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BASIC SCIENCES: Epidemiology

Population Trends in Leisure-Time Physical Activity

Minnesota Heart Survey, 1980-2000

STEFFEN, LYN M.1; ARNETT, DONNA K.2; BLACKBURN, HENRY1; SHAH, GAURANG1; ARMSTRONG, CHRIS1; LUEPKER, RUSSELL V.1; JACOBS, DAVID R. JR.1,3

Author Information
Medicine & Science in Sports & Exercise: October 2006 - Volume 38 - Issue 10 - p 1716-1723
doi: 10.1249/01.mss.0000227407.83851.ba
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Abstract

Physical activity or inactivity is an important modifiable risk factor for cardiovascular disease. Epidemiologic studies have consistently shown an inverse association between participation in regular physical activity and total mortality and morbidity from diabetes, cancer, heart disease, hypertension, hypercholesterolemia, and osteoporosis (35). Compared with sedentary men and women, lower mortality and fewer incident cardiovascular events have been observed among men and women who participate in vigorous and/or moderate physical activities (6,19,23). The proportion of preventable deaths from coronary heart disease attributable to physical inactivity is estimated to be between 27 and 40% (8,21,29).

Physical activity has been defined as "any bodily movement that is produced by the contraction of skeletal muscle and that substantially increases energy expenditure" (7,35). Historically, occupational physical activity was an important source of exercise for many individuals (25). However, leisure-time physical activity (LTPA) serves as a major source of physical activity for many industrialized societies today because contemporary occupations do not involve substantial physical activity (13). Recently, lifestyle physical activity has been described as various household, gardening, and transportation-related physical activities in addition to LTPA (22).

Several studies provide evidence that LTPA increased in the 1950s through the mid 1980s (15,33,35). However, population trends in LTPA have remained constant during the 1990s according to a population-based, random-digit-dialed telephone survey of U.S. adults, the 1990-1998 Behavior Risk Factor Surveillance Survey (BRFSS) (9). More than 22% of adults reported moderate activity on a regular basis, 54% were somewhat active, and 24% or more were inactive (reporting no leisure-time activity during the past month) (30). With the addition of household chores and transportation physical activities to LTPA in the 2001 BRFSS questionnaire, the prevalence of physical activity was 46%, an increase of 20% from previous 2000 BRFSS data that reported only LTPA participation (22). Other BRFSS results show LTPA declining from 30% in 1994 to almost 24% in 2004 (18). Rates from national surveys for participation in LTPA are consistently low for women, minorities, older adults, and persons with low educational attainment (3,30,35).

Many health and fitness organizations in the United States have published recommendations for physical fitness and activity (35). Recommendations from the U.S. Centers for Disease Control and Prevention (CDC) and the American College of Sports Medicine (ACSM) suggest that adults "accumulate at least 30 minutes a day of moderate intensity activity on most days, and preferably all days of the week" (30). Interpreting "moderate" intensity to mean three or more METs per minute and "most days" as at least 5 days per week, this is equivalent to a minimum of 64 physical activity kilocalories expended per day per week. Most recently, the Institute of Medicine (IOM) recommended 1 h of physical activity per day, a minimum of 180 kcal·d−1, for adults to maintain weight and cardiovascular health (26). Yet, a recent national survey reported a declining prevalence of leisure-time physical inactivity (18), and about 24% of U.S. adults remain physically inactive despite the beneficial effects of LTPA (18,24,35).

In this report from the Minnesota Heart Survey (MHS), a population-based surveillance study examining trends of mortality, morbidity, and cardiovascular risk factors in Minneapolis-St. Paul metropolitan area residents, we evaluate gender-specific 20-yr trends in lifestyle physical activity and LTPA, as measured by the Minnesota Leisure-Time Physical Activity (MN LTPA) questionnaire. This questionnaire measures lifestyle physical activities, including LTPA, household and house repair activities, gardening, and other yardwork activities. Unlike previous MHS LTPA reports, we use lifestyle physical activity to refer to the composite of all activities on the questionnaire, and LTPA refers only to specific activities designated as leisure by the CDC (22). We also describe the 20-yr trend of Minneapolis-St. Paul metropolitan area residents who participate in regular physical activity. We interpret these trends in light of the ongoing obesity epidemic.

