health benefits associated with an active lifestyle in different age groups of the population. The quantity and quality of physical activity and possible strategies for promoting cardiovascular health also are discussed.

For this review, physical activity is defined as any bodily movement produced by skeletal muscles that results in increased energy expenditure.[7] Physical activity is closely related to but distinct from "exercise" and "Physical fitness." Exercise is defined as planned, structured, and repetitive bodily movement done to improve or maintain one or more components of physical fitness. Physical fitness is a set of attributes that people have or achieve that relates to the ability to perform physical activity. The term "physically fit" conveys the ability to perform moderate to vigorous levels of physical activity without undue fatigue and the capability for maintaining such ability throughout life.[7] It is important to note that this review is limited by the fact that most of the research related to cardiovascular disease has been conducted on men. Unless otherwise noted, the studies discussed in this article were conducted with male subjects.

Economic Burden of Physical Inactivity

The number of coronary bypass procedures performed each year in the United States is approaching 300,000, with each procedure costing more than $30,000.[8] Based on 1989 mortality estimates for CHD shown in Figure 1, the extrapolated cost of physical inactivity is $5.7 billion[9,10] per year. Among the diverse risk factors for CHD shown in Figure 1, only elevated serum cholesterol levels ([greater than or eqaul to]200 mg/dl) have a higher estimated cost.[10] Hatziandreu et al[11] conducted a cost-effectiveness analysis estimate of the health and economic implications of a physical activity program in preventing CHD using a model with two hypothetical cohorts. Cost-effectiveness is a relative and subjective concept defined as "having an additional benefit worth the additional cost."[11] Data for each of the tao cohorts (one with exercise and the other without exercise) of 1,000 35-year-old men were generated for a 30-year period to observe differences in the number of CHD events, life expectancy, and quality-adjusted life expectancy (measured in quality-adjusted life-years [QALYs]).(*) Physical activity was associated with 78 fewer CHD events and 1,138 QALYs gained during the 30-year period. Exercise had an additional benefit worth the additional cost of $11,313 per QALY, including the costs of time spent exercising and $1,395 in direct costs (excluding employment benefits and other indirect costs). This was a favorable value when compared with the costs of other preventive or therapeutic interventions for CHD such as control of hypertension, which exhibited savings costs of $25,000 per QALY, and smoking cessation, which exhibited savings costs of $21,947 per QALY.[12,13] Shephard[14] has estimated that more than $200 per worker-year of medical and related costs could be saved if persons participated regularly in a physical activity program.

Major Risk Factors for CHD in Children

The mounting evidence of the health benefits of regular exercise coincides with increasing concerns that a large percentage of American youngsters are less physically fit than desirable.[15-17] Youth fitness authorities have contended for more than three decades that a large percentage of American youngsters are less physically fit than is desirable.[18,19] This contention is supported by reports that school-aged children have more body fat and weigh more than children did 20 years ago.[20-22] More alarming is the observation that as many as 60% of children by the age of 12 years in the United States exhibit at least one modifiable adult risk factor for CHD[23] and that the risk factor remains into adulthood.[2-25] Among the more prevalent risk factors that have been found in children that persist into adulthood are dyslipidemia, hypertension, and physical inactivity.[23]

Serum cholesterol levels of children have been tracked in several studies for periods of up to 20 years. These data make it clear that within populations, whether at high or low risk for CHD, individuals tend to stay at a similar level of serum cholesterol for long periods of their lives. Thus, if children were in the 90th percentile when they were first examined, 10 to 20 years later as adults more than 40% of them would still be in the 90th percentile.26,27 Results from the International Atherosclerosis document the presence of fatty streaks by 3 years of age in the aortas of many children. Fat intake during childhood has also been shown to be related to the incidence of fatty streaks in the aortas of individuals killed in accidents during their childhood or adolescence.[28,29] Fatty streaks frequently have been observed in the coronary arteries of children as young as 10 years of age, but more importantly the presence of such lipid deposits is associated with arteriosclerosis in adulthood.[29]

Elevated blood pressures measured in childhood do not identify with precision those adults who will be hypertensive.[30] Blood pressures obtained in childhood, however, tend to retain the same rank within the normal range over time. In particular, systolic blood pressures above the 90th percentile for age remain consistently elevated over time and theoretically allow identification of those children who will ultimately develop hypertension as adults.[31,32] Because blood pressure varies with body size, the level of a given child's blood pressure must be interpreted in terms of age, height, and weight. Upper limits of normative values of blood pressure such as that shown in Figure 2 for boys have been developed and can be useful in identifying children at risk.[30] (See Kavey[30] for a comprehensive review of normal and abnormal values of blood pressure in both male and female children.

