Physical Inactivity as a Risk Factor for Chronic Disease

Krista A Barbour, Timothy T Houle, Patricia M Dubbert. The Health Psychology Handbook: Practical Issues for the Behavioral Medicine Specialist. Editor: Lee M Cohen. Sage Publications. 2003.

The health benefits of physical activity have been demonstrated repeatedly over the past several decades, and engaging in regular physical activity has been recommended as one effective way in which to decrease both morbidity and mortality (U.S. Department of Health and Human Services, 1996). Unfortunately, current guidelines for leisure time physical activity are not met by the majority of individuals in the United States. Indeed, a significant percentage of the population is considered sedentary or insufficiently active (Centers for Disease Control and Prevention [CDC], 2001). Because of its status as a primary risk factor in the development of chronic disease, physical inactivity must be addressed in any effort to reduce rates of illness and early mortality.

Definitions and Descriptions of Physical Activity and Inactivity

In 1996, the U.S. surgeon general issued a report outlining the importance of physical activity for good health. The report included recommendations for the intensity, duration, and frequency of physical activity sufficient for meeting the goal of disease prevention in the general population. A primary conclusion of the report was that a moderate level of activity (e.g., 30 minutes of walking on most days of the week) is an appropriate goal for most Americans in terms of realizing health benefits. However, the report also emphasized that 60% of American adults are not physically active on a regular basis and that 25% are sedentary (e.g., report no leisure time physical activity). A recent, population-based telephone survey of American adults found that rates of physical activity remained stable during the years of 1990 to 1998 (CDC, 2001), a finding that was disappointing and somewhat surprising given the increased emphasis on public health initiatives to increase physical activity rates in the United States.

Perhaps even more discouraging are the results of studies focusing on demographic differences in the prevalence of regular physical activity. These findings indicate that the percentage of active adults is considerably smaller among ethnic minorities (particularly Hispanic Americans) relative to Caucasians. In addition, women are generally less physically active than men, and physical activity decreases with age. Other predictors of an inactive lifestyle include lower income, lesser educational attainment, and living in the southern or midwestern United States (Schoenborn & Barnes, 2002).

According to an American College of Sports Medicine (1998) position stand, healthy adults should engage in 20 to 60 minutes (which may be continuous or accumulated in several shorter bouts) of aerobic activity 3 to 5 days per week to improve cardiorespiratory fitness. For most people, a moderate intensity level of physical activity is recommended to decrease the likelihood of exercise-induced injury. In addition to aerobic exercise, individuals are encouraged to incorporate both resistance (e.g., set of exercises that are designed to condition major muscle groups two or three times per week) and flexibility (e.g., stretch major muscle groups two or three times per week) training into their exercise programs.

However, as already noted, most American adults do not meet this level of physical activity. Given this unfortunate fact, it is important to consider the potential negative consequences of physical inactivity. For example, a lack of physical activity has been clearly linked to an increased risk of all-cause mortality (see, e.g., Blair et al., 1989; Wei et al., 1999). In addition, a sedentary lifestyle has been shown to predict the nation’s number one cause of death: cardiovascular disease (CVD) (Farrell et al., 1998). Indeed, a large percentage of the population is at increased risk of disease morbidity and mortality because of insufficient physical activity. The surgeon general’s report and the body of scientific knowledge it represents suggest that many individuals will acquire illnesses that can be prevented by physical activity. In addition, it is possible that the progression of many diseases and conditions can be slowed or halted by becoming active if one is sedentary or by increasing one’s activity level. For most of the remainder of the chapter, the focus will shift from physical activity as a primary prevention strategy to physical activity as it relates to specific chronic diseases.

To discuss the role that physical activity plays in chronic disease, it is useful to first address the issue of assessment of physical activity. This includes the methods commonly used to measure activity as well as recent trends in assessment (for a review of the major developments in physical activity research during the past decade, see Dubbert, 2002). The following section addresses the numerous ways in which physical activity has been conceptualized by researchers.

Overview of Research in the Assessment of Physical Activity

The measurement of physical activity has received increasing attention during the past several decades. The task of quantifying physical activity across various populations, settings, and purposes has proved to be particularly daunting. In the subsections that follow, the methods that have been used to measure physical activity in both the laboratory and community settings are summarized, with special emphasis on each method’s particular strengths and weaknesses. The issues involved with the assessment of physical activity in specialized populations and settings are also discussed along with future directions of physical activity assessment.

Physical Activity: What to Measure?

Physical activity has been characterized as “any bodily movement resulting in energy expenditure above resting levels” (Freedson & Miller, 2000, p. 21; see also Caspersen, 1989). This definition logically leads to the goal of quantifying energy expenditure as the target of measurement. Indeed, the methods described in this section are often compared with simultaneous measures of energy expenditure as indexes of validity. However, physical activity is more accurately conceptualized as multidimensional in nature, with frequency, intensity, duration, and circumstance as relevant variables (Bassett, 2000). This is intuitively the case given that two very different activities, such as swimming and walking, may have the same net energy expenditure for a given time; even if one is of greater intensity, the other can be engaged in for longer duration or greater frequency.

Given the multidimensional nature of physical activity, no single assessment method provides valid and reliable measurement over the possible range of populations, settings, and uses (Wood, 2000). These limitations are made more complex when combined with the current limitations of technology and logistical concerns, which preclude the use of certain methods when assessing certain activities. Thus, when selecting a method for the assessment of physical activity, a researcher must clearly define the purpose of measurement and the application for which it is to be used.

