Physical fitness as a predictor of herniated lumbar disc disease - A 33-year follow-up in the Copenhagen male study

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DOI: 10.1186/1471-2474-14-86 · Source: PubMed
Abstract
The role of physical fitness (VO2Max (mlO2*min-1*kg-1)) as a risk factor for herniated lumbar disc disease (HLDD) is unknown. The objective of this study was to examine the association between aerobic (physical) fitness and risk of hospitalisation due to HLDD in a long-term follow up. The Copenhagen Male Study is a prospective cohort study established in 1970-71. At baseline, 5,249 men answered a questionnaire about their history of back disease, physical and psychosocial working conditions, lifestyle and social class. Height and weight was measured and aerobic capacity (physical fitness) was estimated based on a submaximal bicycle test. Information about hospitalization due to HLDD was obtained from the National Hospital Register covering the period 1977 - 2003. Hazard Ratios (HR) were calculated by Cox's proportional hazard regression model. Among 3,833 men without history of low back disorders, 64 were hospitalized due to HLDD. The cumulative incidence of HLDD was 1.7% (n=34) among men with low physical fitness (15-32 ml O2*min-1*kg-1), and 1.7% (n=30) among men with high physical fitness (33-78 ml O2*min-1*kg-1). In a final model, adjusted for relevant confounders, the HR (95% CI) for HLDD for those with high physical fitness was 0.88 (0.51-1.50) compared to those with low physical fitness. In the same model, HR for men often exposed to strenuous work compared to those seldom or never exposed to strenuous work was 3.91(1.82-8.38). Also body height was a significant predictor. Physical fitness is not associated with hospitalisation due to HLDD, and the only modifiable risk factor for hospitalisation due to HLDD seems to be strenuousness at work.
RES E AR C H A R T I C L E Open Access
Physical fitness as a predictor of herniated lumbar
disc disease a 33-year follow-up in the
Copenhagen male study
Marie B Jørgensen
1*
, Andreas Holtermann
1
, Finn Gyntelberg
2
and Poul Suadicani
2
Abstract
Background: The role of physical fitness (VO
2
Max (mlO
2
*min
-1
*kg
-1
)) as a risk factor for herniated lumbar disc
disease (HLDD) is unkn own. The objective of this study was to examine the association between aerobic (physical)
fitness and risk of hospitalisation due to HLDD in a long-term follow up.
Methods: The Copenhagen Male Study is a prospective cohort study established in 197071. At baseline, 5,249
men answered a questionnaire about their history of back disease, physical and psychosocial working conditions,
lifestyle and social class. Height and weight was measured and aerobic capacity (physical fitness) was estimated
based on a submaximal bicycle test. Information about hospitalization due to HLDD was obtained from the
National Hospital Register covering the period 1977 2003. Hazard Ratios (HR) were calculated by Coxs
proportional hazard regression model.
Results: Among 3,833 men without history of low back disorders, 64 were hospitalized due to HLDD. The
cumulative incidence of HLDD was 1.7% (n=34) among men with low physical fitness (1532 ml O
2
*min
-1
*kg
-1
),
and 1.7% (n=30) among men with high physical fitness (3378 ml O
2
*min
-1
*kg
-1
). In a final model, adjusted for
relevant confounders, the HR (95% CI) for HLDD for those with high physical fitness was 0.88 (0.51-1.50) compared
to those with low physical fitness. In the same model, HR for men often exposed to strenuous work compared to
those seldom or never exposed to strenuous work was 3.91(1.82-8.38). Also body height was a significant predictor.
Conclusions: Physical fitness is not associated with hospitalisation due to HLDD, and the only modifiable risk factor
for hospitalisation due to HLDD seems to be strenuousness at work.
Keywords: Lumbar disc herniation, Prospective cohort, Physical workload, Physical fitness, Height
Background
Low back disorders are frequent in the western societies
with considerable individual, societal and economical
consequences [1]. Investigation of modifiable risk fac-
tors for development of low back pai n has therefore
been requested [1].
Aerobic capacity (physical fitness) is a highly modifiable
factor depending the on level of physical activity [2], and
low physical fitness ha s previously been shown to be
associated with increased risk of low back disorders
[3-5]. Physical ac tivity is among the most common
recommendations for prevention and rehabilitation of low
back disorders [1], and recently in a 5-year follow-up of 41
individuals Elfering et al. [6] found a higher risk of lumbar
disc degeneration among those who did not participate in
sports activities. A recent cross-sectional study of police
workers by Heneweer et al. [5] found that individuals with
a high physical fitness had a lower prevalence of low back
pain than others. In the same study a U-shaped association
was found between physical activity at work and in leisure
time and low back pain. The authors concluded that phy-
sical fitness, rather than self-reported physical activities, is
more strongly associated with low back pain. However, the
physiological mechanism for the association between low
aerobic capacity and elevated risk of low back disorders is
questionable and needs further investigation with objective
* Correspondence: mbj@nrcwe.dk
1
The National Research Centre for the Working Environment, Lersø Parkallé
105, DK-2100, Copenhagen, Denmark
Full list of author information is available at the end of the article
© 2013 Jørgensen et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the
Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
Jørgensen et al. BMC Musculoskeletal Disorders 2013, 14:86
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data. Potentially, the difference is merely an effec t of
differences in lifestyle.
