The beneficial effects of participation in regular physical activity are widely accepted. Even so, dose response relationships between physical activity and many health benefits remain unclear. The dose of physical activity is described by its duration, frequency, intensity, or mode (7). Previous research suggests that, in general, higher doses of physical activity are associated with increasing benefits to health, but these relationships are not always linear (16). For example, the risk of overuse injuries (5) and upper respiratory tract infections (3) increases with the dose of physical activity, thereby decreasing the overall benefits of physical activity at high doses. Detrimental physical and mental health consequences have also been reported from excessive recreational exercise among “exercise abusers” (13,18) and with overtraining among endurance athletes (14,18). Clearly, dose-response relationships between physical activity and health benefits may vary depending on the health outcome of interest.
Health-related quality of life (HRQOL), an outcome measure increasing in popularity in the health sciences including exercise science (19,22,24), has evolved to include aspects of life that affect perceived physical or mental health, and it is a fundamental measure used to understand a population’s health status (27). Whether relationships between dose of physical activity and HRQOL are similar to those observed for other health outcomes (e.g., upper respiratory tract infections) has not been examined, however. With this in mind, we examined relationships between frequency, duration, and intensity of physical activity and HRQOL among a national sample of adults using data from the 2001 Behavioral Risk Factor Surveillance System (BRFSS).
The BRFSS is a state-based surveillance system that collects data on many of the behaviors and conditions that place adults (age ≥18 yr) at risk for chronic disease. Trained interviewers collect data on a monthly basis using an independent probability sample of households with telephones among the noninstitutionalized U.S. population. In 2001, data were collected from 212,510 persons. A detailed description of the survey design and random sampling procedures is available elsewhere (26). The BRFSS has been approved as exempt research by the Centers for Disease Control and Prevention’s institutional review board.
For this analysis, we examined data for 175,850 persons aged 18 yr or older who had complete information for study variables. Overall, 32.0% (SE = 0.20) of adults were aged 18–34 yr; 21.2% (0.17) 35–44; 30.8% (0.19) 45–64; and 16.0% (0.15) 65 or older. Half (50.3% [0.21]) of the sample were women, and 71.6% (0.21) were white non-Hispanic; 9.2% (0.12) black non-Hispanic; 6.4% (0.13) other non-Hispanic; and 12.8% (0.18) Hispanic. Fifty-eight percent (0.21) of adults had more than a high school education, nearly a quarter were current smokers (22.9% [0.17]), and 21.6% (0.18) were obese (body mass index ≥ 30).
Assessment and definition of physical activity.
Respondents were asked six questions about their participation in moderate or vigorous physical activity during a usual week. Regarding moderate physical activity, they were asked: 1) “Now, thinking about the moderate physical activities you do in a usual week, do you do moderate activities for at least 10 min at a time, such as brisk walking, bicycling, vacuuming, gardening, or any other activity that causes small increases in breathing or heart rate?” For vigorous physical activity, they were asked: 2) “Now, thinking about the vigorous physical activities you do in a usual week, do you do vigorous activities for at least 10 min at a time, such as running, aerobics, heavy yard work, or any other activity that causes large increases in breathing or heart rate?” For each type of activity, respondents indicated the number of days per week and the total time per day they participated in the activity for at least 10 min at a time. For this analysis, frequency of physical activity was categorized as “None,” “1–2,” “3–4,” “5–6,” or “7” days per week. Duration of physical activity was categorized as “None,” “1–19,” “20–29,” “30–59,” “60–89,” or “≥ 90” minutes per day.
Current federal guidelines (17,28) recommend that adults obtain at least 30 min of moderate intensity physical activity on most, preferably all days of the week, or at least 20 min of vigorous physical activity on three or more days per week. The recommendation that people be moderately active most days of the week is typically defined in the scientific literature as five or more days. Consistent with these guidelines, we defined three mutually exclusive and exhaustive groups: adults who engaged in no physical activity (i.e., inactive); adults who engaged in physical activity that was less than the recommended level of moderate or vigorous physical activity but greater than none (i.e., insufficient); and adults who engaged in recommended levels of moderate or vigorous physical activity.
Assessment and definition of health-related quality of life.