Methods for Assessing Physical Activity

Self-Report. Self-report instruments are the most widely used instruments in the assessment of physical activity (Sallis & Saelens, 2000). These instruments include measures such as activity logs, self-administered questionnaires, interview-administered questionnaires, and proxy reports (for detailed lists and reviews, see Kriska & Caspersen, 1997; Montoye, Kemper, Saris, & Washburn, 1996; Sallis, 1991; Sallis & Saelens, 2000). Many of these measures have adequate reliability and validity and also provide assessment of multiple activity modalities over a range of situations at a low cost. In a recent review of self-report measures used during the 1990s, Sallis and Saelens (2000) found that, in general, adult self-report measures were more valid for reports of vigorous activity than for reports of moderate intensity activity. In addition, they found that interview measures had stronger psychometric properties than did self-administered measures and that self-report measures did not provide accurate estimates of absolute amounts of physical activity; in fact, most self-report measures overestimated absolute amounts of physical activity. In general, simple self-report measures, although known to be less than perfectly accurate, remain extremely valuable in many public health surveys and particularly in clinical applications.

Pedometer. A pedometer measures vertical acceleration using a spring-loaded lever arm that records motion either mechanically or by closing an electrical circuit. Sophisticated pedometers have a digital output that can represent activity either in number of steps or in mileage estimates. Electronic pedometers are extremely portable and are available for less than $20 (for a list and review, see Freedson & Miller, 2000). Recent studies have shown that pedometers are fairly accurate at counting steps but cannot distinguish between walking and running (Bassett, 2000). Furthermore, pedometers have little data storage capabilities and do not allow for the recording of specific activity patterns throughout the day. Despite these limitations, pedometers are widely used as a low-cost method of collecting objective physical activity data and in interventions that target increased walking- and running-type activities.

Accelerometer. Accelerometers measure acceleration in either the vertical (uniaxial) or three-dimensional (triaxial) plane. The fundamental assumption behind an accelerometer is that acceleration is directly proportional to muscle forces; the greater the acceleration of the limb, torso, or the like, the more energy expended by the organism. Accelerometers are extremely portable but can be quite expensive, with triaxial accelerometers costing more than $500 (for a list and review, see Freedson & Miller, 2000). Accelerometers have large capacities for data storage and are able to record the amount and intensity of activity as well as the specific activity patterns over days or weeks. Although too expensive for many clinical applications, accelerometers might be of value in specialized programs such as for treatment of chronic pain and cardiopulmonary rehabilitation.

Heart Rate Monitor. Heart rate monitors generally consist of a chest strap transmitter and a wristwatch receiver for storage. Monitors vary in quality but can be purchased for between $200 and $500 (for a list and review, see Freedson & Miller, 2000). Under normal conditions, heart rate is linearly related to energy expenditure, but many sources of error can elevate heart rate even at rest and can obscure the relationship between energy expenditure and heart rate (Freedson & Miller, 2000). Good heart rate monitors have storage capacity to record heart rate over extended periods of time and allow for patterns of activity over time. These monitors might not be practical for many clinical applications but could be valuable for cardiac patients.

Table 9.1 highlights the strengths and weaknesses of each type of field measure described in this section. Recently, researchers aware of the limitations of any single type of assessment modality have focused on combining the methods to sample more of the qualities of physical activity. The use of multiple modalities and complex calibration techniques can increase measurement accuracy, and they are often recommended for the assessment of physical activity.

The complex and multidimensional nature of various forms of physical activity is sufficient to complicate the accurate assessment of physical activity. However, recent attention has been given to the special assessment problems presented by certain subgroups and especially lifestyle-related activities. For example, the assessment of physical activity in older adults using self-report instruments is made more complicated by the fact that this group tends to engage in primarily light- and moderate-intensity activity (Bernstein et al., 1998; Washburn, 2000). There is as yet a paucity of age-specific questionnaires to assess this population (Washburn, 2000). The combination of the types of activities engaged in by older adults, the measures used to assess them, and the potential unreliability of recall for these types of activities supports the need for more refined assessment tools for older adults.

Traditional assessment of physical activity has focused on participation in structured, time-limited bouts of activity (i.e., “exercise”). For example, researchers may measure the number of miles walked on the treadmill or the number of minutes spent on a stationary bicycle. More recently, however, the definition of what should constitute physical activity has been broadened. One such change has been to challenge the notion that physical activity must occur in one episode of long duration (i.e., 20 to 30 minutes) to be beneficial. It is now generally agreed that the accumulation of activity throughout one’s day may be sufficient to realize improved health. Such activities may include walking the stairs instead of riding the elevator and walking short distances instead of driving a car.

In addition to the more inclusive definition of physical activity, other difficulties remain in measurement. For example, occupational activity is often neglected in surveys of physical activity. Because most occupations today are primarily sedentary, most physical activity questionnaires inquire about leisure time activity only. This is problematic because those individuals most likely to be characterized as sedentary during leisure time are also more likely to engage in job-related physical activity (CDC, 2001). Thus, it may be that some respondents who are classified as sedentary are actually active, but only on the job. The failure of many studies to capture physical activity obtained through employment is a weakness that needs to be addressed in future research.

Table 9.1 Field Methods for the Assessment of Physical Activity
Modality Cost Patterns of Activity? Accuracy Comments
Self-report Low Yes Good for relative
amounts
Many different
options available
Pedometer Low No Good for walking Unable to distinguish
instensity of
physical activity
Accelerometer
Uniaxial
Medium Yes Measures both
amount and
intensity
Might not measure
light activities in
multiple planes
Triaxial High Yes As above Measures multiple
planes of movement
Heart rate Medium-High Yes Good Multiple sources of
measurement error

Physical Activity Intervention in Chronic Disease

Cardiovascular Disease and Physical Activity

Physical activity has clearly been shown to reduce the risk of cardiovascular morbidity and mortality (e.g., Berlin & Colditz, 1990) and, as a result, has been recommended in the primary prevention of CVD. However, because most Americans do not acquire a level of physical activity that is sufficient to decrease the risk of CVD, research has focused on secondary prevention following cardiovascular events (e.g., chest pain, heart attack). The purpose of secondary prevention is to prevent further cardiac events in individuals who already manifest some degree of CVD.