Dennison et al[24] suggest that physical inactivity in childhood is a determinant of physical inactivity in adulthood. In a prospective study, the physical activity levels of 453 young men 23 to 25 years of age were compared with physical fitness; test scores obtained when they were children (10-11 years of age). Figure 3 shows the risk of physical inactivity in young adulthood was related to the number of low scores on the 600-yd-run (548.6-m-run) test recorded during childhood.

Because lifestyle habits such as physical activity and exercise that persist into adulthood are established during childhood and adolescence, early identification of children at risk might allow intervention programs to be targeted toward these children. Schools, which commonly administer physical fitness tests, are excellent sites for identifying children who are possibly at risk of becoming inactive adults and for instituting health-related programs.[32] To optimize these opportunities, however, strategies are needed to implement state and school-district policies requiring comprehensive school health education programs that include nutrition education and daily attendance in physical education classes.

Physical Inactivity as a Risk Factor for CHD in Adults

Compelling epidemiological, clinical, and laboratory research indicates that regular physical activity can help to control and decrease the risk of CHD in adults.[33] The most well-known set of studies that suggests that changes in exercise patterns are associated with lower rates of death from CHD is the male Harvard Alumni Study.[34-37] Harvard College archives from 1916 through 1950 provided researchers with physical activity data on thousands of former students who were studied to determine whether their characteristics at college age predisposed them to CHD later in life. For example, in a series of studies, more than 17,000 male Harvard alumni who had not been diagnosed with CHD previously were followed for 16 years to study the relationship between inactivity and incidence of morbidity and mortality. These researchers found that men with weekly energy expenditures of more than 2,000 calories (8,400 J) per week demonstrated a 39% reduction in cardiovascular morbidity and a 24% reduction in cardiovascular mortality.[34-37] Numerous other population and cohort studies also have confirmed a reduction in ischemic events among individuals who perform regular physical activity either at work or during leisure time, including London transportation[38] and civil servants[39,40]; San Francisco longshoremen[41]; railroad workers[42]; white-collar civil service workers[43]; and participants in the Framingham,[44] Multiple Risk Factor Identification Trial;[45] and Lipid Research Clinic studies.[46]

In the past few years, several large, well-conducted prospective studies[47-53] have been published that provide additional strong evidence that the effects of regular physical activity appear to be graded, independent, and long-term predictors of mortality from cardiovascular causes in asymptomatic, middle-aged men. Low disease rates in men who are regularly physically active are evident in the presence as well as in the assence of classical and other established risk factors. For example, two recent prospective studies in Britain[47-49] showed that the incidence of CHD in physically active men of middle and early old age is approximately halved in comparison with the incidence of CHD in sedentary men. Morris et al,[47] in a 9-year follow-up study of 9,376 male British civil servants aged 45 to 64 years at entry into the study, showed that those who participated in vigorous sports had less than half the nonfatal and fatal coronary disease of those who were sedentary. Likewise, Wannamethee and Shaper,[48,49] in a 9 1/2-year follow-up study of 7,735 men aged 40 to 59 years who had participated in the British heart study, found that physical activity reduced the risk of stroke and heart attacks in men both with and without preexisting ischemic heart disease.

Lakka et al[53] studied the relation of self-reported leisure-time physical activity and cardiorespiratory fitness to the risk of acute myocardial infarction in 1,453 men without diagnosed cardiovascular pathology or cancer between the years 1985 and 1989. After adjustments for age and the year of the medical examination, the relative risk ratio of myocardial infarction in the patients who practiced intense physical activity (more than 2.2 hours per week) was 0.31 in comparison with that of the men who indicated the lowest degree of physical activity.[dagger]

