American Journal of Epidemiology
Copyright © 2001 by the Johns Hopkins University Bloomberg School of Public Health
All rights reserved
Vol. 154, No. 1
Printed in U.S.A.
Physical Activity and Changes in Relation to Hip Fracture Risk Høidrup et al.
Leisure-time Physical Activity Levels and Changes in Relation to Risk of Hip
Fracture in Men and Women
Susanne Høidrup,1,2Thorkild I. A. Sørensen,1Ulla Strøger,1Jes Bruun Lauritzen,3Marianne Schroll,4and
The authors prospectively studied the effect of leisure-time physical activity level on hip fracture risk along
with the influence of within-subject changes in activity levels, while taking possible confounding by other health
behaviors and poor health into account. Analyses were based on pooled data from three population studies
conducted in Copenhagen, Denmark. Among 13,183 women and 17,045 men, 1,121 first hip fractures were
identified during follow-up. In comparison with being sedentary, the relative risk (RR) of hip fracture associated
with being moderately physically active 2–4 hours per week was 0.72 (95% confidence interval (CI): 0.59, 0.89)
in women and 0.75 (95% CI:0.55, 1.03) in men after adjustment for confounders.Being in the most active leisure
activity category did not decrease the risk of hip fracture further. Adjustment for poor health affected the risk
estimates only modestly. Subjects who, during follow-up, reduced their physical activity level from the highest or
the intermediate activity level to a sedentary level had a higher risk of hip fracture than did those who remained
moderately physically active at the intermediate level (multivariate adjusted RR = 2.19, 95% CI: 1.00, 4.84 and
RR = 1.89, 95% CI: 1.21, 2.95, for reduction from the highest and intermediate levels, respectively).There was
no evidence of a fracture-protective effect from increasing physical activity. In conclusion, moderate levels of
physical activity appear to provide protection against later hip fracture. Decline in the physical activity level over
time is an important risk factor for hip fracture. Am J Epidemiol 2001;154:60–8.
exercise; hip fractures; osteoporosis; prospective studies; risk factors
Received for publication February 7, 2000, and accepted for pub-
lication November 29, 2000.
Abbreviations: CCCPM, Copenhagen County Center for
Preventive Medicine; CI, confidence interval; CMS, Copenhagen
Male Study; ICD-8, International Classification of Diseases, Eighth
Revision; RR, relative risk.
1The Copenhagen Center for Prospective Population Studies,
Danish Epidemiology Science Center at the Institute of Preventive
Medicine, Copenhagen University Hospital, Copenhagen, Denmark.
2Copenhagen County Center for Preventive Medicine, Unit for
Dietary Studies, Glostrup University Hospital, Glostrup, Denmark.
3Department of Orthopedic Surgery, Copenhagen University
Hospital Hvidovre, Copenhagen, Denmark.
4 1Department of Geriatrics, Copenhagen University Hospital
Bispebjerg, Copenhagen, Denmark.
The Copenhagen Center for Prospective Population Studies con-
sists of the Copenhagen County Center for Preventive Medicine, the
Copenhagen City Heart Study, and the Copenhagen Male Study.
Reprint requests to Dr.Susanne Høidrup, Unit for Dietary Studies,
Copenhagen County Center for Preventive Medicine, Glostrup
University Hospital, DK-2600 Glostrup, Denmark (e-mail:
A general adaptation to a sedentary lifestyle during the
past century is thought to be one of the causes responsible
for the steep rise in hip fracture incidence observed in most
Western countries during the last 5 decades (1–3). This
assumption rests on the observation of an osteogenic effect
of physical activity on human and animal bones (4–6) and
on findings that physical activity improves muscle strength,
balance, and physical function, thereby reducing the risk of
falling (7–10). Furthermore, numerous epidemiologic stud-
ies seem to confirm the existence of an inverse relation
between physical activity and hip fracture risk (11–26).
Considering the observational design of these studies, selec-
tion bias, recall bias, and confounding by other health
behaviors or by functional status cannot, however, be ruled
out as alternative explanations of the apparent fracture-
protective effect of physical activity. Intervention studies
may clarify the existence of a causal relation between phys-
ical activity and hip fracture, but the long lag time to occur-
rence of hip fracture makes such studies difficult to conduct.
Valuable information toward making causal inference as
well as assessing public health implications may be obtained
by observing the consequences of changes in physical activ-
ity level on hip fracture risk.