Although herniated lumber disc disease (HLDD) is only
the cause of low back pain in a s mall number of individuals
with low back pain, in contrast to the unspecific nature of
low back pain generally, it is a well-defined p athologic con-
dition. Also, HLDD should be considered a serious mani-
festation of low back disorder, making it relevant to study if
low physical fitness may be a risk factor for developing
HLDD. No previous studies ha ve addressed this potentially
important issue. A previous paper from the Copenhagen
Male Study showed that frequent exposure to strenuous
physical activity at work was a strong risk factor for later
hospitalisation due to HLDD. In that paper, the potential
role of physical fitness was not considered [7].
Inthepresentstudywetestedthehypothesisthatlow
physical fitness (objectively measured estimation of aerobic
capacity) is a risk factor for hospitali sation due to HLDD.
Methods
Study population
The Copenhagen Male Study was established in 19701971
as a prospective cohort study of physical fitness and cardio-
vascular disease in employees in 14 private and public
companies in Copenhagen. All men aged 4059 years
were invited; 5,249 men, 87% of potential participant s,
took part in the examinations at baseline [8,9]. In th e
present study, only men without a history of back
disorders were included.
Baseline data were obtained by interview based on a pre-
cedent structured questionnaire, and a clinical examination
including measurement of height and weight [10], and
indirect measurement of aerobic capacity (physical fitness)
was performed with a bicycle ergometer [11]. Thirty-five
men with orthopaedic problems unable to perform the
bicycle test were excluded from the study. Information on
working conditions , lifestyle, and general health wa s
obtained from the questionnaire. Variables that could
represent potential risk factors for lumbar inte rverte-
bral disc herniation were chosen [12-17]. Details on
the questionnaire have pre viously been published [10].
Physical fitness Heart rate was measured during
submaximal bicycle work in steady state with the aid of a
stopwatch and stethoscope. The loads used were 100, 150
and 200 W. One, two or in a few cases three different
loads were used. The loads chosen in each case were de-
termined from the weight and age of the person or heart
rate during the first minute of the test, and the estimation
of VO
2
Max was accomplished with the aid of Åstrands
nomogram [18]. The correlation between directly and in-
directly measured physical fitness is high. The method
used has previously been described in detail [8].
Physical occupational workload wa s assessed from
one question addressing ergonomic load to the back: Is
your work a) sedentary, b) slightly physical without lifting,
c) physical with some lifting, or d) hard physical with heavy
lifting, shovelling or the like? The two first categories were
condensed in the analyses. Additionally, a more general
question about physical workload was asked: Do you
perform strenuous work (regularly resulting in sweating)?
Answer options were: often, occasionally and seldom
or n ever.
Physical activity in leisure time was classified into one of
four different groups: Predominantly sedentary, slightly
active, fairly active or very active. For analytical purposes
answers in the two middle groups were condensed.
Mental stress at work and during leisure time was
evaluated separately by asking: Do you feel under mental
stress when performing your job/during leisure t ime?
Answer options were: Seldom and Regularly.In
addition it was asked whether sedatives were taken
regularly, occasionally or never.
Lifestyle factors included current and previous tobacco
and alcohol consumption.
Health related factors included questions on present
or previous episodes of back pain or back injury. Answer
options were yes and no.
Anthropometrics included height and weight based on
measurements and body mass index (BMI) calculated as
kg * m
2
. In separate analyses of height and weight the study
population was divided into thirds based on tertile values.
BMI was divided into three groups: Normal weight or lean:
BMI < 25, overweight: BMI 2529.9, and obese: 30.
Social class wa s grouped in five categories based on
educational le vel, job type and position in terms of
number of subordinates [8,19]. Typical jobs in the co-
hort were as follows. Social cla ss I: head of depart-
ment, officer, civil engineer. Social class II: head clerk ,
engineer, resident archite ct. Social class III: engine
driver, train guard. Social class IV: skilled workers.
Social class V: unskilled worker, driver.
End-point was hospitaliz ation due to herniated lum-
bar disc disease identified in the National Hospital
Register between 1977 and 2003. Code 725.11 from the
International Classification of Diseases (ICD), 8
th
Revision
was applied from 1977 to 1994, and code M51.1 from ICD
10
th
Revision from 1994 to 2003.