Respondents were asked the following questions related to HRQOL: 1) “Now, thinking about your mental health, which includes stress, depression, and problems with emotions, for how many days during the past 30 d was your mental health not good?” and 2) “Now, thinking about your physical health, which includes physical illness and injury, for how many days during the past 30 d was your physical health not good?” Respondents were not asked for specific underlying reasons of any reported unhealthy days. These questions and their construct validity are described elsewhere (15,27). We calculated overall unhealthy days as the sum of physically and mentally unhealthy days, not to exceed 30 d. We defined a dichotomous HRQOL variable as <14, ≥ 14 unhealthy days. A total of 14 unhealthy days is a meaningful cut point for those reporting substantially impaired HRQOL and corresponds to the upper 10–15% of the distribution for each Healthy Days measure in the BRFSS.
Because the prevalence of physical activity differed substantially across age groups, we stratified the primary analysis by age (18–44, 45–64, ≥ 65 yr). Means and percentages were age-standardized to the 2000 U.S. standard population (8). We used logistic regression to obtain odds ratios (OR) and 95% confidence intervals (CI) adjusted for age (continuous), gender, race/ethnicity (white, non-Hispanic; black, non-Hispanic; other, non-Hispanic; Hispanic), education (< high school, high school, > high school), smoking status (smoker, nonsmoker), and body mass index (<25.0, 25.0–29.9, ≥ 30.0 kg·m−2). Parameter estimates were obtained by maximum likelihood techniques, and 95% CI were based on the SE of the model coefficients. We used SUDAAN 8.0 (Research Triangle Institute, 2001) to account for the survey’s complex sampling design. Statistical inferences were based on a significance level of P (two-sided) ≤ 0.05.
Regarding moderate physical activity, 18% (N = 33,261) reported none; 66% (N = 115,872) reported at least 30 min·d−1 (mean [SE] = 58.8 [0.33] min·d−1); and the mean (SE) days per week of moderate physical activity was 4.7 (0.01) (Table 1). Similarly, for vigorous physical activity, 56% (N = 102,506) reported none; 42% (N = 68,789) reported at least 20 min·d−1 (mean [SE] = 32.4 [0.27] min·d−1); and the mean (SE) days per week of vigorous physical activity was 2.4 (0.01).
Overall, the average (SE) number of unhealthy days (physical or mental) during the previous 30-d period was 6.0 (0.04); 48% of respondents reported having no unhealthy days and the median number of unhealthy days was 1.0 (data not shown). As a group, persons aged 65 yr or older reported one additional unhealthy day on average than those aged 18–44 yr (6.6 [0.10] vs 5.6 [0.05], P ≤ 0.01). The age-standardized prevalence of 14 or more unhealthy days was 17.8% (0.16).
In Figure 1, we present (within the boxes) the age-standardized mean number of unhealthy days (physical or mental) and the age-standardized proportion of adults with ≥ 14 unhealthy days by frequency and duration of moderate or vigorous physical activity. The age-standardized prevalence (SE) of 14 or more unhealthy days (physical or mental) was 28.4% (0.50) among physically inactive adults, 16.7% (0.27) among those with insufficient levels of physical activity, and 14.7% (0.22) among adults who met recommended levels. The age-standardized mean (SE) number of unhealthy days was 8.7 (0.12) for physically inactive adults, 5.7 (0.07) for adults with insufficient levels of physical activity, and 5.1 (0.05) for adults meeting physical activity recommendations.
The proportion of adults with 14 or more unhealthy days is presented by age and frequency of physical activity (both moderate and vigorous) (Fig. 2). The prevalence of 14 or more unhealthy days was higher among older adults. Across all age groups for both moderate and vigorous physical activity there was a curvilinear relationship between frequency of physical activity and having 14 or more unhealthy days. Thus, adults with no physical activity as well as those participating in 7 d·wk−1 of physical activity had poorer HRQOL compared with those with 1–6 d·wk−1. In Figure 3, we present the proportion of adults with 14 or more unhealthy days by age and duration of physical activity. As in Figure 2, the relationship between duration of physical activity and having 14 or more unhealthy days was nonlinear.