Traditionally, individuals who had experienced heart attacks were prescribed bed rest as a significant part of their cardiac rehabilitation. However, as the medical and psychological benefits of being active were documented for this population (for a review of this evidence, see Dubbert, Rappaport, & Martin, 1987), physical activity became a primary focus of treatment for those with CVD. The recommended intensity, duration, and frequency of physical activity to be prescribed in cardiac rehabilitation programs are described in Leon (2000). In sum, sessions of aerobic exercise should include both warm-up and cool-down periods and should be of an intensity of between 40% and 85% of VO2max (a widely used measure of maximal oxygen consumption at a given workload). These sessions should occur three times per week and last for at least 20 to 60 minutes each session. When health psychologists consider physical activity in CVD rehabilitation, they must keep in mind that participation in a physical activity program should be supervised by a medical expert such as a physician or an exercise professional (American Association of Cardiovascular and Pulmonary Rehabilitation, 1999; Dubbert etal., 1987). This is a necessity because although events are infrequent, patients with CVD are at increased risk for experiencing cardiac events as a result of participation in physical activity relative to their disease-free counterparts.

Many observational studies have demonstrated a significant relationship between physical activity and cardiovascular mortality. In a large-scale prospective investigation, Wannamethee, Shaper, and Walker (2000) examined the association between physical activity and mortality in older men with CVD. These men were followed up approximately 13 years subsequent to a baseline assessment. Men who participated in physical activity experienced significant reductions in both all-cause and cardiovascular mortality relative to sedentary individuals. Importantly, several types of physical activity (e.g., gardening, walking) were shown to have a beneficial effect on mortality rates. In addition, those men who were sedentary at baseline but who later became active demonstrated significantly lower levels of mortality than did those men who remained inactive throughout the follow-up period.

Randomized clinical trials of physical activity in the treatment of CVD have generally shown that physical activity is effective at reducing cardiovascular mortality (for a review of these studies, see Oldridge, Guyatt, Fischer, & Rimm, 1988). In addition, physical activity interventions have resulted in increased exercise endurance, decreased chest pain, and reduced progression of atherosclerosis (Leon, 2000). Finally, as discussed elsewhere in this chapter, physical activity also improves several conditions that are known risk factors for CVD such as obesity and hypertension.

In conclusion, physical activity has been shown to be effective in the secondary prevention of CVD (i.e., reduction of cardiac events). Regular physical activity should be recommended for all individuals with CVD who do not have conditions that would limit or prohibit their participation. Because most cardiac rehabilitation research has focused on Caucasian men, future research should address the effectiveness of physical activity in ethnic minorities and women who have developed CVD.

Hypertension and Physical Activity

Hypertension, or elevated blood pressure (BP), affects approximately 50 million adults in the United States (Kaplan, 1998). Hypertension is a major health issue in the United States, representing the most significant risk factor for the development of CVD, including both coronary heart disease and stroke (MacMahon et al., 1990). Current guidelines (Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure [JNC], 1997) dictate that the optimal level of BP should be 120/80 mm Hg (millimeters of mercury) or lower. Values of 140/90 or higher are considered to be high or in the hypertensive range.

Given that many hypertensive patients’ BP levels are not consistently controlled with antihypertensive medication (most likely due to poor compliance with medications [Mancia, Sega, Milesi, Cesana, & Zanchetti, 1997]), decades of research have been devoted to the investigation of the nonpharmacological treatment of hypertension. Examples of widely studied treatments include relaxation, biofeedback, and stress management (for a review of these treatments, see Linden & Chambers, 1994).

Physical activity has also been examined as a treatment for hypertension. It has been recommended that for individuals classified as mildly hypertensive, initial treatment consisting of “lifestyle changes,” including physical activity, should be implemented for the first 6 to 12 months (JNC, 1997). In this subsection, the efficacy of physical activity for the treatment of hypertension is explored. In general, the desired outcome of a physical activity intervention would be the decreased use of medications used to control BP or the elimination of the need for antihypertensive medication altogether, along with a possible improvement in other CVD risk factors related to physical activity.

In a recent review of 15 studies of exercise training in hypertensive individuals, Hagberg, Park, and Brown (2000) found that approximately 75% of participants experienced significant decreases in BP. In addition, the hypotensive effect of physical activity was shown to occur quickly (within 1 to 10 weeks) and at low to moderate levels of intensity. Interestingly, the reduction in BP levels was not related to the amount of weight lost during the various training programs.

In a meta-analysis of 29 randomized clinical trials of aerobic physical activity, it was demonstrated that participation in a training program averaging 19 weeks resulted in a 4-point decrease in systolic BP and a 3-point decrease in diastolic BP (Halbert et al., 1997). Although this change in BP was significantly more than that achieved by participants in the control groups, it is lower than that reported in other studies. Consistent with the Hagberg and colleagues (2000) review, however, the improvement in BP levels occurred in the absence of weight loss.

Thus, aerobic exercise training appears to have a beneficial effect on reducing BP in individuals with hypertension. (However, not all studies have found significant effects [Blumenthal, Siegal, & Appelbaum, 1991; Nami et al., 2000].) Although the majority of training programs examined have aerobic exercise (e.g., walking, cycling), recent evidence suggests that nonaerobic forms of physical activity, such as resistance training, may also be useful in the treatment of hypertension. A recent meta-analysis (Kelley & Kelley, 2000) showed a small but statistically significant decrease in BP following resistance training two to five times per week for an average of 14 weeks.