RESEARCH DESIGN AND METHODS

To assess time trends in physical activity participation, a series of five population-based cross-sectional surveys were conducted in 1980-1982 (N = 1609), 1985-1987 (N = 2290), 1990-1992 (N = 2435), 1995-1997 (N = 2900), and 2000-2002 (N = 3089). The study design and operation of MHS have been described elsewhere (20). Briefly, the survey was designed to produce a representative sample of 25- to 74-yr-old residents of the seven-county Minneapolis-St. Paul metropolitan area. A two-stage self-weighting cluster design was employed to select households at random; within each household, one individual was randomly selected to participate in the survey except for the surveys in 1980-1981, 1995-1997, and 2000-2002, when all age-eligible household members were invited to participate (20). After completing a written informed consent form, selected individuals then completed a home interview and were invited to attend a clinic visit for measurement of height, weight, blood pressure, blood lipids, physical activity, and other health characteristics. Overall participation rates for the five surveys in 1980, 1985, 1990, 1995, and 2000 were 69.1, 68.1, 68.4, 64.8, and 64.0%, respectively (2,20). The study protocol was approved by the committee for the protection of human subjects in research at the University of Minnesota.

Assessments of lifestyle physical activity and LTPA were obtained using the interviewer-administered MN LTPA questionnaire in each survey year (34). The MN LTPA questionnaire, developed in the 1970s, measures participation in 63 activities during the previous year, including frequency and duration. The questionnaire has been validated indirectly against duration of treadmill exercise (12,16,34). Lifestyle physical activity, including LTPA, household chores, house repairs, gardening, and yardwork, was characterized as total minutes spent in physical activity and summary physical activity scores. Summary lifestyle physical activity scores and LTPA scores were calculated in units of kilocalories per day, an approximate measure of energy expenditure estimated as the product of the activity intensity code and duration of physical activity in minutes (34). The intensity code is equivalent to units of kilocalories per minute in an individual having a basal metabolic rate of 60 kcal·h−1. Thus, the lifestyle physical activity score closely approximates kilocalories expended over time. All activities named specifically in the MN LTPA questionnaire, as well as occasional additional activities volunteered by the participant, went into the computation of the lifestyle physical activity summary scores, whereas LTPA scores excluded activities such as house repair and maintenance, carpentry, gardening, and yard work. An intensity code was assigned to each activity based on the ratio of work metabolic rate to basal metabolic rate (34). The light-, moderate-, and heavy-intensity physical activity scores were calculated using activity scores of 2.5 (only three such activities queried), from 3.0 to 5.5 and ≥ 6.0 METs, respectively. Light-intensity activities include mowing the lawn with a riding mower and duck hunting; moderate-intensity activities include walking, carpentry in the workshop, bicycling for pleasure, mowing the lawn with a power mower, weeding and cultivating the garden, and home exercise; and heavy-intensity activities include health club exercise, weight lifting, aerobic dance, ballet, jog/walk combination, and running.

During the interviewer-administered MN LTPA interview in the clinic, participants reviewed the list of 63 activities and manually checked off whether or not they had performed any of the 63 activities or "any other activities" during the past 12 months. For all activities performed, an interviewer asked about the months in which the activities were performed, the number of times per month, and the number of hours and minutes per occasion. During an initial separate home interview session (not done in 1980), participants were asked whether they had "engaged in physical exercise during leisure time on a regular basis"; and, if so, how much time had been devoted during each exercise session (< 15 min, 15-30 min, 31-60 min, or ≥ 61 min) and how many exercise sessions they had engaged in per week.

The Health Insurance Plan of New York Occupational (HIP Job) questionnaire (1) consists of six questions about time spent sitting and walking at work, modes of transportation to and from work, and the frequency of heavy lifting at work. It was administered in 1980-1982 (N = 1204), 1995-1997 (N = 1870), and 2000-2002 (N = 2492) to employed study participants who also completed the LTPA questionnaire.

