Context Physical activity has been related to reduced mortality, but it is not
clear whether changes in physical activity affect mortality among older women.
Objective To examine the relationship of changes in physical activity and mortality
among older women.
Design, Setting, and Participants Prospective cohort study conducted at 4 US research centers (Baltimore,
Md; Portland, Ore; Minneapolis, Minn; and Monongahela Valley, Pa) among 9518
community-dwelling white women aged 65 years or older who were assessed at
baseline (1986-1988), 7553 of whom were reassessed at a follow-up visit (1992-1994;
median, 5.7 years later).
Main Outcome Measures Walking and other physical activities at baseline and follow-up; vital
status, with cause of death confirmed by death certificates/discharge summaries,
tracked for up to 12.5 years after baseline (up to 6.7 years after the follow-up
visit).
Results Compared with continually sedentary women, those who increased physical
activity levels between baseline and follow-up had lower mortality from all
causes (hazard rate ratio [HRR], 0.52; 95% confidence interval [CI], 0.40-0.69),
cardiovascular disease (HRR, 0.64; 95% CI, 0.42-0.97), and cancer (HRR, 0.49;
95% CI, 0.29-0.84), independent of age, smoking, body mass index, comorbid
conditions, and baseline physical activity level. Associations between changes
in physical activity and reduced mortality were similar in women with and
without chronic diseases but tended to be weaker among women aged at least
75 years and among those with poor health status. Women who were physically
active at both visits also had lower all-cause mortality (HRR, 0.68; 95% CI,
0.56-0.82) and cardiovascular mortality (HRR, 0.62; 95% CI, 0.44-0.88) than
sedentary women.
Conclusion Increasing and maintaining physical activity levels could lengthen life
for older women but appears to provide less benefit for women aged at least
75 years and those with poor health status.
Physically active lifestyles have been consistently related to reduced
mortality and morbidity from cardiovascular disease (CVD), diabetes, physical
disability, and certain cancers.1-9 Although
primarily observed in middle-aged male populations,1-4 these
benefits have also been demonstrated among older men and women.10-16 Additional
studies have shown that persons who increase their physical activity or fitness
levels over time reduce their risk of cardiovascular and all-cause mortality.17-21 However,
these studies of changes in physical activity have mostly examined middle-aged
populations and men, and the single study that examined the relationship between
changes in physical activity and mortality among older women did not find
a reduced mortality risk among women who increased their physical activity
levels.21 Thus, it remains unclear whether
adoption of a physically active lifestyle by previously sedentary older women—particularly
those with chronic conditions such as CVD, diabetes, and physical frailty—leads
to similar benefits. Since more than one third of older adults are sedentary22 and most of them have 1 or more chronic conditions,23 the effects of changes in physical activity on mortality
for this segment of the population have important public health implications.
In the present study, we examined whether changes in physical activity
levels were associated with reduced mortality among a large population of
older community-dwelling women.
Study Design and Population
The study population consisted of 9704 women aged at least 65 years
who were participating in the Study of Osteoporotic Fractures (SOF), a prospective
observational cohort study originally designed to assess the incidence and
risk factors for fractures among older women.24 Women
were recruited from population-based lists in 4 US communities: Baltimore,
Md; Portland, Ore; Minneapolis, Minn; and Monongahela Valley, Pa. The SOF
excluded women if they were institutionalized or unable to walk without the
assistance of another person. African American women were excluded from the
SOF because of their lower fracture incidence.
All participants attended a baseline visit between 1986 and 1988, when
physical activity levels and other behavioral and health status variables
were available for 9518 women (186 women [2%] were excluded because of missing
data). Physical activity levels were reassessed among 7553 nondisabled women
who attended a follow-up visit 4.0 to 7.7 years later (1992-1994; median,
5.7 years). This sample served as the basis for analysis of change in physical
activity, among whom vital status was subsequently tracked for up to 6.7 years
after the second visit (through 1998-1999). In addition, we examined the association
between baseline physical activity and mortality on the entire sample from
baseline over 12.5 years of follow-up.