The ability of a physical activity program to decrease BP may depend, in part, on individual characteristics such as gender and ethnicity (Lesniak & Dubbert, 2001). Regarding gender differences in the effectiveness of physical activity in treating hypertension, the data have generally shown no significant differences between women and men. Studies examining the effectiveness of physical activity in lowering BP in African Americans are particularly important because of the high prevalence of hypertension within this group. In addition, hypertension in African Americans is more severe and less well controlled relative to other ethnic groups. Unfortunately, the relationship between hypertension and physical activity in African Americans has not been studied adequately (Lesniak & Dubbert, 2001). Encouragingly, the data that do exist suggest that physical activity is an effective treatment for hypertension in African Americans (e.g., Kokkinos et al., 1995).

In summary, physical activity has been shown to be an effective treatment for hypertension. However, it should also be noted that the hypotensive effect of physical activity has been shown to be modest in several studies (for a review, see Blumenthal, Sherwood, Gullette, Georgiades, & Tweedy, 2002). Although the BP reduction achieved by physical activity may appear to be minimal, epidemiological data indicate that small drops in BP are accompanied by an impressive decrease in the risk of stroke and CVD. For example, a 5- to 6-point reduction in diastolic BP has been associated with a 35% to 40% decrease in stroke and with a 20% to 25% decrease in heart disease (Collins et al., 1990).

Chronic Obstructive Pulmonary Disease and Physical Activity

Chronic obstructive pulmonary disease (COPD) is a condition in which there is an impaired ability of the lungs to take in sufficient air. The primary symptoms of the disease consist of difficulty in breathing and a long-term cough. COPD may result from a variety of lung disorders such as asthma and emphysema. A common corollary of COPD is exercise intolerance. This often is a result of a vicious cycle in which the individual, on experiencing breathing difficulties (dyspnea) while exercising, perceives the dyspnea as threatening and so avoids engaging in physical activity. Eventually, deconditioning occurs and leads to exercise intolerance.

In an effort to target the exercise intolerance that often accompanies the disease, many treatment programs for COPD include a physical activity component. The physical activity offered is typically in the context of a pulmonary rehabilitation program (which might also include topics such as health education and stress management). There is much evidence showing that physical activity improves the exercise tolerance of COPD patients. In addition, improvement can be demonstrated after 4 weeks of training (Lacasse et al., 1996). Specifically, physical activity decreases dyspnea, decreases leg fatigue, and enhances health-related quality of life (for a review of this literature, see Bourjeily & Rochester, 2000).

COPD is a disease that is characterized by intermittent exacerbations (e.g., upper and lower respiratory tract infections). In general, the available evidence suggests that participation in a pulmonary rehabilitation program that includes a physical activity component is associated with a reduction in COPD-related exacerbations and hospitalizations (Berry & Walschlager, 1998). Unfortunately, it has been difficult to identify the specific components of rehabilitation programs that may lead to this improvement in health. Further research is necessary to determine whether or not physical activity alone leads to better health outcomes.

In addition to the physical limitations experienced by COPD patients, impaired cognitive functioning is sometimes associated with the disease (Prigatano, Parsons, Wright, Levin, & Hawryluk, 1983) and may result from decreased blood oxygenation levels (Grant et al., 1987).

Physical activity has been shown to improve the cognitive test performance in COPD patients. For example, Emery, Schein, Hauck, and MacIntyre (1998) found that, in addition to improvement in exercise endurance, some aspects of cognitive functioning (e.g., verbal fluency) were improved in a sample of community adults (over age 50 years) diagnosed with COPD. In this study, the physical activity component consisted of 45 minutes of aerobic activity on 5 days per week for 5 weeks (followed by a 5-week period of lower intensity activity on 3 days per week). As with the improvement exhibited in physical health, these changes in cognitive functioning were demonstrated after a relatively brief physical activity intervention.

In sum, the evidence clearly supports the use of physical activity in the treatment of COPD. Inclusion of a physical activity component in pulmonary rehabilitation programs has been shown repeatedly to increase exercise endurance and may result in fewer complications and hospitalizations related to the disease. In addition, there is some evidence to suggest that physical activity improves the cognitive functioning of individuals with COPD. Further research is needed to determine the optimal dose of physical activity necessary to produce sufficient improvements in both physical health and cognitive performance.

Obesity and Physical Activity

Obesity in the United States has achieved the status of a public health crisis (for a recent review of the problem, see Wadden, Foster, & Brownell, 2002). This is due to the fact that excess weight represents a major risk factor for chronic disease development (e.g., hypertension, CVD, type 2 diabetes mellitus). Most clinicians are now encouraged to evaluate weight status using the body mass index (BMI). BMI, the most commonly used measure of healthy versus unhealthy weight, assesses body weight in relation to height and is defined as weight in kilograms divided by height in meters squared. Current guidelines define “overweight” as BMI values in the range of 25.0 to 29.9, with “obesity” considered to be any value of 30.0 or higher (National Institutes of Health, 1998). It has been estimated that approximately 50% of U.S. adults are overweight or obese (World Health Organization, 1998), and the prevalence appears to be rising (Bouchard, 2000).

Body weight is determined by energy (food) intake and energy expenditure; however, this is a simplified definition of an extremely complex set of biological, behavioral, and environmental variables (the complexities of the issue are addressed in Salbe & Ravussin, 2000). If energy intake and expenditure are in balance, no significant weight loss or gain should occur. However, a situation in which food intake is consistently higher than energy expended will result in weight gain. Two primary contributors to the energy imbalance evident in the United States are a sedentary lifestyle and poor dietary habits. The focus of the remainder of this section is on the relationship between obesity and the expenditure of energy, that is, physical activity (or exercise).