A recent meta-analysis by Berlin and Colditz[55] combined all the available occupational and nonoccupational cohort studies that investigated the relationship between exercise and CHD. Berlin and Colditz concluded that a sedentary lifestyle nearly doubles the risk for heart attack in both men and women. In addition, these investigators state that "the methodologically stronger studies tended to show a larger benefit of physical activity than less well-designed studies."[55](p612) Centers for Disease Control and Prevention, after reviewing 43 studies that provided associations between physical activity and CHD, has concluded that physical inactivity is, by itself, a significant risk factor for the development of CHD.[56] The relative risk ratio was 1.9 in sedentary people as compared with active people after adjustment for other CHD risk factors.[56] This relative risk is only slightly less than the relative risks of elevated serum cholesterol levels, hypertension, and smoking shown in Figure 4. Not only is physical inactivity now recognized as an independent risk factor for CHD, but physical activity appears to have a favorable effect on conventional risk factors and an additional effect on clotting mechanisms.[57] Physical exercise improves functional work capacity and usually lowers heart rate and blood pressure response to a give work load. In addition, exercise helps reduce weight, lowers serum triglyceride levels, increases high-density lipoprotein levels, decreases platelet adhesiveness, enhances fibrinolysis, and lessens adrenergic responses to stress.[58,59]

Of the independent risk factors for CHD, sedentary lifestyle has been reported to be the most prevalent modifiable risk factor, followed by cigarette smoking, elevated serum cholesterol levels, and hypertension. Paffenbarger et al[35] have estimated the population-attributable risk[double dagger] for CHD mortality associated with physical inactivity to be 14%, in comparison with 20% for hypertension, 13% for cigarette smoking, and 20% for a positive family history of premature parental death.

Physical Activity and Elderly Individuals

In 1988, more than 11% of the US population (30.4 million persons) was older than age 65 years.60 The percentage of those aged 65 years and older is expected to grow to 12.8% by the year 2000 and to over 20% by the year 2030 when the children of the post-world War II "baby boomers" will have reached old age.[61-63] Since 1980, the percentage of the population in the 65- to 74-year age range has increased by 11% and the percentage of the population in the 75- to 84-year age range has increased by more than 17%. The fastest-growing segment of the population is the group older than age 85 years, which has increased 24.8% during the past 10 years.[61]

Three to four percent of persons aged 55 to 64 years are too disabled to walk 800 m or climb stairs, but at age 75 to 84 years more than 15% have difficulties in walking and more than half cannot lift an object weighing 4.5 kg.64 Although epidemiological interest in physical activity in the elderly population is a fairly recent phenomenon, the available data suggest that benefits of physical activity may extend to the elderly population.[65,66] Kaplan et al,[65] for example, studied the association of physical inactivity and 17-year mortality in subjects in the Alameda County (Calif) Study who were 60 to 94 years of age at baseline. Little or no leisure-time physical activity was associated with a consistently increased risk for premature death at all ages. Individuals who were over the age of 60 years at baseline and who were physically inactive had 1.5 times the mortality risk of those who maintained a moderate level of physical activity.

Donahue et al[66] examined the relationship of physical activity to the development of CHD in elderly men (65-69 years of age) participating in the Honolulu Heart Program. After 12 years of follow-up, results indicated that the high levels of physical activity at the beginning of the study were inversely related to the risk of CHD. The rate of CHD in men who led active lifestyles was less than half the rate experienced by those who led more sedentary lifestyles (relative risk=0.43).

Persons older than 60 years appear to benefit from exercise training at least as much as younger adults, and maintaining a physically active lifestyle can reduce the risk of CHD and extend the active life span (ie, life characterized by independence).[64,67,68]

Physical Activity and Individuals With Cardiovascular Disease

About 6 million people have been estimated to be living with diagnosed CHD, and large numbers of others are believe to be living with significant, but as of yet undiagnosed, silent myocardial ischemia.[69] The World Health Organization has attributed a primary role to physical exercise and rehabilitation programs for patients with heart conditions, calling for "the best possible physical, mental, and social conditions for heart patients so that they are able to maintain or resume as far as possible their normal role in society."[59(pS7)]

In a recent review of exercise conditioning programs, Leon[70] reported that these programs led to improved functional capacity and cardiovascular efficiency as well as enhanced psychological well-being. Evidence, however, does not indicate whether an exercise conditioning program can independently (without the influence of other risk factors such as lowering blood pressure) reduce recurrence of fatal or nonfatal CHD events. Although more than 4,700 patients have been studied in randomized trials of cardiac rehabilitation, individual studies have had sample sizes that were too small to distinguish reliably between any moderate (eg, 15%-30%), yet worthwhile, reduction in morbidity and mortality.[71] Although the value of rehabilitation programs has not been thoroughly assessed, support for the role of exercise in reducing the risk for cardiovascular disease comes from the demonstration of a favorable "trend" for lower mortality rates in individuals who exercise.[72-78]