All statistical analyses were conducted using SAS, version 8.2 (SAS Institute; Cary, NC). Summary physical activity scores and minutes per day spent in physical activity for total lifestyle physical activity and LTPA and light-, moderate-, and heavy-intensity activities were transformed using the natural logarithm to normalize skewed distributions. Age-adjusted, gender-specific geometric means (computed as exponentiated log means) of energy expended and minutes of lifestyle physical activity and LTPA were calculated for each survey period for men and women. Gender-specific trends in total minutes per day spent in lifestyle physical activity and LTPA were examined by 10-yr age groups, by age-adjusted education levels, by status of employment, and by BMI across the five survey periods. The total energy expended and minutes per day spent in physical activity were analyzed using a mixed-model linear regression analysis with random effects to account for intraclass correlations in the cluster design. For each survey period, the prevalence was estimated of men and women participating in physical activity for at least 30 min·d−1 on five or more days per week and regular physical activity for 60 min·d−1, similar to recommended national physical activity guidelines (30,31). Frequency of participation in recommended levels of physical activity was calculated both from the direct questions about regular exercise and from average minutes per week of LTPA computed using the extensive MN LTPA questionnaire. The prevalence of individual employment activities reported on the HIP Job questionnaire was determined, and an overall job activity score was created based on the sum of 1 point apiece for not sitting, standing, lifting, and walking half a mile or more to work. Mixed-model linear regression analysis was used to evaluate the relation of job score and achievement of the physical activity guidelines with BMI. We used linear models for continuous as well as dichotomous dependent variables. In the latter case, the primary interest was in survey-specific prevalence estimates, which are biased in logistic regression (the estimated logits are unbiased). We verified that reported P values were very similar whether derived using linear regression (SAS proc mixed) or logistic regression (SAS proc genmod).

RESULTS

As shown in Table 1, the average age of study participants increased steadily over survey periods from 43.1 and 43.7 yr in 1980-1982 to 46.5 and 46.0 yr in 2000-2002 for men and women, respectively. Additionally, the proportion of college graduates increased for both men and women, whereas the prevalence of men and women who did not complete high school decreased in subsequent survey periods. The proportion of employed men decreased across survey years, and employment among women increased from 1980-1982 to 1995-1997 but then decreased in 2000-2002. Among both men and women, average BMI increased across survey years.

T1-5
TABLE 1:
Characteristics of men and women living in the seven-county metro area of Minneapolis-St. Paul: Minnesota Heart Survey (MHS) 1980-1982 to 2000-2002.

Energy expenditure (kcal·d−1) from total lifestyle physical activity and LTPA significantly increased over the five surveys for both men and women (Figure 1). Of total lifestyle physical activity reported, about 60 and 70% of energy expended was spent in LTPA for men and women, respectively. Men expended almost twice the number of kilocalories per day in lifestyle physical activity and about 50% more energy in LTPA than women in each survey year. Figures 2 and 3 show sex-specific daily energy expenditure of moderate- and vigorous-intensity physical activity across survey years. During the 20-yr period, moderate lifestyle physical activity and LTPA did not consistently change for men (P > 0.05), although there was change between individual survey years. Moderate lifestyle physical activity increased slightly for women in the 20-yr period (P = 0.02) but moderate LTPA did not. Vigorous-intensity lifestyle physical activity and LTPA increased between the 1980-1982 and 2000-2002 surveys for both men and women (P < 0.001). Because only three activities were rated at less than 3 METs in intensity, daily energy expenditure from reported light-intensity activities was minimal for both men and women (data not shown). Younger men and women spent more time in lifestyle physical activity than older adults across survey years (Table 2). From 1990-1992 to 1995-1997, there was a downward trend in minutes spent in physical activity among younger women and an upward trend in minutes spent in physical activity among older women. Minutes spent in physical activity were greater with increasing level of education, with a significant increasing linear trend for each level of education among women, except for women who did not graduate from high school. The linear trends over five survey periods between 1980-1982 and 2000-2002 for men in all education levels were not significant. Among normal-weight women and overweight men and women, minutes of physical activity increased across survey years (Ptrend < 0.01), but not for normal-weight men (P = 0.20). Compared with the other survey years, the amount of time spent in lifestyle physical activity was greater in 2000-2002 among all age groups, education levels, employment, and weight groups for both men and women. Overall, the pattern of minutes spent in LTPA by age, education, employment, and weight group was similar to that of lifestyle physical activity (data not shown).