Of the 1965 women who were not part of the follow-up physical activity
analysis, the following reasons for exclusion/loss to follow-up were noted:
1050 (11%) had died before the end of the visit period; 203 (2%) refused or
were unable to attend the visit; 365 (4%) were excluded because they reported
much difficulty or inability to walk at baseline; and 347 (4%) had missing
data at follow-up. Women who did not attend the follow-up visit tended to
be older at baseline (74.1 vs 71.0 years among those with follow-up data)
and had a higher prevalence of hypertension (46.4% vs 36.3%), diabetes (11.2%
vs 6.0%), stroke (6.5% vs 2.1%), and smoking (13.1% vs 9.2%) than those who
attended both visits.
Assessment of Physical Activity
Physical activity was assessed using a modified version of the Harvard
Alumni Questionnaire.25 Women were asked about
the number of city blocks or equivalent (12 blocks = 1 mile [1.6 km]) they
walked each day for exercise or as part of their normal routine and about
frequency and duration of other leisure activities (eg, dancing, gardening,
aerobics, swimming) during the past year. Walking for exercise was attributed
an intensity of 8.3 kcal/block (equivalent to 5 kcal/min) and nonexercise
walking was attributed 5 kcal/block (equivalent to 3 kcal/min). Other low-
and moderate-intensity activities were attributed 5 kcal/min and 7.5 kcal/min,
respectively. A summary estimate of physical activity was calculated and expressed
in kilocalories per week.26,27 For
analyses of changes in physical activity levels, we excluded information about
high-intensity activities (eg, racquetball, running, team sports) to make
computation of physical activity consistent across visits, because this information
was collected only at baseline and high-intensity activities were reported
by only 300 women (4.0%).
Classification of Physical Activity
For analyses of the association between baseline physical activity and
mortality, we categorized women by quintile of total physical activity at
baseline. For analyses of changes in physical activity levels, we compared
mortality risks among 4 groups: those sedentary at both baseline and follow-up,
defined as being in the lowest 40% (<595 kcal/wk); those physically active
at baseline and sedentary at follow-up (ie, moved from highest 60% to lowest
40%); those sedentary at baseline and active at follow-up (ie, moved from
lowest 40% to highest 60%); and those physically active at both visits. We
also examined the amount of absolute kilocalorie change in physical activity
(rather than quintile shifts relative to other women, as in our main analysis).
For these analyses, the quintile with the least physical activity change was
the referent group (referred to as maintainers); women with decreases and
increases in physical activity were divided evenly into categories.
Smoking; history of physician-diagnosed medical conditions, including
coronary heart disease (myocardial infarction, angina, or congestive heart
failure), diabetes, cancer, chronic obstructive pulmonary disease, and stroke;
and self-rated health status were assessed by interview. Hip fractures that
occurred between the 2 study visits were ascertained by postcard or telephone
follow-up every 4 months and confirmed by radiographic report. Hypertension
was defined as treatment with diuretic medications or having a blood pressure
greater than 160/90 mm Hg. Body weight and height were measured at clinic
visits and used to compute body mass index (BMI, calculated as weight in kilograms
divided by the square of height in meters).
The methods of determining deaths have been published.28 Participants
were contacted every 4 months, with 99% follow-up. Causes of death were confirmed
by death certificates, and, when available, hospital discharge summaries were
obtained. The underlying cause of death was coded by a clinical epidemiologist
using the International Classification of Diseases, Ninth
Revision, Clinical Modification, and categorized as due to all causes,
CVD (ICD-9-CM codes 401 to <405, 410 to <415,
425, 428, 429.2, 430 to <439, 440 to <445, and 798), and cancer (ICD-9-CM codes 140 to 239).
Age-adjusted mortality rates were computed using direct adjustment to
the SOF sample population. Cox proportional hazards regression was used to
assess the association of baseline physical activity levels (quintiles) and
changes in physical activity (shifts in quintiles and kilocalorie change)
with mortality. These analyses controlled for age, smoking, BMI, stroke, diabetes,
coronary heart disease, hypertension, cancer, chronic obstructive pulmonary
disease, hip fracture, and baseline physical activity level (kilocalories
per week). Covariates were based on assessments from the follow-up visit rather
than baseline for all variables except BMI, in which case the most recent
measurement was used (82% of the sample had BMI values at the follow-up visit
and 18% [n = 1337] had BMI measured at an interim visit). Although statistical
assumptions underlying the proportional hazards regression were met, we present
our primary analyses in three 2.2-year segments because we found a tendency
for stronger effects during the early years of follow-up. To explore potential
bias due to underlying illness, we also conducted analyses in which we excluded
data from the first 2 years of follow-up and present findings for which we
found notable differences.