For example, the National Exercise and Heart Disease Project studied 651 men aged 30 to 64 years with diagnosed CHD in a randomized 3-year clinical trial of the effects of prescribed supervised exercise on morbidity and mortality.[79] The men in the exercise group exercised intensively in the laboratory for 8 weeks and then in a gymnasitim, 3 days per week for 34 months. The cumulative rate of recurrent myocardial infarction was 7% in the control group and 5.3% in the intervention group, and the total mortality was 7.3% in the control group and 4.6% in the intervention group. Mortality rates in the two groups did not differ, but the data were consistent with the author's hypothesis that exercise would result in a substantial benefit.

Oldridge et al[80] reported the results of a meta-analysis on the combined results of 10 randomized clinical trials that investigated the effect of exercise on the rehabilitation of patients who had undergone a myocardial infarction. Of the total of 4,347 patients in the Analysis, 2,202 patients were randomly assigned to a cardiac rehabilitation program and 2,145 patients served as controls. Cardiac rehabilitation began within 8 weeks to as late as 3 years after myocardial infarction. Duration of the programs were as short as 6 weeks to as long as 4 years. comprehensive cardiac rehabilitation resulted in a beneficial effect on mortality but not on nonfatal recurrent myocardial infarction. All-cause mortality (death from all causes [eg, cancer, stroke, infection] combined) was reduced by 24% and cardiovascular mortality was reduced by 25% for the patients in the cardiac rehabilitation program, a risk reduction as great as that reported for beta-blockade regimens used in clinical practice.[81]

In a meta-analysis evaluating the effects of exercise training of patients who had an acute myocardial infarction, O'Connor et al[71] reported a 20% reduction in both overall mortality and cardiovascular mortality. The results were apparent as early as 1 year after randomization and persisted for at least 3 years after infarction. The relative risk ratio for sudden death showed a large reduction of 0.63 in risk during the first year. Based on the inferential evidence in these reviews, physical exercise appears to be a promising yet unproven intervention for patients who have had a myocardial infarction.

Trends in Physical Activity in Children

Despite more than four decades of compelling epidemiological, clinical, and laboratory research indicating that regular physical activity can help to prevent and control CHD, Americans remain largely inactive.[82,83] Despite the highly visible "fitness boom" that occurred in the 1970s and early 1980s, the prevalence of individuals who perform physical activity has changed little since that time.[82,83]

The percentage of children who are unfit is not clear because different criterion-referenced standards are used in various fitness programs.[84] In virtually all the programs, however, 20% or more of the children fail to meet recommended fitness standards in one or more areas.[17,22,85] The overall trend in physical fitness appears to have declined in the 1980s. For example, measurements of physical fitness as measured with the 1.6-km (1-mile) run test[86] indicate that boys aged 6 to 15 years scored lower in 1985 than in 1980, with a difference of more than 20 seconds in 6 of the 10 age groups. Scores of 12- to 17-year-old girls also declined on the 1.6-km run between the period 1979 to 1981 and 1985. Results of a decade-long study that evaluated fitness annually using the Physical Fitness Test Battery in a sample of 12,000 boys and girls indicate an overall decline in aerobic fitness of approximately 10% as measured by distance runs of 0.4 km (0.25 mile) for 6- to 7-year-olds, 0.8 km (0.5 mile) for 8- to 9-year-olds, 1.2 km (0.75 mile) for 10-to 11-year-olds, and 1.6 km for children 12 years of age and older.[22]

Similar declines in physical activity also have been noted in other large surveys. In 1984, the National Children and Youth Fitness Survey showed that a total of 65% of 9th- to 12th-grade students were enrolled in physical education classes.[87] In 1991, this figure had declined to 48.9%.[88] Of the students reporting being enrolled in physical education classes, only 41% reported attending daily. Figure 5 illustrates that the decline becomes even more striking when grouped by grade. Daily attendance decreased substantially from the 9th grade through the 12th grade. Among the students attending physical education classes, almost 50% indicated that they do not spend at least 30 minutes exercising or playing sports during the class.[88]