Statistical analyses
The analyses were performed using SPSS for Windows
[20,21]. Relative risks were estimated by exp(β), where β is
the hazard coefficient for the variable of interest in a Coxs
proportional hazards regression model [22] with the max-
imum likelihoo d ratio method and the use of a backward
stepwise elimination procedureinanalysesadjustedforage
only, and in a final model including also other potentially
relevant variables. Assumptions for use of Coxspropor-
tional hazards wer e met by inspection of the log minus log
Jørgensen et al. BMC Musculoskeletal Disorders 2013, 14:86 Page 2 of 6
http://www.biomedcentral.com/1471-2474/14/86
function at the covariate mean. A two-sided probability
value of p 0.05 was a priori taken as significant.
Ethics
The ethics committee for medical research in the County
of Copenhagen approved the study in connection with a
follow-up of the cohort in 198586; in 1970 no committee
had been established.
Results
At baseline, 1,412 of the men (26.9%) reported a history
of back disorder. The 3,833 individuals without such a
history were entered in the study. A number of 108 men
were hospitalized due to herniated lumbar disc disease
during the study period 1977 to 2003; of those, 64 men
without previous history of low back pain or injury were
hospitalized the group analysed in the present stud y.
Table 1 shows potential predictors of hospitalisation for
Herniated lumbar disc disease among the 3,833 men with-
out history of any back disease at baseline in 197071. In
analyses adjusted for age only, significant associations were
found for occasional as well frequent exposure to strenuous
work, and for increasing height and weight. Physical fitness
was not significantly associated with HLDD.
Table 2 shows characteristics of men with low and
high physical fitness (cut-off at median=32) among men
without history of low back pain/injury. Included in the
table are only factors significantly associated with HLDD
risks in Table 1. Men with low physical fitness were
slightly older, and they weighed more. These two factors
could thus be regarded as conventional potential
confounders and were included in the final model show n
in Table 3.
Table 3 shows the result of the Cox proportional hazards
regression analysis using backward elimination. In addition
Table 1 Potential predictors of hospitalisation for Herniated
lumbar disc disease during the period 19772003 among
3,833 men without history of any back disease at
baseline in 197071
Herniated lumbar
disc disease, n (%)
Age-adjusted
HR (95% CI)
Physical fitness (VO
2
Max)
Low, - 32 (n= 2055) 34 (1.7%) 1 (reference)
High, 33 + (n= 1778) 30 (1.7%) 0.85 (0.51-1.40)
Ergonomic load to the back
Low (n=3,073) 47 (1.5%) 1 (reference)
Medium (n=582) 13 (2.2%) 1.54 (0.83-2.84)
High (n = 79) 3 (3.8%) 2.80 (0.87-9.0)
Strenuous work
Seldom/never (n = 2,328) 26 (1.1%) 1 (reference)
Occasionally (n= 1,186) 26 (2.2%) 2.09 (1.21-3.61)
Often (n= 247) 10 (4.0%) 3.95 (1.90-8.20)
Leisure time physical activity
Low (n= 647) 13 (2.0%) 1 (reference)
Medium (n = 2,710) 43 (1.6%) 0.72 (0.38-1.33)
High (n = 365) 7 (1.9%) 0.78 (0.31-1.97)
Social class
I (n = 676) 8 (1.2%) 1 (reference)
II (n = 395) 5 (1.3%) 1.06 (0.34-3.24)
III (n = 717) 13 (1.8%) 1.58 (0.65-3.82)
IV (n = 1,542) 28 (1.8%) 1.57 (0.71-3.46)
V (n = 497) 10 (2.0%) 2.22 (0.87-5.65)
Height, cm
- 171 (n = 1,283) 12 (0.9%) 1 (reference)
172-177 (n = 1,355) 29 (2.1%) 2.06 (1.05-4.05)
178 + (n = 1,186) 21 (1.8%) 1.60 (0.78-3.26)
Weight, kg
- 72 (n = 1,282) 14 (1.1%) 1 (reference)
73-80 (n = 1,278) 21 (1.6%) 1.51 (0.77-2.98)
81 + (n = 1,271) 29 (2.3%) 2.15 (1.13-4.07)
BMI, kg*m
2
- 25 (n = 1,858) 28 (1.5%) 1 (reference)
>2529.9 (n = 1,712) 31 (1.8%) 1.30 (0.78-2.17)
30 + (n = 250) 3 (1.2%) 0.95 (0.28-3.12)
Under mental stress at work
Seldom (n = 3,002) 54 (1.8%) 1 (reference)
Regularly (n = 821) 10 (1.2%) 0.64 (0.32-1.26)
Under mental stress in leisure time
Seldom (n = 3,574) 62 (1.7%) 1 (reference)
Regularly (n= 249) 2 (0.8%) 0.47 (0.11-1.94)
Use of sedatives
Never (n = 2,998) 51 (1.7%) 1 (reference)
Table 1 Potential predictors of hospitalisation for Herniated
lumbar disc disease during the period 19772003 among
3,833 men without history of any back disease at
baseline in 197071 (Continued)
Regularly/occasionally (n=827) 13 (1.6%) 1.08 (0.58-2.00)
Smoking
Current (n = 2798) 41 (1.5%) 1 (reference)
Previously (n = 705) 17 (2.4%) 1.43 (0.56-3.63)
Never (n = 328) 6 (1.8%) 0.95 (0.40-2.23)
Daily alcohol use, beverages
0 (n = 1,271) 21 (1.7%) 1 (reference)
1-2 (n = 1,815) 32 (1.8%) 1.08 (0.62-1.87)
3-5 (n = 611) 9 (1.5%) 1.03 (0.47-2-26)
>= 6 (n = 123) 2 (1.6%) 1.22 (0.28-5.21)
Adjusted for age only. Significant associations (p < 0.05) are highlighted (bold).