Observational studies have demonstrated a significant relationship between physical activity and obesity, with more active individuals being less likely to be obese. In addition, there exist prospective studies suggesting that a lack of physical activity is a predictor of obesity (for a review, see Jebb & Moore, 1999). However, isolating the effects of physical activity interventions for obesity is difficult because most studies also include a dietary modification component such as a low-calorie diet. It has been shown that weight loss programs using physical activity alone are effective in producing modest weight loss (as compared with control groups) but do not result in as much weight loss as does exercise combined with dietary changes (Wing, 1999). One possible explanation for the relatively minimal effect of physical activity alone on body weight is the duration of most exercise programs. It may be that the 4 to 6 months of training typical of physical activity studies is not sufficient for realizing the benefits of physical activity (Wing, 1999). Findings from longitudinal investigations of physical activity and weight have suggested that long-term physical activity is effective in slowing and minimizing subsequent weight gain (but might not result in weight loss or even prevent weight gain [Grundy et al., 1999]).

Thus, maintaining participation in an exercise program is key to successful weight loss. In general, treatment programs for obesity (typically including physical activity accompanied by dietary changes) induce an initially rapid weight loss followed by a steady reduction in the amount of weight lost long term (Jeffery et al., 2000). Perri and colleagues (2001) compared a standard weight loss intervention (20 weeks of educational sessions, dietary changes, and moderate-intensity physical activity) with extended treatment. Extended treatment occurred for 12 months following the completion of the standard intervention and included the use of exercise diaries. It was found that participants assigned to extended treatment (after completing the standard program) maintained the weight lost during the standard treatment, whereas those completing the standard treatment alone regained approximately half of the weight initially lost (Perri et al., 2001).

There is also evidence that tailoring a physical activity program to an individual’s specific needs may result in higher levels of exercise maintenance. For example, Bock, Marcus, Pinto, and Forsyth (2001) found that participants who received individualized feedback regarding their exercise program were significantly more likely to maintain treatment levels of physical activity at follow-up (12 months) than were participants who completed a standard physical activity intervention.

In conclusion, weight loss programs that include a physical activity component have generally been shown to lead to a clinically significant weight reduction. However, the challenge of maintaining this weight loss following treatment remains. Future research should focus on the factors related to continued participation in physical activity. For example, understanding the behavioral variables that differentiate individuals who maintain physical activity (and weight loss) from those who regain the weight initially lost during treatment is a high priority (Marcus et al., 2000).

Osteoarthritis and Physical Activity

Arthritis is one of the leading causes of chronic pain and often results in disability and decreased quality of life. Osteoarthritis can be divided into two conceptual types: primary osteoarthritis, which is thought to be related to aging and heredity, and secondary osteoarthritis, which is caused by conditions such as obesity, joint trauma, and repetitious joint use (Cheng et al., 2000). It is estimated that the prevalence of arthritis is increasing, with 15.0% of the population affected in 1990 but an expected prevalence of 18.2% by 2020 (Wang, Helmick, Macera, Zhang, & Pratt, 2001). Furthermore, the disability rates produced from arthritis also appear to be growing, with those affected reporting more suffering from arthritis (Wang et al., 2001; Yelin, 1992).

Traditionally, physicians have suggested the avoidance of vigorous activity and encouraged physical inactivity for the treatment of osteoarthritis, reflecting a belief that joint use exacerbates the condition. This “wear-and-tear” hypothesis persists even today, as studies examining the role of physical activity in producing osteoarthritis have been somewhat inconsistent (Sutton, Muir, Mockett, & Fentem, 2001). The potential role of physical activity in the development of osteoarthritis has increased in importance with the surgeon general’s report calling for adults to increase their levels of physical activity (U.S. Department of Health and Human Services, 1996).

Recent research has shown that engaging in low- to moderate-intensity levels of physical activity does not increase the risk of the development of osteoarthritis in the knee or hip (Cheng et al., 2000; Sutton et al., 2001). The risk of developing osteoarthritis from high-intensity activities is not so clear, however, with some studies showing increased risk (Cheng et al., 2000; Cooper et al., 1998; Cooper, McAlindon, Coggon, Egger, & Dieppe, 1994) and others not showing increased risk (White, Wright, & Hudson, 1993). What is clear from the studies involving intense physical activity is that joint injury greatly increases the risk of osteoarthritis (Cheng et al., 2000; Sutton et al., 2001) and that perhaps the higher risk of developing osteoarthritis associated with intense physical activities may be explained by those activities’ greater association with injury.

There are several reasons to believe that increased physical activity is beneficial in preventing and reducing the symptoms of osteoarthritis (Cheng et al., 2000). Activity strengthens the muscular support surrounding joints and consequently reduces the risk of injury. Furthermore, physical activity improves and maintains joint mobility. Physical activity is also effective at reducing many of the other risk factors of osteoarthritis such as obesity, hypertension, hypercholesterolemia, and high blood glucose (Hart, Doyle, & Spector, 1995). Finally, physical activity increases the nourishment of joint cartilage through the diffusion of nutrients via joint fluid (Cheng et al., 2000; Hall, Urban, & Gehl, 1991).

In 1999, the Arthritis Foundation, CDC, and 90 other organizations released the National Arthritis Action Plan: A PublicHealth Strategy (Meenan, Callahan, & Helmick, 1999; Wang etal., 2001). Among the recommendations included in this publication was the need to decrease the rates of physical inactivity in adults with arthritis (34.8%), which are higher than those in adults without arthritis (27.7%) (Wang et al., 2001). Others have also recommended the use of physical activity in the management of osteoarthritis, and its positive effects have received empirical support (Ettinger & Afable, 1994; Ettinger et al., 1997; Minor, 1991).

Cancer and Physical Activity

Recently, Friedenreich (2001) summarized the current literature about the association between physical activity and cancer. She concluded that there is growing evidence for a protective effect resulting from physical activity. However, the evidence considered was entirely dependent on epidemiological research. Friedenreich expounded on the need for randomized, controlled intervention trials, which will allow the underlying mechanisms of the association between physical activity and cancer to be better understood. Yet even with the lack of controlled research, the negative association between physical activity and certain cancers supports the role of physical activity in the prevention of many forms of cancer.