Time spent watching television also has been implicated as a contributing factor in the. decline in physical activity among America's youth.[89-91] Dietz and colleagues[89,90] reported that among 6,965 children aged 6 to 17 years surveyed in the National Health and Nutrition Examination Survey (NHANES) Cycle II (1963-1965) and Cycle Ill (1966-1970), children aged 6 to 11 years watched television an average of more than 23 hours per week and adolescents spent approximately the same amount of time (22 hours per week) viewing television. As Figure 6 illustrates, there is an association between time spent in television viewing and the prevalence of obesity. The prevalence of obesity was found to increase by 2% for each additional hour of television viewed.

These trends are alarming and suggest the need for additional efforts and greater attention if the Healthy People 2000 national health objectives of decreasing obesity and increasing physical activity are to be met.[8] The Healthy People 2000 goals for children include (1) no more than 15% obesity among adolescents, (2) increasing to more than 50% the proportion of youth in grades I through 12 who engage in daily school physical education, and (3) increasing from 27% to more than 50% the share of physical education class time actually devoted to physical activity.

If approximately 20% of America's children are at risk because of low fitness, that means that nationally between 8 and 9 million school-age children are at risk. Responding to such a need and opportunities could be today's "best investment" in public health.[92]

Strategies to Increase Physical Activity in Children

Addressing the subject of physical activity should begin in the preschool age group. Most children of this age are intrinsically active, but a few are sedentary by nature.[85] Parents should be encouraged to introduce the importance of exercise by promoting regular activity. Because time spent in physical activity correlates inversely with time spent watching television, a time limit on watching television could be important.[30]

The American Heart Association in the special report "Integrated Cardiovascular Health Promotion in Childhood" provides the following recommendations that can be used by families to promote physical activity and improve physical fitness in children[93]:

* Regular walking, bicycling, and backyard play; use of stairs, playgrounds, and gymnasiums; and interaction with other children

* Less than 2 hours per day of television watching or computer and video games

* Weekly participation in age-appropriate organized sports, lessons, clubs, or sandlot games

* Daily school or day-care physical education that includes a minimum of 20 minutes of coordinated large-muscle exercise

* Regular participation in household chores

* Weekly family outings that involve walking, cycling, swimming, or other recreational activities

* Positive role modeling for a physically active lifestyle by parents, other caretakers, physicians, and school personnel

Trends in Physical Activity in Adults

In 1985, 22% of adults ranging in age from 18 to 74 years engaged in light to moderate exercise five or more times per week.[8] In 1991, this percentage had increased only slightly to 24%.[83]. Light to moderate physical activity requires sustained, rhythmic muscular movements, at least equivalent to sustained walking, and is performed at less than 60% of maximum heart rate for age, five or more times per week. Maximum heart rate equals approximately 220 bpm minus age.[8] Likewise, there was only a slight increase in the percentage of persons 18 years of age and older performing vigorous exercise, from 12% in 1985 to 14% in 1991.[83] Vigorous physical activities are rhythmic, repetitive physical activities that use large muscle groups at 60% or more of maximum heart rate for age.[8]

More alarming, however, was the report that 58.5% of adults 18 years of age and older have a sedentary lifestyle, representing no change from 1986[94] (Fig. 7). A sedentary lifestyle is defined as having fewer than three 20-minute sessions of leisure-time physical activity per week.[8] It has been estimated that less than 10% of the American adult population participates in sustained large-muscle activities at least three times per week at an intensity sufficient to improve or maintain cardiorespiratory fitness.[94]

Because of the strong association between physical activity and cardiorespiratory fitness and optimal health and because of the positive impact that exercise can have on cardiovascular disease progression and risk factors, it is not surprising that physical activity was made the first priority of the 22 priority areas of the Healthy People 2000 initiative.[8] Given the current trends of physical inactivity, however, the Healthy People 2000 goal of having no more than 15% of the US population aged 6 years and older being sedentary will not be met.[82] In 1986, 31.9% of adults 18 years of age and older reported engaging in no exercise, recreation, or physical activities other than those in their jobs. In 1990, this figure had changed little, decreasing slightly to 28.7%.[94] This trend is unfortunate because Powell and Blair[54] theorized that if everyone were highly active, the death rate from CHD would be only two thirds of the current rate. These investigators noted that even though it is unlikely that everyone will become highly active, increases in physical activity practices that are less than highly active could reduce mortality from CHD as much as 5.6%, or 28,500 deaths per year.[54]