By using pooled data from three large, population-based
follow-up studies, we evaluated the effect of leisure-time
physical activity level on hip fracture risk while taking pos-
sible confounding introduced by other health behaviors and
functional status into account. Furthermore, we assessed the
influence of within-subject changes in physical activity
level on hip fracture risk.
MATERIALS AND METHODS
This study is based on data from The Copenhagen Center
for Prospective Population Studies, which compiles data
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Physical Activity and Changes in Relation to Hip Fracture Risk
Am J EpidemiolVol. 154, No. 1, 2001
from three longitudinal population studies conducted in the
Copenhagen, Denmark, area between 1964 and 1992: The
Copenhagen County Center for Preventive Medicine
(CCCPM) (formerly the Glostrup Population Study), with
10,191 randomly selected men and women from six birth
cohorts from Copenhagen suburbs; the Copenhagen City
Heart Study, with 15,786 randomly selected, age-stratified
men and women from central Copenhagen; and the
Copenhagen Male Study (CMS), which sampled 5,246 men
from 14 major work sites in the Copenhagen area (27–30).
The overall attendance rate at the first examination was 77
percent (range, 69–88 percent). After the exclusion of
subjects with missing information on leisure-time physical
activity, subjects with previous hip fracture, and double par-
ticipants, 30,228 subjects were available for analysis of the
effect of physical activity levels on hip fracture risk.
Characteristics of the study population are outlined in table 1.
To study the influence of changes in physical activity lev-
els on hip fracture risk, we selected subjects who attended
the first and second examinations of the Copenhagen City
Heart Study and the CCCPM. Because of a very short inter-
val between the first and second examinations of the CMS
and the lack of analog information on leisure-time physical
activity from the third CMS examination, subjects who
attended the first and the fourth examinations of this study
were selected for the analysis of changes in physical activ-
ity. The combined population for the study of changes in
physical activity levels totaled 17,285 subjects, of whom
15,498 (8,431 women and 7,067 men) gave information on
their level of leisure-time physical activity at both examina-
tions. A total of 13,487 subjects (45 percent of the entire
study population) were not eligible for the analysis of
changes in physical activity due to the following circum-
stances: 1,311 subjects (10 percent) died between the base-
line examination and the time of the second (CMS: fourth)
examination, 3,113 subjects (23 percent) did not respond the
invitation to the second (CMS: fourth) examination, 3,416
subjects (25 percent) participated in a substudy in which no
reexaminations were performed during follow-up, and 5,647
subjects (42 percent) were recruited and/or examined for the
first time at one of the reexaminations.
Examination procedure and ascertainment of leisure-
time physical activity
As part of a general health examination, all three popula-
tion studies used a self-administered questionnaire with
detailed questions regarding lifestyle habits and other health-
related items. The phrasing of questions differed slightly in
the various subcohorts, but the covariates used for this study
could be harmonized without substantial loss of information.
Participants were asked to place themselves into one of
the four following categories of leisure-time physical activ-
ity levels: 1) sedentary, i.e., physically inactive, performing
mainly sedentary tasks such as watching television, reading,
or performing moderate physical activities such as light
housekeeping, light gardening, biking, or walking less than
2 hours per week; 2) moderately physically active 2–4 hours
per week; 3) moderately physically active more than 4 hours
per week or energetically physically active 2–4 hours per
week, including energetic activities such as running, brisk
walking or biking, heavy gardening, playing tennis etc.; and
4) energetically physically active more than 4 hours per
week or participating in sports competitions. Owing to the
small number of subjects in the last category, it was neces-
sary to collapse physical activity levels 3 and 4 into a single
category in all analyses. For the analyses of changes in
physical activity levels, subjects were categorized into the
nine combinations of physical activity for the first and sec-
Studies, Copenhagen, Denmark, 1964–1992
Characteristics of the study population,The Copenhagen Centre for Prospective Population
Copenhagen City Heart Study
Copenhagen Male Study
Copenhagen County Center of
1970, 1971, 1976, 1985
1964, 1974, 1984, 1989
1976, 1981, 1987
* Subjects with hip fracture before entrance into the study (n = 88), double participants (n = 246), subjects with
International Classification of Diseases, Eighth Revision, diagnosis code modifications indicating previous hip
fracture (n = 54), and subjects lost to follow up (n = 63) were excluded.
† MONICA, monitoring of trends and determinants in cardiovascular disease (World Health Organization
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Høidrup et al.
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