The independent associations between having 14 or more unhealthy days and frequency and duration of moderate physical activity are shown in Table 2. Across all age groups, persons who did not engage in moderate physical activity were more likely to experience 14 or more unhealthy physical or mental days than were persons who engaged in some moderate physical activity even after adjustment for duration of moderate physical activity or numerous other variables including participation in vigorous physical activity (see footnote to Table 2). Overall and among adults aged 45–64 yr or aged ≥ 65 yr, those with only 1–2 d of moderate activity per week were more likely to experience 14 or more unhealthy days than were those who participated in 5–6 d of moderate activity. Similarly, across all age groups the likelihood of poor HRQOL (≥ 14 unhealthy days) was greater among adults who participated in moderate physical activity every day of the week than those participating on 5–6 d. Finally, independent of frequency, participation in moderate physical activity for < 20 min or ≥ 90 min·d−1 was associated with an increased likelihood of poor HRQOL when the referent was participation for 30–59 min·d−1 for all groups except those aged ≥ 65 yr (≥ 90 min).
Overall, adults who participated in no vigorous activity and those who engaged in 7 d of this activity were more likely to have 14 or more unhealthy days after multivariable adjustment (Table 3). In addition, those who participated in < 20 or ≥ 60 min·d−1 of vigorous physical activity (vs 20–29 min) were more likely to have 14 or more unhealthy days. Similar relationships were observed among adults aged < 45 yr. The estimated relationships between frequency or duration of vigorous activity and HRQOL among adults aged ≥ 45 yr are somewhat unstable due to small sample sizes in some cells. The results, although nonsignificant, are in the direction of the overall findings, however, except for persons aged 45–64 yr obtaining 30–59 or 60–89 min·d−1 of vigorous activity.
Currently available evidence supports the beneficial effects of participation in regular physical activity; but our understanding of dose-response relationships between physical activity and many health outcomes is less clear. An inverse linear relationship has been reported between volume of physical activity and all-cause mortality (10), but there is insufficient evidence regarding dose-response relationships between physical activity and such other health measures as glucose homeostasis (6), obesity (20), blood lipids (11), cardiovascular disease (9), and depression and anxiety (2).
Given the sedentary lifestyle of the U.S. population, communication of the importance of regular physical activity to the general public should include a discussion of the frequency, duration, and intensity of activity or exercise necessary to achieve health benefits (16). Current physical activity recommendations suggest a threshold of physical activity exists for conferring health benefits (1). Although convenient for public health policy recommendations, conceptualizing the effects of physical activity as an “all or nothing” phenomenon is overly simplistic and may not be supported by current evidence (1).
In this cross-sectional analysis of data from a national sample of men and women, we found that physically inactive adults suffer more unhealthy days than either adults who engage in some physical activity but below recommended levels or those who meet recommendations. When we examined the relationships between physical activity and HRQOL separately for moderate and vigorous activity both overall and by age group, we found curvilinear relationships as a poor HRQOL was always more likely among those with no physical activity, usually more likely among those who had daily (7 d a week) activity, almost always more likely for those with activity of short duration (<20 min·d−1), and more likely more than half the time for those with very long duration (≥ 90 min·d−1).
Our findings are comparable to earlier studies that examined the association between physical activity or exercise dose and physiologic measures or other health outcomes and showed diminished benefits associated with higher amounts of training. For example, prolonged endurance exercise is associated with adverse immune response reactions (21). And, in a study of 530 runners, Heath and colleagues (3) reported a strong relationship between running mileage during the previous 12 months and upper respiratory tract infections. In a study of approximately 400 collegiate swimmers, Morgan and colleagues (14) found that mood disturbances increased in response to an increasingly greater training stimulus or volume (yardage) among both women and men. Finally, in a study of 5001 men and women from the Aerobics Center Longitudinal Study, Hootman and associates (4) identified an increased risk of musculoskeletal injuries with increased duration of physical activity among men.
This study possesses several strengths. Studies of athletes and of people enrolled in fitness centers, such as described above, may not translate to the general population of people defined as inactive, insufficiently active, or physically active at recommended levels according to population-based national surveys. Findings from the present study add to the knowledge base in this regard. We should also note that few studies have documented the health outcomes associated with insufficient physical activity. A common belief is that doing some physical activity is better than doing none, and the classification of people doing some yet “insufficient” activity is being used more frequently in the physical activity and public health literature. We found that insufficient activity is associated with better HRQOL than that for inactive persons but not quite as good as that for those who attain recommended amounts of physical activity. We also found that doing too much activity may attenuate benefits related to HRQOL. For some health outcomes, it may be informative to dichotomize the group of people obtaining recommended amounts of physical activity into two groups such as active and very active.