Many older people are obese, are unable to tolerate intense or prolonged physical activity, and are susceptible to cardiovascular disease, hypertension, and diabetes.[64] The goal for promoting physical activity for people aged 65 years and older is to reduce the prevalence of those in this group who engage in no leisure-time physical activity to no more than 22%.[8] The prevalence of sedentary lifestyles in people aged 65 years and older has remained relatively the same since 1986. In 1986, 65.4% of this age group reported having a sedentary lifestyle, and in 1990, this figure had decreased only slightly to 63.9%.[94] Clearly, aggressive measures to increase physical activity are needed if the Healthy People 2000 health goals for physical fitness are to be met to improve the health and longevity of elderly persons.

Quantity and Quality of Physical Activity

Frequently, the question is asked, "How much exercise is enough?" or "How intensely and how often should a person exercise to develop and maintain fitness?" Recognizing that the term "physical fitness" is a multifaceted term composed of a variety of characteristics, including cardiorespiratory fitness, strength, and flexibility, the American College of Sports Medicine (ACSM) has published recommendations for the quantity and quality of exercise for developing and maintaining cardiorespiratory and muscular fitness in asymptomatic adults.[95,96] The recommendations are shown in the Table.

Table. Exercise Guidelines for Asymptomatic Adults

Variable           Prescription

Intensity          60%-90% of maximum heart rate
                   (HRmax), or 50%-85% of maximal
                   oxygen consumption or heart rate
                   (HR) reserve (HR reserve is derived
                   by the formula: [(HRmax-resting
                   Hr) + resting HR])

Duration           20-60 min/session of continuous
                   aerobic activity (optimal)
                   30 min of accumulated short bouts of
                   brisk activity (minimal)

Frequency          3 to 5 d/wk

Mode               Any activity that uses large muscle
                   groups, can be maintained
                   continuously, and is rhythmic and
                   aerobic in nature (eg, walking,
                   jogging, biking, swimming)

Training effects   improved cardiovascular function
                   Increased oxidative capacity of muscle
                   increased capillarization of muscle

Recent data suggest that moderate physical activity levels, which are lower than the recommendations of the ACSM, may reduce the risk for death from CHD, even though these levels may not improve maximal oxygen consumption. For example, in a 7-year follow-up of 12,138 middle-aged men in the Multiple Risk Factor Intervention Trial, Leon and coworkers[45,97] observed similar decreases in relative risk of CHD and sudden death for individuals exercising at only modest levels and for individuals exercising more vigorously. Participants were classified into tertiles according to their self-reported level of leisure-time physical activity calculated in kilocalories,, per day. The lowest tertile (ie, the lower one third of the levels) exercised at an intensity that used only 310.8 kJ (74 kcal) per day, whereas the middle tertile exercised a level that used 936.6 kJ (223 kcal) per day and the tipper tertile exercised at a level that used 2,679.6 k (640 kcal) per day. The highest tertile was more than twice as active as the middle tertile and five times more active than the lowest tertile. The relative risk ratios for CHD in the middle- and upper-level fitness groups, however, were nearly the same, ranging from 0.63 to 0.64.

Paffenbarger and colleagues[35-37] have reported similar findings from data collected as part of the Harvard Alumni Study. These authors followed the association between self-reported physical activity levels and subsequent mortality in 10,269 alumni from 1977 to 1985. A physical activity index (in kilocalories per week) was computed in which walking 1 km was assigned a value of 252 kJ (60 kcal), ascending and descending stairs was assigned a value of 168 kJ (40 kcal), and sports play and other recreational activities were scored according to intensity of effort as a ratio of activity metabolic rate to resting metabolic rate. Compared with those individuals who reported expending less than 2,100 kJ (500 kcal) per week, individuals expending only 2,100 to 8,400 kJ (500-2,000 kcal) per week exhibited a relative risk ratio for death (relative risk ratio=0.7) similar to that of individuals expending 8,400 to 14,700 kJ (2,000-3,500 kcal) per week (relative risk ratio=0.68).