F1-5
FIGURE 1:
Age-adjusted 20-yr trends of daily lifestyle and leisure-time physical activity among men and women: Minnesota Heart Survey (MHS) 1980-1982 to 2000-2002. * P < 0.05 when compared to the preceding survey year; † P < 0.001 for linear trend.
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FIGURE 2:
Age-adjusted 20-yr trends of daily lifestyle and leisure-time physical activity (LTPA) by intensity level among men living in the seven-county Minneapolis-St. Paul metro area: Minnesota Heart Survey (MHS) 1980-1982 to 2000-2002. * P < 0.05; ** P < 0.01; *** P < 0.001 when compared to the preceding survey year; † P < 0.001 for linear trend.
F3-5
FIGURE 3:
Age-adjusted 20-yr trends of daily lifestyle and leisure-time physical activity (LTPA) by intensity level among women living in the seven-county Minneapolis-St. Paul metro area: Minnesota Heart Survey (MHS) 1980-1982 to 2000-2002. * P < 0.05; ** P < 0.01; *** P < 0.001 when compared to the preceding survey year; † P < 0.001 for linear trend.
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TABLE 2:
Average age-adjusted geometric mean (SE) minutes of lifestyle (leisure, household, and transportation) physical activity by age, education level, employment, and weight status among men and women living in the seven-county metro area of Minneapolis-St. Paul: Minnesota Heart Survey (MHS) 1980-1982 to 2000-2002.

Two methods for estimating the proportion of Minneapolis-St. Paul adults participating in regular physical activity are reported (Table 3). From the direct questions about regular exercise, 46-57% of men (Ptrend < 0.001) and 47-58% of women (Ptrend < 0.001) reported participation in regular exercise across 20 yr. However, the proportion of regular exercisers was reduced when duration of exercise session was also considered. For those reporting 31 min or more of regular exercise, the time trend increased from 33% in 1980-1982 to 40% in 2000-2002 for men (Ptrend > 0.05) and from 23 to 35% for women (Ptrend < 0.05). Taking the number of exercise sessions per week into account further reduced the proportion of adults participating in regular exercise sessions. The proportion of MHS men and women who reported participation in five exercise sessions per week for at least 31 min ranged from 8 to 11% and 9 to 12%, respectively, with no trend over survey years. Using the MN LTPA questionnaire, the proportion of men and women who reported an average 30 min or more per day of LTPA ranged from 41 to 57% in men (Ptrend < 0.001) and 31 to 47% in women (Ptrend < 0.001) over the five survey years. Regular participation in approximately 60 min·d−1 could also be calculated using the two methods. Using the direct questions, the proportion exercising for 61 min or more per session in seven regular exercise sessions per week was 1-2% in both men and women across survey years. Using the MN LTPA questionnaire, the proportion of men and women participating on average in 60 min of LTPA per day increased over survey years from 17 to 30% in men (Ptrend < 0.001) and 11 to 20% in women (Ptrend < 0.001).

T3-5
TABLE 3:
Age-adjusted proportion of men and women living in the Minneapolis-St. Paul seven-county metro area who are regularly active according to selected measurements: MHS 1980-2000.

Despite the increase in average BMI across survey years (Table 1), BMI was somewhat lower among those who were regularly physically active (e.g., by 1 kg·m−2, varying across criteria and survey years), and the rate of BMI increase was less among physically active compared with inactive adults. Among men and women who engaged in regular exercise across survey years, BMI increased 1.2 and 1.8 kg·m−2, respectively, whereas the BMI increase from 1980 to 2000 was 2.6 and 4.4 kg·m−2 among men and women, respectively, who did not exercise regularly across survey years (Pinteraction< 0.05). For men who achieved an average of 30 min or more five times per week, their BMI increased by 1.6 kg·m−2 compared with the nonachievers' 2.2 kg·m−2 (Pinteraction = 0.15); women who met this goal increased 2.2 kg·m−2 versus 3.8 kg·m−2 across 20 yr (Pinteraction = 0.001).

Results from the Hip Job questionnaire indicate decreasing physical activity at the workplace over time (Table 4). The proportion of workers sitting more than half the time or walking less than half the time in their jobs increased, and the proportion of workers frequently lifting decreased between the 1980-1982 and 2000-2002 survey years for employed men and women. The proportion walking more than half a mile to work decreased over the survey years in women. The average job activity score across survey years remained about 1.2 ± 0.06 for men and significantly decreased from 1.2 ± 0.04 to 0.98 ± 0.03 for women. For each positive job activity performed (such as walking more than half the time or lifting frequently), BMI was 0.25 kg·m−2 lower than if no positive job attribute was performed. Thus, BMI was 1 kg·m−2 lower for those performing all four job-related activities.