The strongest evidence of the negative association between physical activity and cancer exists for colon cancer and, to a somewhat lesser degree, breast cancer (Friedenreich, 2001). In both forms of cancer, the risk reduction for the most physically active has been as high as 70% (for reviews, see Colditz, Cannuscio, & Frazier, 1997; Friedenreich, 2001; Friedenreich, Thune, Brinton, & Albanes, 1998; Gammon, John, & Britton, 1998; Lattika, Pukkala, & Vihko, 1998). The average risk reduction in colon cancer is 40% to 50%, with risk for both forms of cancer showing a negative dose-response to increasing levels of physical activity (Friedenreich, 2001).

The evidence supporting risk reduction from physical activity in other cancers is not as compelling, but there is substantial evidence supporting the association with prostate cancer (Friedenreich, 2001). Furthermore, preliminary evidence has been gathering to indicate that physical activity may be negatively related to lung cancer, testicular cancer, ovarian cancer, and endometrial cancer (Friedenreich, 2001). However, there is also compelling evidence to indicate that physical activity is not at all associated with rectal cancer (Friedenreich, 2001).

The role of exercise in cancer treatment has not been well researched, yet physical activity can help to reduce the loss of lean muscle mass during treatment, improve functional capacity, increase appetite, and enhance quality of life (for a review, see Oliveria & Christos, 1997). Furthermore, physical activity has been shown to be helpful in reducing other forms of risk in the development of cancer such as obesity and other lifestyle-related health conditions.

Diabetes Mellitus and Physical Activity

Diabetes is a heterogeneous group of disorders characterized by hyperglycemia or higher than normal levels of blood glucose. Type 1 diabetes, commonly referred to as “insulin-dependent diabetes,” occurs as a result of autoimmune destruction of the pancreas, leading to a deficiency in insulin production (Peirce, 1999). Type 2 diabetes, which accounts for 90% to 95% of all diabetic cases in the United States (Kriska, Blair, & Pereira, 1994), occurs as a result of altered insulin secretion, elevated hepatic glucose production, and/or diminished glucose use in skeletal muscle (Wallberg-Henriksson, Rincon, & Zierath, 1998). Prolonged hyperglycemia leads to the glycation of tissues, causing organ damage and other negative health effects.

The past decade witnessed an increase in the prevalence of diabetes. It is estimated that in 1995, 4.0% of the world’s population was afflicted, and this prevalence was projected to increase to 5.4% by 2025 (Peirce, 1999). However, these reported case estimates may be low given that there may be no symptoms at the onset of the disease, causing many early cases to elude detection (Kriska et al., 1994). The incidence of type 2 diabetes increases with age and is greatly increased in obese individuals, with a reported 60% to 90% of type 2 diabetic patients being obese at onset (Kriska et al., 1994).

Physical inactivity has been shown to affect the physiological mechanisms thought to underlie diabetes. A total of 35 days of induced physical inactivity caused a decrease in glucose tolerance in eight healthy males (Lipman et al., 1972). Furthermore, individuals with spinal cord injury had higher blood glucose levels than did age-matched controls (Duckworth et al., 1980). Although a complete review of the physiological effects of physical activity on glucose regulation is beyond the scope of this chapter, substantial evidence exists supporting the use of physical activity in the management and prevention of type 2 diabetes (for reviews, see Peirce, 1999; Wallberg-Henriksson et al., 1998).

Exercise has been shown to lower blood glucose levels in diabetics (Hubinger, Franzen, & Gries, 1987; Peirce, 1999). It is estimated that 90% of glucose clearance occurs in skeletal muscle, and this process can be greatly enhanced with increased energy use created through physical activity (Peirce, 1999). Exercise can enhance insulin-independent transport of glucose into cells (Peirce, 1999) and can increase insulin sensitivity (Burstein, Epstein, Shapiro, Charuzi, & Karnieli, 1990). Furthermore, regular exercise programs have been shown to improve metabolic control, especially in the young (Wallberg-Henriksson, 1992). Exercise-related decrease in body weight can also have a beneficial effect on glucose regulation (Wallberg-Henriksson et al., 1998).

Increasing physical activity in patients with type 1 diabetes is a complicated issue because of the necessary self-regulation of insulin levels. Hypoglycemia is a potentially life-threatening state that can be induced after exercise. Thus, the use of physical activity in the regulation of type 1 diabetes must be conducted with careful consideration of the potential for exercise-induced hypoglycemia or ketosis. However, exercise has been shown to decrease the daily insulin regimens of type 1 diabetics by increasing insulin-independent glucose transport and by increasing insulin sensitivity (Peirce, 1999).

An overwhelming body of evidence exists to suggest that increased physical activity should be recommended not only as a fundamental management strategy for physician-diagnosed diabetes but also as a strategy for the prevention of type 2 diabetes.

Psychological Functioning and Physical Activity

In addition to the widely studied effects of physical activity on physical functioning, participation in physical activity has been shown to play a role in emotional well-being. When discussing the relationship between physical activity and emotions, it is necessary to consider both acute and chronic effects. Regarding the improvement of mood (e.g., depression) following a single bout of exercise, the evidence is mixed, with some studies finding no effect and others demonstrating significantly enhanced mood. However, the results do indicate that dose of exercise (i.e., physical activity of varying intensities) does play a role in subsequent mood (for a review of these studies, see Rejeski, 1994).

In addition to mood improvement, one must consider the possible inducement of negative mood following a session of physical activity. Results of several studies have shown that high-intensity exercise bouts may lead to an increase in feelings of anxiety and depressed mood in some individuals (Rejeski, 1994). There is some evidence to suggest that individuals who are predisposed to negative mood states are more likely to experience anxiety as a result of engaging in physical activity (Cameron & Hudson, 1986). However, this issue is complex, and further research is needed to increase our understanding of the role that physical activity plays in short-term changes in mood. It has been found that in women with normal mood at baseline, mood after exercise improves most for those who felt worse previous to engaging in physical activity (Rejeski, Gauvin, Hobson, & Norris, 1995).