In a follow-up study, Lee et al[98] examined the independent association of vigorous ([greater than or equal to]6 resting metabolic rate [MET] score) and nonvigorous (<6 MET score) physical activity with longevity using prospective data collected as part of the Harvard Alumni Study. They followed 17,331 alumni from 1962 or 1966 through 1988. These investigators found that men who reported expending 6,300 kJ/wk or more in vigorous exercise had 0.75 to 0.87 times the risk of dying during follow-up compared with those who expended less than 630 kJ/wk. This difference in mortality risk was of approximately the same magnitude as that between alumni who were 20% or more overweight and those of ideal weight and of approximately the same magnitude as that between alumni who smoked one pack of cigarettes or less daily and nonsmokers.[98]

Studies investigating levels of physical fitness and the risk of CHD by measuring cardiovascular fitness during exercise testing support the concept that even modest levels of exercise are advantageous in protecting against CHD.[46.50,52.99] For example, the relationship between physical fitness and CHD and all-cause mortality was studied in 2,014 male industrial workers in Oslo, Norway, over a 7-year period.[99] Physical fitness was assessed in relation to a near-maximal bicycle exercise test. The investigators subdivided the workers into quartiles of physical fitness within each 5-year age group and studied levels of CHD risk factors and CHD deaths within the 16 subgroups over the 7-year period. They noted that resting heart rate, blood pressure and serum lipids, smoking, maximal heart rate and blood pressure during exercise, and death from myocardial infarction were inversely associated with levels of physical fitness. Individuals in the highest quartile of fitness exhibited one fifth the incidence of fatal CHD exhibited by those in the lowest fitness quartile, whereas individuals in the middle quartiles exhibited a 60% reduction in the incidence of fatal CHD.

Sandvik et al[50] in a 16-year follow-up study of 1,960 Norwegian men showed a dose-response effect for cardiovascular mortality that became evident between the 8th and 16th years of follow-up. Conventional coronary risk factors and physical fitness measured as the total work performed on a bicycle ergometer during a symptom-limited exercise tolerance test were assessed. The relative risk ratio for death from cardiovascular causes in those men in the highest quartile of physical fitness as compared with men in the lowest fitness quartile (quartile 1) was 0.41. The corresponding relative risk ratios for middle fitness quartiles 3 and 2 as compared with quartile 1 were similarly lower at 0.45 and 0.59.

Blair et al[52] studied physical fitness and all-cause mortality in 10,224 middle-aged men and 3,120 middle-aged women who were given preventive medical screenings at the Cooper Clinic at Dallas (Tex). The subjects were divided into five fitness levels based on maximum treadmill performance and were followed for 8 years for all-cause mortality. The death rate in the most-fit group of men was 70% less than that in the least-fit group. Similarly, the death rate in the most-fit group of women was 78% less than that in the least-fit group. An increasing mortality gradient was noted in both men and women from the most-fit group to the least-fit group, with the middle group showing 45% less mortality than the lowest-risk group for men and 43% less mortality than the lowest-risk group for women. Using the most-fit quintile as the reference category, Figure 8 shows that in men the age-adjusted all-cause death rate was 3.4 times higher in the less-fit quintile group but only 1.37 and 1.46 times higher in the moderate-fitness groups. The age-adjusted all-cause death rate in women was 4.65 times higher in the less-fit quintile group but only 2.42 and 1.43 times higher in the moderate-fitness groups. These trends persisted in both men and women even when cigarette smoking, elevated blood pressure, elevated cholesterol, elevated glucose, family history of heart disease, and body mass index were taken into account.

In a more recent prospective study conducted over a 4.9-year period, Blair et all[100] evaluated the relationship between changes in physical fitness and the risk of mortality from all causes. The authors evaluated the health status of 9,777 men and administered a maximal exercise test at the beginning and end of the study. The men who were unfit at both examinations exhibited a death rate that was 67% higher than that of men who were fit at both examinations. Men who improved from unfit to fit between the first and second examinations exhibited a reduction in risk of mortality of 44% relative to men who remained unfit from the first to the second examination. For each minute increase in maximal treadmill time between examinations, there was a corresponding 7.9% decrease of mortality. Similar results were seen when the group was stratified by health status and for cardiovascular disease mortality.