We also estimated propensity scores, evaluated the balance of covariates
across 5 strata defined by propensity scores, and evaluated our primary exposure
groups (sedentary vs increasers), stratified by propensity score. Propensity
score methods reduce the bias in comparing exposure groups in observational
studies29 and are an alternative to traditional
covariance analysis adjustments (eg, regression models). Analyses were conducted
using SAS version 8.2 (SAS Institute Inc, Cary, NC).
Among 9518 participants followed for up to 12.5 years (median, 10.6
years) from baseline, there were 2218 deaths, with 826 due to CVD and 633
due to cancer. Among the 7553 women in the analysis of the effects of change
in physical activity, there were 1029 deaths (386 due to CVD and 264 due to
cancer) over 6.7 years (median, 5.1 years) after the follow-up visit.
Table 1 shows study population
characteristics at the follow-up visit by change in physical activity. Median
walking distance at follow-up was 1.2 mile/wk for women who reported being
sedentary at both visits, 1.8 mile/wk for those who became sedentary between
visits, 8.2 mile/wk for women who became active, and 9.3 mile/wk for those
active at both points. Women who reported being sedentary at both visits tended
to be older, have a higher BMI, and were more likely to be smokers and have
comorbid conditions than women who became active or were active at both study
visits. Women who were active at baseline but became sedentary were also older
and more likely to have comorbid conditions than those who became active or
remained active.
Baseline Physical Activity and Mortality
Higher levels of total physical activity and walking at baseline were
associated with lower all-cause and CVD mortality rates, controlling for age,
BMI, smoking, and comorbid conditions (Table 2). The magnitude of risk reduction associated with physical
activity was greatest for CVD mortality; compared with the lowest quintile,
hazard rate ratios (HRRs) were lowest for women in the highest quintile (HRR,
0.58; 95% confidence interval [CI], 0.46-0.74 for total physical activity
and HRR, 0.61; 95% CI, 0.49-0.78 for walking). Although women in the second
and fourth quintiles of total activity had reduced cancer mortality risk (HRR,
0.77 and 0.62, respectively), associations were less consistent and walking
was not associated with cancer mortality risk. The HRRs were not appreciably
altered when we examined the subsample for whom coronary heart disease data
were available (see Table 2 footnote).
Changes in Physical Activity
Compared with women who were sedentary at both visits, sedentary women
who became active had significantly reduced rates of mortality due to all
causes (HRR, 0.52; 95% CI, 0.40-0.69), CVD (HRR, 0.64; 95% CI, 0.42-0.97),
and cancer (HRR, 0.49; 95% CI, 0.29-0.84) after controlling for age, BMI,
smoking, comorbid conditions, and baseline physical activity (Table 3 and Figure 1).
Adjusting for propensity scores yielded similar results. The HRRs associated
with increasing physical activity were lowest in the first 2 years of follow-up
for each outcome (HRR range, 0.38-0.44 across the 3 outcomes) and increased
slightly during the middle years (HRR range, 0.48-0.79) and later years (HRR
range, 0.60-0.72).
Mortality due to CVD tended to be lowest in women with the greatest
increases in physical activity; compared with women who were sedentary at
both visits, all-cause HRRs for those who increased from the bottom 2 quintiles
to the third, fourth, and fifth quintiles were 0.85 (95% CI, 0.48-1.50), 0.69
(95% CI, 0.36-1.30), and 0.32 (95% CI, 0.12-0.87), respectively. There was
little dose-response relationship between physical activity change and all-cause
and cancer mortality, however; HRRs for those who increased to the third through
fifth quintiles, respectively, were 0.54 (95% CI, 0.36-0.83), 0.58 (95% CI,
0.38-0.89), and 0.43 (95% CI, 0.25-0.73) for all-cause mortality and 0.59
(95% CI, 0.28-1.28), 0.37 (95% CI, 0.14-1.00), and 0.51 (95% CI, 0.20-1.25)
for cancer mortality.
Women reporting high levels of total physical activity at both visits
also had significant reductions in mortality due to all causes (HRR, 0.68;
95% CI, 0.56-0.82) and CVD (HRR, 0.62; 95% CI, 0.44-0.88) but not cancer (Figure 1). Mortality rates for women who
decreased their activity levels did not differ significantly from those of
continually sedentary women, except for cancer mortality, for which they had
a lower mortality rate (HRR, 0.61; 95% CI, 0.42-0.90).