Given that the U.S. population is aging, the treatment of depressive disorders in older adults is becoming increasingly important. Blumenthal and colleagues (1999) examined the use of physical activity as a treatment for major depressive disorder in older adults. In their work, they compared participation in a physical activity program with the use of a commonly prescribed antidepressant medication (sertraline hydrochloride). Participants in the study were randomly assigned to receive either the medication, a physical activity intervention, or a combination of physical activity and medication. The 16-week physical activity treatment consisted of three 45-minute sessions of aerobic activity per week. It was found that both the medication group and the physical activity group experienced a reduction in their levels of depression (Blumenthal et al., 1999). The two types of treatment did not differ significantly from one another in effectiveness. These results suggest that physical activity is a viable alternative to medication in the treatment of depression in older adults.

In a 6-month follow-up study of the same participants, it was demonstrated that those individuals who were assigned to the physical activity treatment experienced lower rates of depression than did those receiving medication (30% vs. 50%, respectively). In addition, participants in the physical activity group were significantly more likely to have recovered from major depressive disorder (partially or fully) than were those in the medication group (Babyak et al., 2000). Regarding maintenance of physical activity, 64% of participants who received the physical activity treatment continued to exercise following completion of the 16-week program.

In sum, it appears that physical activity has the potential to improve mood both immediately following a bout of exercise (i.e., acute effects) and after participation in a long-term program (i.e., chronic effects). Thus, physical activity may be an effective means of enhancing mental health as well as physical health. In individuals experiencing a mood disorder (e.g., depression), physical activity has been shown to be a practical alternative to medication and should be recommended as either a primary treatment or an adjunctive treatment (assuming no physical limitations that would contraindicate exercise).

Conclusions

The purpose of this chapter was to review the evidence that physical activity is beneficial in terms of the prevention and treatment of disease. It was shown that the literature supports engaging in physical activity as an effective means to reducing morbidity and mortality. Unfortunately, most individuals do not achieve an adequate level of physical activity. Understanding this pervasive lack of physical activity participation requires consideration of the role that environmental factors play in our society.

As indicated in a recent review (Dubbert et al., 2002), it is clear that the inactive lifestyle that characterizes many individuals in the United States is due partly to a decrease in activity required on the job as well as to the availability of sedentary leisure time activities for both children and adults. Americans are also dining out more often and are consuming larger portions during meals. In addition, the physical environment plays a role in physical activity participation. For example, in a sample of women age 40 years or over, it was found that the lack of enjoyable scenery and hills in the neighborhood was associated with less leisure time activity (King et al., 2000).

Case Study

“L. B.” was a 56-year-old married Caucasian male who was referred for evaluation by his primary care physician. At the time of the evaluation, L. B. was 40 pounds overweight and was at the borderline of requiring medication to control his blood glucose levels. He was also experiencing low to moderate levels of chronic pain in his lower back, and this pain was reportedly aggravated by exertion. He was referred for evaluation of potential behavioral and lifestyle interventions to place his blood sugar levels under better control and help him to lose weight.

  1. B. was screened for depression, anxiety, and other psychopathology during his initial visit using a standard battery of self-report assessment instruments in combination with a clinical interview. The assessment revealed that L. B. was generally well adjusted but that he was reporting low levels of dysphoria and poor mood. Specifically, he reported a lack of energy, difficulty in sleeping, and a loss of interest in pleasurable activities. These symptoms were severe enough to warrant discussion but appeared to be on the subthreshold of a clinical diagnosis. L. B. appeared to be motivated to address his current medical problems, stating that he was quite concerned about developing diabetes.
  2. B.’s lifestyle was initially assessed during the interview using questions such as “Describe a typical day.” From this line of questioning, it was apparent that his lifestyle largely consisted of eating meals out, working long hours at his desk, and enjoying his weekend and after-work time by watching sports and being sedentary. When asked about his levels of physical activity, L. B. reported that he used to enjoy being active but that his lower back pain had forced him to “take it easy” because it usually hurt when he engaged in even moderate levels of activity. To further examine his levels of physical activity, he was given a pedometer to wear throughout the next week and was instructed to engage in his usual schedule. It should be noted that L. B.’s primary care physician had cleared him medically for all forms of physical activity and that L. B. had no detectable structural damage in his lower back. In addition, circulation and sensation in his lower extremities and feet were not impaired (walking ability may be limited in those with foot complications secondary to diabetes). He agreed to record the number of steps each evening in a log as well as to keep a food diary.

Figure 9.1 displays L. B.’s baseline levels of physical activity as recorded by a pedometer. These baseline levels of activity were discussed in the session, and L. B. reported surprise at the low levels of activity in which he was engaging. His beliefs about increasing his levels of physical activity were discussed. The rationale behind the positive benefits of physical activity for mood, glucose regulation, and chronic pain was explained. L. B. felt that walking would not aggravate his back and stated that he was willing to begin a walking regimen. It was decided that L. B. would attempt to walk during his lunch hour but that if he were unable to do so, he would walk after work with his wife. He also agreed to continue to monitor his activity but decided to discontinue his food log.

Figure 9.1 L. B.’s Steps as a Function of Time

  1. B. called the clinic after his first day of walking to report that he had greatly increased pain (which he described as 8 on a 10-point scale) and that he wanted to discontinue the regimen. He further stated that he was not wearing the pedometer (he had called in sick to work because of the pain) and that he planned on not moving for the remainder of the day. After a discussion of some of L. B.’s frustrations, he was persuaded to continue to wear the pedometer for the remainder of the week, even if he did not continue to implement a walking regimen.