The BRFSS HRQOL items used in this study have been shown to be valid and reliable (15,27), and our study uses these items to build upon earlier research. Previous studies differ in how they have defined quality of life, and many studies have used quality of life measures with poor psychometric qualities (24). The present study stratified by age to determine whether the association between physical activity and quality life differed across age groups. Spirduso and Cronin (22) have pointed out that physical activity goals and quality of life indicators take on different meanings at different stages of life, and controlling for age-related differences is important in physical activity and quality of life research.
The results of this analysis are also subject to some limitations. Because the analysis was cross-sectional, determining cause and effect was impossible. Although physical activity can improve HRQOL, it is possible that persons with impaired HRQOL are less likely to participate in physical activity. Data from clinical trials, however, support the notion that physical activity is associated with improvements in HRQOL (25) and perceived health status (12). The BRFSS is a telephone-based survey. Persons of low socioeconomic status, who are more likely to be physically inactive and have poor quality of life, are also less likely to have a telephone and to be included in the BRFSS. Finally, these data are self-reported. Conceivably, some respondents participated in regular physical activity but did not perceive it sufficient to be considered moderate or vigorous, and thus we may have underestimated the prevalence of physical activity. Similarly, some respondents may have misreported their levels of physical activity to provide a socially desirable response (23), and thus we might have overestimated the prevalence of physical activity.
Although these results may suggest that persons who participate in at least some regular physical activity have better HRQOL than those who are essentially inactive, it is also possible that HRQOL may be related to unmeasured lifestyle characteristics among these persons. Previous research has demonstrated the clustering of health behaviors including diet, tobacco use, and physical activity (29). Also, respondents were not asked for possible reasons of unhealthy physical days which may provide further insight into relationships between physical activity and HRQOL. Nonetheless, these results highlight the possible benefit of regular physical activity.
In summary, participation in currently recommended levels of moderate or vigorous physical activity is independently associated with higher levels of HRQOL among adults. Our results suggest, however, that participation in moderate or vigorous physical activity every day of the week or for extended periods may not be optimal, inasmuch as these practices are associated with poorer HRQOL. Prospective studies of the relationship between dose of physical activity and HRQOL, specifically using the Healthy Days measures from this analysis, are needed. The relationship between physical activity and health outcomes is complex. Although engaging in 7 d a week in moderate or vigorous physical activity may be associated with poor HRQOL, research is needed to determine whether this is true for other health outcomes. Thus, future research examining the relationship between dose of physical activity and HRQOL should also include other health outcomes.
1. Blair, S. N., and J. C. Connelly. How much physical activity should we do? The case for moderate amounts and intensities of physical activity. Res. Q. Exerc. Sport
2. Dunn, A. L., M. H. Trivedi, and H. A. O’Neal. Physical activity dose-response effects on outcomes of depression and anxiety. Med. Sci. Sports Exerc.
3. Heath, G. W., E. S. Ford, T. E. Craven, C. A. Macera, K. L. Jackson, and R. R. Pate. Exercise
and the incidence of upper respiratory tract infections. Med. Sci. Sports Exerc.
4. Hootman, J. M., C. A. Macera, B. E. Ainsworth, M. Martin, C. L. Addy, and S. N. Blair. Association among physical activity level, cardiorespiratory fitness, and risk of musculoskeletal injury. Am. J. Epidemiol.
5. Jones, B. H., D. N. Cowan, and J. J. Knapik. Exercise
, training and injuries. Sports Med.
6. Kelley, D. E., and B. H. Goodpaster. Effects of exercise
on glucose homeostasis in type 2 diabetes mellitus. Med. Sci. Sports Exerc.
7. Kesaniemi, Y. A., E. Danforth, Jr., M. D. Jensen, P. G. Kopelman, P. Lefebvre, and B. A. Reeder. Dose-response issues concerning physical activity and health: an evidence-based symposium. Med. Sci. Sports Exerc.
8. Klein, R. J., and C. A. Schoenborn. Age Adjustment Using the 2000 Projected U.S. Population: Healthy People Statistical Notes
, no. 20. Hyattsville, MD: National Center for Health Statistics, 2001, pp. 1–9.