Stratification by propensity score quintile allowed for examination
of the association of increasing physical activity within strata of women
who had relatively homogeneous health status (Table 4). We found that the association between physical activity
changes and lower mortality were generally consistent regardless of risk status
(propensity score tertile). However, exclusion of the first 2 years of follow-up
led to a notable attenuation of the association among the highest-risk women
(ie, those with worse health status). The association between increasing activity
level and mortality reduction also tended to be stronger among women younger
than 75 years than among women aged at least 75 years (Table 5), particularly when we excluded women with less than 2 years
of follow-up (HRR, 0.23; 95% CI, 0.10-0.53 for <75 years; HRR, 0.77; 95%
CI, 0.55-1.08 for ≥75 years).
When we examined the relationship between absolute changes in physical
activity levels (rather than quintile shifts), women who increased their activity
levels had a 36% lower mortality (HRR, 0.64; 95% CI, 0.53-0.77) than women
who had maintained their physical activity levels (Table 5); this association was stronger among women with large increases
in physical activity (HRR, 0.54; 95% CI, 0.41-0.70) and existed in both age
strata. However, the association between increases in physical activity and
lowered mortality was attenuated among older women when we excluded the first
2 years of follow-up. Additionally, large decreases in physical activity were
associated with increased mortality among women aged at least 75 years (HRR,
1.43; 95% CI, 1.07-1.90) but were not associated with mortality among women
younger than 75 years.
In this large population of older white women, both being physically
active and becoming active were associated with substantially lower mortality
rates. Notably, sedentary white women who increased their physical activity
levels to the equivalent of about 1 mile/d of walking between baseline and
a follow-up visit 6 years later had approximately 40% to 50% lower all-cause,
CVD, and cancer mortality rates than chronically sedentary white women. We
found similar reductions in all-cause and CVD mortality rates among white
women who were consistently active throughout the study. However, recent physical
activity levels were a more important predictor of longevity than past levels;
previously sedentary white women who became active had a similar mortality
rate as those who were already active, whereas active white women who became
sedentary had a mortality risk similar to those who were sedentary all along.
Increased longevity associated with increasing physical activity could
arise from many factors, including reductions in CVD risk factors and events,
improved cardiorespiratory fitness,4,9,19,20 and
reduced risk of falls, osteoporotic fractures, and physical disability.9,27,30,31 We
also found that increasing physical activity was associated with mortality
reductions in all but the highest-risk white women. However, even the low-
and moderate-risk women in the study sample had a high prevalence of comorbid
conditions such as hypertension, diabetes, CVD, and functional difficulties.
Thus, our study may indicate that physical activity works as much by slowing
decline, enhancing recovery, and extending life in those who already have
chronic conditions as by preventing onset of new disease. Such a finding is
consistent with previous meta-analyses relating exercise-based cardiac rehabilitation
to CVD mortality32 as well as observational
studies among diabetic persons relating higher physical activity and fitness
to fewer cardiovascular events and deaths.33,34
Whether increasing physical activity has a similar association with
lower mortality across the full age range of older women is less clear, as
we found a slightly weaker relationship among white women aged at least 75
years. This may be due in part to a survival bias, wherein the oldest white
women are particularly hardy and among whom variation in physical activity
may play a lesser role in influencing longevity. Additionally, our physical
activity questionnaire may have been less sensitive to physical activities
commonly performed by the oldest white women. These concerns, combined with
the lack of comparable studies, indicate a need for future studies to more
closely examine the effects of physical activity among the oldest segment
of the population.