The results of these Studies suggest that moderate physical activity below the level recommended for cardiorespiratory fitness can reduce the risk for CHD. In 1994, a group of experts were brought together by the Centers for Disease Control and Prevention and the ACSM to review evidence and to develop practical guidelines for the public regarding physical activity. This group concluded that regular, moderate levels of physical fitness, which are attainable by most adults, appear to protect against early mortality from CHD and to provide substantial health benefits.[101] The panel recommended that every adult should engage in 30 minutes or more of moderate-intensity physical activity on most, and preferably all, days of the week. Adults who engage in 30 minutes of moderate-intensity physical activity during the course of a day (ie, 840 kJ [200 kcal] or more per day) can expect to have many of the health benefits of more intense exercise performed for longer periods of time. The recommended 30 minutes of activity can even be intermittent and can be accumulated by performing short bouts of such activities as walking up a flight of stairs, gardening, or raking leafs if conducted at an intensity corresponding to that of a brisk walk.

The Challenge

The challenge for physical therapists is: How can physical therapists contribute to the promotion of a cardiovascularly healthy America? All persons, regardless of age, should be encouraged to develop physically active lifestyles because of the low risks and proven diseaseprevention health maintenance benefits. They should be encouraged to increase physical activity to levels appropriate to their capacity, needs, and interest. Physical therapists may need to become more skilled at motivating people to initiate and maintain regular exercise activities. This will be a challenge because approximately 50% of individuals who start an aerobic exercise program will stop within the first 6 months.102

Oldridge[103] reported that adherence dropped from 59% in a supervised program to 29% after 6 months of unsupervised exercise. Practically, adherence may take the form of return visits in which the patient knows that fitness changes will be assessed and discussed, or more ideally, it could involve developing exercise goals, graded activities to achieve those goals, and success indicators that the patient would use to monitor progress. Martin et al,[104] for example, examined different combinations of strategies for enhancing adherence to a series of 3-month exercise programs. In this series of studies, techniques such as feedback and praise during exercise, goal setting, lottery reinforcement, cognitive strategies during exercise, and relapse prevention training were used. Martin and colleagues concluded that the combination of personalized feedback and praise, flexible exercise goals set by the individual, and training in cognitive dissociation could result in maximal adherence rates of 80% to 85% for the 3-month program.

To initiate a program of exercise, a person needs strong reasons to change and help surmounting the barriers to starting and maintaining an exercise program. One barrier that therapists can control is patients' confusion or lack of understanding of what they are to do and why.[105] Clear instructions combined with sufficient practice and feedback to develop the patients' confidence in their ability to do the exercise correctly could develop the patients' self-assurance that they can do the exercise and that they can expect positive outcomes from the exercise.[106]

Summary

Data support the conclusion that an inverse association exists between physical activity and CHD. Sedentary persons make up a large proportion of the population of the United States, and the incidence of CHD attributable to insufficient physical activity remains unacceptably high. A sedentary lifestyle should be considered one of the more important modifiable risk factors for CHD. Physical therapists' encouragement of regular physical activity should represent a major goal in the primary prevention of CHD.

The quantity and quality of exercise needed to attain health-related benefits may differ from what is recommended for physical fitness benefits. This is fortunate because exercise requiring moderate endurance and performed on an almost daily basis is more likely to be adopted and maintained than vigorous physical activity. Regular activity requiring moderate endurance is probably the most feasible exercise prescription, with considerable potential to reduce CHD while increasing the likelihood of long-term compliance in individuals who are completely sedentary.

Transformation from a sedentary state to a more active lifestyle could pay large dividends to the individual and to society. Motivating people to make a substantial investment of personal time in increased physical activity may be America's "best investment," which could mean a significant reduction in risk of CHD, a decrease in demand for medical services, and a reduction of health care costs.

[Figures 1-8 ILLUSTRATION OMITTED]

(*) Quality-adjusted life expectancy is a relative and subjective cost-effectiveness concept based on the question: Is having an additional benefit, such as an added year of productive work, worth the additional cost of the activity?[11] ([dagger]) The strength of the relationship is often expressed as a relative risk ratio or odds ratio, depending on whether the observation study was cohort study for which rates of risk can be calculated but for which rate ratios can be estimated. The relative risk ratio is calculated by dividing the rate of the outcome (eg. CHD, myocardial infarction) among the exposed (sedentary) persons by the rate of the outcome among the unexposed (active) persons.[54]

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