At the next clinic visit, L. B.’s log was reviewed, and as can be seen in Figure 9.1, his increased pain was attributed to his “overdoing it” on the first day of his regimen. Many of his pain beliefs regarding his physical limitations were discussed in the context of pacing himself. L. B. was persuaded to attempt his walking regimen again, but this time with some restrictions set by the therapist. Specifically, during the first week, he should not walk beyond 4,500 steps (as indicated on the pedometer). He agreed to this but felt that this walking regimen was too little to be of benefit.

Throughout the next few weeks, L. B.’s therapist-imposed upper limits of pedometer readings were allowed to increase (Figure 9.1). L. B. reported success with the regimen, stating that it allowed him extra time with his wife, who was also enjoying walking with him. The goal of 10,000 steps daily was discussed, and L. B. felt that this goal could be accomplished. He soon reached the goal and discontinued recording his activity, although he purchased his own pedometer. He was also implementing changes in his diet that greatly facilitated his sense of efficacy in changing his lifestyle.

By the end of treatment, L. B. had succeeded in losing 15 pounds and was extremely proud of this accomplishment. He had increased his activity to 10,000 steps daily, with no increase in lower back pain. (It should be noted that individuals with foot or knee pain might not be able to attain this level of activity.) In fact, he reported that his back “has not felt better in 15 years.” Furthermore, his primary care physician believed that if L. B. were able to maintain his lifestyle changes, he would not have to pharmacologically manage L. B.’s blood sugar levels. L. B. was confident that he would be able to maintain his increased levels of physical activity, stating that he particularly liked the accompanying increased levels of energy and would like to lose more weight.

Although more recent studies have included men and women of color, the majority of research has been with Caucasians. Because certain diseases are more prevalent in ethnic minorities (e.g., hypertension in African Americans), it is crucial to focus attention on these groups. In addition, consideration should be given to subgroups within ethnic minority groups. Crespo, Smit, Carter-Pokras, and Andersen (2001) found that degree of acculturation (i.e., the extent to which an ethnic group adopts the customs and traditions of the majority culture) was associated with leisure time physical activity in Mexican Americans. Specifically, it was shown that inactivity was significantly more likely in those individuals who spoke primarily Spanish in the home (a sign of less acculturation to American society). In contrast, those who spoke primarily English had physical activity rates that were similar to those of the general (majority) population. Thus, it may be useful to tailor physical activity promotion in such a way as to target less acculturated individuals (e.g., by increasing the availability of informational materials in languages other than English).

Recent studies have examined physical activity versus physical fitness. Researchers have attempted to make this distinction because it is possible, for example, that two individuals engaging in the same dose (including intensity, duration, and frequency) of physical activity may have different levels of physical fitness. Blair, Cheng, and Holder (2001) sought to address the issue of whether physical activity or fitness is more important for good health. After reviewing the evidence from 67 studies that assessed physical activity, fitness, and a health outcome, they concluded that it is not possible to determine whether activity or fitness is more important in terms of producing health benefits. Instead, they found a consistent dose-response relation between physical activity and fitness and health outcomes across studies included in the review. That is, at both higher activity and fitness levels, there was a reduction in disease morbidity and mortality.

Blair and colleagues (2001) recommended that researchers begin to define more specifically the nature of the dose-response relationship. For example, what would be the difference in health outcomes in an individual who exercises for 15 minutes per day versus an individual who engages in 60 minutes of physical activity per day? Whether or not such a difference in activity duration translates into significantly different health outcomes is unknown. In addition, although the focus has been on cardiorespiratory fitness, other possible types of fitness should be considered and studied more widely. These might include metabolic, flexibility, cognitive functioning, and mental health. As other types of fitness become better defined, it may be possible to tailor physical activity to an individual’s needs. For example, an individual might be prescribed a specific type of physical activity to address his or her particular risk factors.

Another important area of continued research should focus on the maintenance of physical activity. Why is it so difficult to maintain this behavior? Why is engaging in physical activity pleasurable for some individuals but not for others? One step in answering the question of why some individuals are more likely to maintain a program of physical activity might be to examine emotions related to exercise. Although much has been written of the emotions that occur as a result of physical activity participation, one aspect of the relationship between physical activity and emotion that has received little attention is the importance of the emotional changes that occur during physical activity (Rejeski, 1994).

Researchers should also continue to investigate the role of medical professionals in physical activity participation. In a recent telephone survey of nearly 2,000 U.S. adults, only 28% of the respondents reported receiving advice from their physicians to increase physical activity (Glasgow, Eakin, Fisher, Bacak, & Brownson, 2001). Although there has been increased interest in this area and several studies have been published, further examination of the factors that may increase physician prescription of physical activity is needed.

Finally, more research is needed on effective strategies for promoting physical activity in adults. As illustrated in the preceding case study, cognitive-behavioral strategies such as self-monitoring and goal setting can be very effective in helping individuals to change physical activity behaviors. In behavioral medicine settings with chronic disease patients, health psychologists work with exercise professionals and other experts who prescribe the appropriate activity regimens. Psychologists are often responsible for assisting patients in developing motivational strategies and self-management programs to build physical activity into their lifestyles and sustain the change over time. Psychologists may also assist with assessing mood and cognitive states that could affect ability to carry out prescribed physical activity programs.

Recent reviews (e.g., Blair & Morrow, 1998; Dubbert, 2002; Sallis & Owen, 1998; U.S. Department of Health and Human Services, 1996) have described successful physical activity promotion projects and intervention strategies in a variety of populations and settings. Participation in physical activity is a powerful tool in the prevention of many of the diseases covered in this chapter. By increasing the numbers of individuals who regularly engage in physical activity, we could significantly reduce the chronic disease burden and improve the quality of life of millions of Americans.