Few studies have examined the association between late-life physical
activity changes and mortality. In a population-based sample of 1405 Swedish
women,21 changes in physical activity levels
were positively associated with longevity, but this appeared to be driven
primarily by women who decreased their physical activity levels having increased
mortality. Women who increased their physical activity levels did not have
reduced mortality, leading the authors to conclude that it may be the maintenance
of physical activity (ie, preventing decreases) that is most important for
longevity in older age. One other study conducted among a small sample of
older men (aged 65-74 years) found that those who increased their physical
activity levels had an increased risk of ischemic heart disease mortality.35 Results from at least 4 other younger sample populations,
however, have associated increases in physical activity or fitness levels
during middle age with reduced all-cause and CVD mortality.17-20
Previous studies have indicated that observational studies do not systematically
overestimate effects relative to randomized controlled trials.36,37 Notable
exceptions to this, however, are the recently published large randomized controlled
trials that showed no benefit of hormone replacement therapy on CVD outcomes,
contradicting earlier observational studies.38,39 The
controversy over hormone replacement therapy may stimulate a closer examination
of the large body of observational studies associating physical activity levels
with longevity as well. Like most studies of physical activity and mortality,
these were secondary analyses using data from a large cohort study. Accordingly,
our study also cannot rule out the influence of selection biases, residual
confounding, or findings due to chance. We found changes in physical activity
to be more consistently associated with cancer mortality than baseline physical
activity, raising the question of whether a higher prevalence of undetected
cancer among chronically sedentary white women explained the association of
physical activity increases with reduced mortality. Recent systematic reviews
suggest that regular physical activity is causally associated with reduced
colon and breast cancer risk40,41 but
that data related to other cancer sites for women are insufficient to make
conclusions.
The potential for similar biases and residual confounding also exists
for CVD mortality, as we found weaker effects in later years of follow-up
than in the early years. However, mortality reductions remained substantial
in later years, and our analytic approach was conservative in that we controlled
for comorbidities at the follow-up visit rather than the baseline visit. We
also found similar results when we controlled for and stratified by propensity
scores. However, these analyses cannot account for variation in health not
measured by the study or control for other subclinical disease that could
have influenced both the ability to increase physical activity levels and
mortality risk. Considering these limitations and the widespread credit given
to physical activity's health influence,9 it
is unfortunate that there has never been an adequately powered randomized
controlled trial to verify the independent influence of physical activity
on major disease outcomes or death and determine whether selection factors
influence observational studies of physical activity.
Our study may have been limited by our dependence on self-reported physical
activity and our inability to assess the effects of lower-level nonleisure
activities. However, we observed broad distributions for physical activities
that are common among older women (walking, gardening, nonleisure walking)
and estimates of physical activity using these measures have been shown to
be reliable among postmenopausal women.42 There
was also slight variation in the physical activity questionnaires used at
baseline and follow-up. Although we excluded information about high-intensity
activities to make the measures of physical activity equivalent, there may
have been some difference in the absolute levels of physical activity that
resulted from these slightly different formats. Another potential limitation
is that changes in physical activity levels could indicate a regression to
the mean. If so, the group reporting increasing physical activity levels may
include habitually active women who were simply misclassified as sedentary
at baseline. Previous studies have provided some validation for repeated physical
activity assessments, however, showing that reported changes in physical activity
are accompanied by significant improvements in cardiorespiratory fitness.19 Accordingly, it seems unlikely that changes in physical
activity are simply due to reporting error or misclassification. Finally,
we had less power for our evaluation of cancer mortality than for all-cause
and CVD mortality. In post hoc power calculations for analyses of changes
in physical activity, we estimate that we had 80% power to identify reductions
of 23%, 38%, and 50% for all-cause mortality, CVD, and cancer mortality, respectively,
and 46% and 28% all-cause mortality reductions among those younger than and
at least 75 years of age, respectively.
Our finding that being consistently physically active and adopting a
physically active lifestyle are each associated with longevity in older white
women are important because the population of older women in the United States
is projected to double in the next 30 years43 and
more than one third are now sedentary.22 Modest
increases in physical activity could have wide-ranging benefits ranging from
improved risk factors to reduced disability. Our findings suggest these benefits
may translate into substantial reductions in mortality, indicating a need
to identify effective modes of enhancing walking and other low-intensity activities
among older white women as well as effective health system–, community-,
and environmentally based approaches to enhance physical activity. Examination
of these issues in the form of randomized controlled trials would help translate
these findings to the clinical setting, determine the mechanisms for such
effects, and rule out the influence of self-selection factors.
1.Paffenbarger RS, Hyde RT, Wing AL, Hsieh CC. Physical activity, all-cause mortality and longevity of college alumni.
N Engl J Med.1986;314:605-613.Google Scholar 2.Leon AS, Connett J, Jacobs DR, Rauramaa AR. Leisure-time physical activity levels and risk of coronary heart disease
and death: the Multiple Risk Factor Intervention Trial.
JAMA.1987;258:2388-2395.Google Scholar 3.Lee IM, Hsieh CC, Paffenbarger Jr RS. Exercise intensity and longevity in men: the Harvard Alumni Health
Study.