9. Kohl, H. W. III. Physical activity and cardiovascular disease: evidence for a dose response. Med. Sci. Sports Exerc.
10. Lee, I. M., and P. J. Skerrett. Physical activity and all-cause mortality: what is the dose-response relation?Med. Sci. Sports Exerc.
11. Leon, A. S., and O. A. Sanchez. Response of blood lipids to exercise
training alone or combined with dietary intervention. Med. Sci. Sports Exerc.
12. McMurdo, M. E., L. Burnett. Randomised controlled trial of exercise
in the elderly. Gerontology
13. Morgan, W. P. Negative addiction in runners. Physician Sportsmed.
14. Morgan, W. P., D. R. Brown, J. S. Raglin P. J. O’Connor, and K. A. Ellickson. Psychological monitoring of overtraining and staleness. Br. J. Sports Med.
15. O¸unpuu S., L. W. Chambers, D. Chan, and S. Yusuf. Validity of the US Behavioral Risk Factor Surveillance System’s health related quality of life survey tool in a group of older Canadians. Chronic Dis. Canada
16. Pate, R. R. Physical activity and health: dose-response issues. Res. Q. Exerc. Sport
17. Pate, R. R., M. Pratt, S. N. Blair, et al. Physical activity and public health: a recommendation from the Centers for Disease Control and Prevention and the American College of Sports Medicine. JAMA
18. Raglin, J. S., and L. Moger. Adverse consequences of physical activity: when more is too much. In:Lifestyle Medicine
, J. M. Rippe, (Ed.). Malden, MA: Blackwell Science Inc., 1999, pp. 998–1004.
19. Rejeski, W. J., L. R. Brawley, and S. A. Schumaker. Physical activity and health-related quality of life. In:Exercise and Sport Sciences Reviews
, Vol. 24. J. O. Holloszy (Ed.). Baltimore: Williams & Wilkins, 1996, pp. 71–108.
20. Ross, R., and I. Janssen. Physical activity, total and regional obesity: dose-response considerations. Med. Sci. Sports Exerc.
21. Shephard, R. J., S. Rhind, and P. N. Shek. Exercise
and training: influences on cytotoxicity, interleukin-1, interleukin-2 and receptor structures. Int. J. Sports Med.
22. Spirduso, W. W., and D. L. Cronin. Exercise
dose-response effects on quality of life and independent living in older adults. Med. Sci. Sports Exerc.
23. Stewart, A. L., R. D. Hays, and J. E. Ware. Methods of validating MOS health measures. In:Measuring Functioning and Well-Being
, A. L. Stewart and J. E. Ware (Eds.). Durham, NC: Duke University Press, 1992, pp. 309–324.
24. Trine, M. R. Physical activity and quality of life. In:Lifestyle Medicine
, J. M. Rippe (Ed.). Malden, MA: Blackwell Science Inc., 1999, pp. 989–997.
25. Tsai, J. C., P. Chan, C. H. Wang, et al. The effects of exercise
training on walking function and perception of health status in elderly patients with peripheral arterial occlusive disease. J. Intern. Med.
26. U. S. Centers for Disease Control and Prevention. Behavioral Risk Factor Surveillance System User’s Guide.
Atlanta, GA: U. S. Centers for Disease Control and Prevention, 1998, pp. 30–43.
27. U. S. Centers for Disease Control and Prevention. Measuring Healthy Days: Population Assessment of Health-Related Quality of Life.
Atlanta, GA: U. S. Centers for Disease Control and Prevention, 2000, pp. 1–44.
28. U. S. Department of Health and Human Services. Healthy People 2010
,. 2nd Ed. With Understanding and Improving Health and Objectives for Improving Health
, 2 Vols. Washington, DC: U. S. Government Printing Office, 2000, pp. 1–39.
29. Wankel, L. M., and J. M. Sefton. Physical activity and other lifestyle behaviors. In:Physical Activity, Fitness, and Health: International Proceedings and Consensus Statement
, C. Bouchard, R. J. Shephard, and T. Stephens (Eds.). Champaign, IL: Human Kinetics Publishers, Inc., 1994, pp. 530–550.
Keywords:©2004The American College of Sports Medicine
EXERCISE; EPIDEMIOLOGY; CROSS SECTIONAL; LOGISTIC REGRESSION