T4-5
TABLE 4:
Age-adjusted reported activity in the workplace among men and women living in the seven-county metro area of Minneapolis-St. Paul: Minnesota Heart Survey (MHS) 1980-1982 to 2000-2002.

DISCUSSION

The consistent use of the same questionnaires to measure LTPA over five survey periods is a major strength in this study, which focuses on secular trends in physical activity of residents living in the seven-county metro area of Minneapolis-St. Paul. Despite the greater proportion of men and women participating in moderate and vigorous activity patterns in 2000-2002, but consistent with the trends towards less job-related activity, BMI increased more consistently across survey years than did physical activity. Nevertheless, BMI increased at a slower rate in adults who regularly participated in physical activity compared with those who did not.

Physical activity questionnaires are diverse and measure multiple but nonoverlapping dimensions of physical activity (16). Generally, light- and moderate-intensity physical activities are less accurately and reliably measured than heavy-intensity activities. Therefore, these activities are not captured easily when queried. This is unfortunate from the obesity perspective because a large difference in daily energy expenditure can result from subtle shifts in low-intensity activities. For example, sitting quietly expends energy at just over 1 MET·min−1, whereas standing requires approximately 2 MET·min−1. Standing rather than sitting for 8 h would require more than 400 kcal of extra energy expenditure. It is significant that one easily identifiable sedentary activity, namely, sitting at work (instead of standing), increased markedly over 20 yr and that, interestingly, Minneapolis-St. Paul men and women who sat less than half the time at work had a 0.25 kg·m−2 lower BMI than those who sat more than half the time. Even though overall BMI increased by survey year 2000-2002, in each survey adults who were moderately or vigorously physically active for 30 min or more per day had a lower BMI than those who did not participate in as much or as intense physical activity. Furthermore, normal-weight women reported greater participation in physical activity than women with higher BMI, although normal-weight and overweight men reported similar activity participation.

Despite the generally documented beneficial effects of physical activity on health, national data suggest that most U.S. adults remain inactive or do not participate in regular LTPA on most days of the week. Analysis of BRFSS data showed that trends in LTPA remained relatively constant between 1990 and 1998: regular participation in physical activity increased slightly from 24.3% in 1990 to 25.4% in 1998 (9). In 2001, the BRFSS physical activity questions changed by differentiating moderate and vigorous physical activity and, in addition to LTPA, including other activities such as household chores and transportation-related activity. With these additional activities, the prevalence of regular participation in lifestyle physical activity increased from 26.2% in 2000 to 45.4% in 2001 (22). The dramatic change in participation in regular physical activity most likely reflects the change in questions and not an actual increase in physical activity. Meanwhile, the prevalence of physical inactivity remained relatively unchanged between 2000 (27.4%) and 2001 (26%), but decreased to 25% in 2002 (18).

Different measures quantifying physical activity participation have yielded very different results, which make comparison of such data difficult (5). The indirect approach of asking about a list of individual activities and associated frequency and duration has a tendency to overstate, but this approach also might discover and properly weight many activities that the individual would not have thought of during direct questioning. The MN LTPA questionnaire queries average duration and frequency (per day, week, or month) for numerous leisure-time physical activities performed during the past year, including 63 activities on a precoded list in the questionnaire, but study participants also have the opportunity to report other activities not listed on the questionnaire, such as rollerblading, curling, broomball, and racketball. Therefore, it is not surprising the proportion of MHS adults who report participation in 30 min of LTPA (calculated from the average daily energy expenditure of LTPA) was 20-30% higher than the 2000 BRFSS survey data, in which U.S. residents, including Minnesota residents, were asked to quantify the two most frequently performed physical activities during the previous month (22). Another form of physical activity question was included in the MHS home interview, which directly queried participation in regular exercise and (when applicable) the typical duration and frequency of such exercise. It is doubtful that individuals would interpret this direct question about "regular exercise" in the same way that they would interpret questions about specific lifestyle physical activities. Based on the MHS "regular exercise" question, the prevalence of participation in 31 or more minutes of physical activity was 10% lower than that of the U.S. BRFSS 1990-1998 data and MN BRFSS 2000 data (9,22). In another national survey, the NHIS physical activity questionnaire queries performance of physical activity in the past 2 wk, including type of activity, frequency, duration, and heart rate outcome (27). Among 12,272 men and 16,890 women participating in the 1990 NHIS, the prevalence was 34 and 30%, respectively, for those meeting the CDC/ACSM recommendation of 30 min on five or more days per week (17). Clearly, the impression one gets of population-wide physical activity levels depends on the questionnaire used to obtain this information.