JAMA.1995;273:1179-1184.Google Scholar 4.Lakka TA, Venalainen JM, Rauramaa R, Salonen R, Tuomilehto J, Salonen JT. Relation of leisure-time physical activity and cardiorespiratory fitness
to the risk of acute myocardial infarction.
N Engl J Med.1994;330:1549-1554.Google Scholar 5.Manson JE, Hu FB, Rich-Edwards JW.
et al. A prospective study of walking as compared with vigorous exercise in
the prevention of coronary heart disease in women.
N Engl J Med.1999;341:650-658.Google Scholar 6.Powell KE, Thompson PD, Caspersen CJ, Kendrick JS. Physical activity and the incidence of coronary heart disease.
Annu Rev Public Health.1987;8:253-287.Google Scholar 7.Helmrich SP, Ragland DR, Leung RW, Paffenbarger RS. Physical activity and reduced occurrence of non-insulin-dependent diabetes
mellitus.
N Engl J Med.1991;325:147-152.Google Scholar 8.Hu FB, Stampfer MJ, Colditz GA.
et al. Physical activity and risk of stroke in women.
JAMA.2000;283:2961-2967.Google Scholar 9.US Department of Health and Human Services. Physical Activity and Health: A Report of the Surgeon
General. Atlanta, Ga: National Center for Chronic Disease Prevention and Health
Promotion, Centers for Disease Control and Prevention; 1996.
10.Hakim AA, Petrovitch H, Burchfiel CM.
et al. Effects of walking on mortality among nonsmoking retired men.
N Engl J Med.1998;338:94-99.Google Scholar 11.Kushi LH, Fee RM, Folsom AR, Mink PJ, Anderson KE, Sellers TA. Physical activity and mortality in postmenopausal women.
JAMA.1997;277:1287-1292.Google Scholar 12.Bijnen FC, Caspersen CJ, Feskens EJ, Saris WH, Mosterd WL, Kromhout D. Physical activity and 10-year mortality from cardiovascular diseases
and all causes: the Zutphen Elderly Study.
Arch Intern Med.1998;158:1499-1505.Google Scholar 13.Morgan K, Clarke D. Customary physical activity and survival in later life: a study in
Nottingham, UK.
J Epidemiol Community Health.1997;51:490-493.Google Scholar 14.Rakowski W, Mor V. The association of physical activity with mortality among older adults
in the Longitudinal Study of Aging (1984-1988).
J Gerontol.1992;47:M122-M129.Google Scholar 15.Kaplan GA, Seeman TE, Cohen RD, Knudsen LP, Guralnik J. Mortality among the elderly in theAlameda County Study: behavioral
and demographic risk factors.
Am J Public Health.1987;77:307-312.Google Scholar 16.Stessman J, Maaravi Y, Hammerman-Rozenberg R, Cohen A. The effects of physical activity on mortality in the Jerusalem 70-Year-Olds
Longitudinal Study.
J Am Geriatr Soc.2000;48:499-504.Google Scholar 17.Wannamethee SG, Shaper AG, Walker M. Changes in physical activity, mortality, and incidence of coronary
heart disease in older men.
Lancet.1998;351:1603-1608.Google Scholar 18.Paffenbarger RS, Hyde RT, Wing AL, Lee IM, Jung DL, Kampert JB. The association of changes in physical activity level and other lifestyle
characteristics with mortality among men.
N Engl J Med.1993;328:538-545.Google Scholar 19.Blair SN, Kohl HW, Barlow CE, Paffenbarger RS, Gibbons LW, Macera CA. Changes in physical fitness and all-cause mortality: a prospective
study of healthy and unhealthy men.
JAMA.1995;273:1093-1098.Google Scholar 20.Erikssen G, Liestol K, Bjornholt J, Thaulow E, Sandvik L, Erikssen J. Changes in physical fitness and changes in mortality.
Lancet.1998;352:759-762.Google Scholar 21.Lissner L, Bengtsson C, Bjorkelund C, Wedel H. Physical activity levels and changes in relation to longevity: a prospective
study of Swedish women.
Am J Epidemiol.1996;143:54-62.Google Scholar 22.Kamimoto LA, Easton AN, Maurice E, Husten CG, Macera CA. Surveillance for five health risks among older adults—United
States, 1993-1997.
MMWR CDC Surveill Summ.1999;48:89-130.Google Scholar 23.Parker CJ, Morgan K, Dewey ME. Physical illness and disability among elderly people in England and
Wales: the Medical Research Council Cognitive Function and Ageing Study.
J Epidemiol Community Health.1997;51:494-501.Google Scholar 24.Cummings SR, Black DM, Nevitt MC.
et al. Appendicular bone density and age predict hip fracture in women.
JAMA.1990;263:665-668.Google Scholar 25.Paffenbarger RS, Wing AL, Hyde RT. Physical activity as an index of heart attack risk in college alumni.
Am J Epidemiol.1978;108:161-175.Google Scholar 26.Pereira MA, Fitzgerald SJ, Gregg EW.
et al. A collection of physical activity questionnaires for health-related
research.
Med Sci Sports Exerc.1997;29(6 suppl):S1-S205.Google Scholar 27.Gregg EW, Cauley JA, Seeley DA, Ensrud KE, Bauer DG. Physical activity and osteoporotic fracture risk in older women: the
Study of Osteoporotic Fractures.
Ann Intern Med.1998;129:81-88.Google Scholar 28.Cauley JA, Seeley DG, Browner WS.
et al. Estrogen replacement therapy and mortality among older women: the Study
of Osteoporotic Fractures.
Arch Intern Med.1997;157:2181-2187.Google Scholar 29.D'Agostino Jr RB. Propensity score methods for bias reduction in the comparison of a
treatment to a non-randomized control group.
Stat Med.1998;17:2265-2281.Google Scholar 30.Campbell AJ, Robertson MC, Gardner MM, Norton RN, Tilyard MW, Buchner DM. Randomised controlled trial of a general practice programme of home
based exercise to prevent falls in elderly women.
BMJ.1997;315:1065-1069.Google Scholar 31.Leveille SG, Guralnik JM, Ferrucci L, Langlois JA. Aging successfully until death in old age: opportunities for increasing
active life expectancy.
Am J Epidemiol.1999;149:654-664.Google Scholar 32.Oldridge NB, Guyatt GH, Fischer ME, Rimm AA. Cardiac rehabilitation after myocardial infarction: combined experience
of randomized clinical trials.
JAMA.1988;260:945-950.Google Scholar 33.Wei M, Gibbons LW, Kampert JB, Nichaman MZ, Blair SN. Low cardiorespiratory fitness and physical inactivity as predictors
of mortality in men with type 2 diabetes.
Ann Intern Med.2000;132:605-611.Google Scholar 34.Hu FB, Stampfer MJ, Solomon C.
et al. Physical activity and risk for cardiovascular events in diabetic women.
Ann Intern Med.2001;134:96-105.Google Scholar 35.Hein HO, Suadicani P, Sorensen H, Gyntelberg F. Changes in physical activity level and risk of ischaemic heart disease:
a six-year follow-up in the Copenhagen Male Study.
Scand J Med Sci Sports.1994;4:57-64.Google Scholar 36.Benson K, Hartz AJ. A comparison of observational studies and randomized, controlled trials.
N Engl J Med.2000;342:1878-1886.Google Scholar 37.Concato J, Shah N, Horwitz RI. Randomized, controlled trials, observational studies, and the hierarchy
of research designs.
N Engl J Med.2000;342:1887-1892.Google Scholar 38.Writing Group for the Women's Health Initiative Investigators. Risks and benefits of estrogen plus progestin in healthy postmenopausal
women: principal results from the Women's Health Initiative randomized controlled
trial.
JAMA.2002;288:321-333.Google Scholar 39.Stampfer MJ, Colditz GA. Estrogen replacement therapy and coronary heart disease: a quantitative
assessment of the epidemiologic evidence.
Prev Med.1991;20:47-63.Google Scholar 40.Thune I, Furberg A. Physical activity and cancer risk: dose-response and cancer, all sites
and site specific.
Med Sci Sports Exerc.2001;33(suppl):S530-S550.Google Scholar 41.Friedenreich CM. Physical activity and cancer prevention: from observational to intervention
research.
Cancer Epidemiol Biomarkers Prev.2001;10:287-301.Google Scholar 42.Cauley JA, LaPorte RE, Sandler RB.
et al. Comparison of methods to measure physical activity in postmenopausal
women.
Am J Clin Nutr.1987;45:14-22.Google Scholar 43.US Bureau of the Census. Statistical Abstract of US 1999. Washington, DC: US Dept of Commerce; 1999.