A strength of this study is its use of the MN LTPA questionnaire, which has been validated in adult populations, is highly reliable, and measures participation in specific activities rather than general activities (12,16,34). Furthermore, data on the frequency, duration, and intensity of activities are included in the MN LTPA measure, which allows for the calculation of minutes of activity or actual energy expenditure. Additionally, the ability to differentiate total lifestyle (LTPA, household, gardening, and other yardwork activities) and only LTPA is an advantage for comparison to other data. In this study, rather than relying solely on categorical definitions of active or sedentary activity, continuous measures of time spent in total activities and intensity-specific activities were reported. Another strength of this study is the probability sampling design of the survey.

Although the physical activity trend information is limited to the seven-county Minneapolis-St. Paul metropolitan area, the MHS "regular exercise" data confirm the generally stable physical activity trends observed for both BRFSS and NHIS, suggesting that findings of associations between physical activity and BMI may be generalized more broadly to the U.S. population. However, the continuous lifestyle physical activity and LTPA energy expenditure data show an increasing trend of physical activity during 20 yr in the Minneapolis-St. Paul residents. One limitation of our study was that physical activity estimates are based on self-reported activity and may be biased by over reporting of participants providing socially desirable responses. Limited information about housecleaning and related chores and occupational physical activities is another limitation of the study. A full evaluation of physical activity should include LTPA, activity performed on the job, and in-household chores. However, the Hip Job questionnaire administered in 1980, 1995, and 2000 provided evidence that activity on the job was decreasing.

Because greater participation in physical activity is related to lower levels of coronary risk factors (19,23,25,30,35), it is important that policy promote increased participation of physical activity among all individuals (28). Our data support physical activity as part of the solution to the obesity epidemic because physically active adults have increased BMI at a slower rate than inactive adults, although even the active people have increased BMI. It has been stated that 60 min·d−1 of moderately intense physical activity is necessary to prevent weight gain (4,26,31) and that, given energy intake, energy expenditure of 1500-2000 kcal·wk−1 from LTPA may be necessary to maintain energy balance and prevent weight gain (11). However, moderate physical activities of low intensity, such as MET level 3 and 3.5 activities, may not be sufficient to prevent weight gain even if performed daily for 60 min; that is, approximately 180-210 kcal·d−1 energy expenditure, especially because there is probably 300-500 kcal·d−1 less unmeasured low-intensity activity now than there was 30-50 yr ago. Thus, population-wide attempts to combat obesity by increasing leisure activity may be insufficient in the face of societal changes that result in less work, household, and transportation activity. Personal barriers have long been cited as influencing physical activity patterns; such barriers include time restrictions, lack of motivation and social support, and issues related to safety (10,32). The built environment encourages or discourages physical activity levels; relevant issues include access to walking or bike paths, neighborhood safety, or urban versus suburban living (32). There has been an overall shift away from public transit, walking, and biking and toward the use of private cars for several decades (14), which may be attributable to an increasing proportion of the population residing in the suburbs (32). Similarly, there has been a pervasive shift toward labor-saving devices, such as snowblowers and other devices that reduce effort in yardwork as well as the use of e-mail and desktop printers that eliminate walks down the hall in the office or other job-related physical activity. These physical activity time trend data presented here, together with considerations of unmeasured low-intensity physical activity in daily life and the logic of energy balance, suggest a need to develop a comprehensive public health strategy, such as reengineering our culture or environment to include more physical activity opportunities in routine leisure, job, and transportation activities to facilitate participation in daily physical activity, especially among women, the elderly, and less educated individuals.

The Minnesota Heart Survey was funded by grant R01 HL023727 from the National Heart, Lung, and Blood Institute, Bethesda, Maryland.

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Keywords:

SURVEILLANCE; PHYSICAL ACTIVITY RECOMMENDATIONS; PHYSICAL ACTIVITY INTENSITY; CARDIOVASCULAR RISK FACTORS

©2006The American College of Sports Medicine