HR = hazard ratio; CI = confidence interval.
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to physical fitness, age and weight, we included in the
analysis the two factors previously shown to be risk factors
for HLDD in the Copenhagen Male Study. In the fully
adjusted model, only strenuous work (HR (95% CI) 2.37
(1.36-4.13) and 3.91 (1.82-8.38) for occasionally and often,
respectively)andbodyheight(HR(95%CI)2.18(1.10-4.32)
and 1.88 (0.92-3.86) for 172-177 cm a nd 178+ cm,
respectively) remained significantly associated with
HLDD. Men with high physical fitness had a slightly
lower risk for HLDD, far from reaching statistical
significance, (HR = 0.88 and 95% CI = 0.51-1.50).
Discussion
This study is the first to investigate the relation between
objective ly measured physical fitness and hospitalization
due to HLDD in a prospective design. The hypothesis that
low physical fitness is a risk factor for hospitalization due
to HLDD was not supported.
As men tioned in the introduction, a beneficial effect
on low back disorders should be expected with increas-
ing leisure time physical activity (LTPA) sufficiently
intense to increase cardio-respiratory fitness. However,
in this study, neither self-reported LTPA nor the objec-
tively measured physical fitness was predictive of later
hospitalization due to HLDD. From a theoretical point
of view, physical activity should increase the resistance
of the intervertebral discs as discussed by Nachemson
[19]. So, it cannot be ruled out that the negative finding
in our study might be caused by a lack of statistical
power (a type 2 error). Still, not even a tendency of an
association was observed with an incidence of HLDD of
1.7% among men with a rather low physical fitness
(mean (SD) 27.5 (3.5)), and 1.7% among men with a
quite high physical fitness level (mean (SD) 38.8 (5.4)).
Also, age-adjusted mean baseline values of physical
fitness were practically identical for those who were later
hospitalized due to HLDD and those who were not,
32.74 and 32.75, respectively (not shown in table).
Furthermore, to gain an even larger exposure contrast
between men with a high and a low physical fitness, we
compared the incidence of HLDD between the lowest and
the highest quintiles of physical fitness (not shown in table).
The incidence among men with a fitness level of 1526
(lowest quintile) was 1.9%; among men in the highest
quintile 3978 (highest quintile) it was 2.1%. Based on the
above reasoning and the results, we find it unlikely that a
lack of statistical power explains our findings.
Since no previous studies have investigated the associ-
ation between physical fitness and HLDD, we are not able
Table 2 Characteristics of men with low and high physical
fitness (cut-off at median=32) among men without
history of low back pain/injury
Level of physical fitness
Up to median Above median p
a
VO
2
MAX range:
15-32
VO
2
MAX range:
33-78
n=2055 n=1778
Physical fitness
VO
2
MAX (mlO
2
/kg/min) 27.5 (3.5) 38.8 (5.4) -
Strenuous work, %
Seldom/never 62.4 61.3
Occasionally 31.0 32.2 0.72
Often 6.6 6.5
Height, %
- 171 cm 33.5 33.6
172-177 cm 35.8 35.0 0.83
178 + cm 30.7 31.4
Height, mean (SD) 174.3 (6.5) 174.3 (6.5) 0.87
Weight, %
- 72 kg 30.3 49.5
73-80 kg 30.7 30.7 <0.001
81 + kg 39.0 19.9
Weight, mean (SD) 79.2 (10.4) 74.1 (9.1) <0.001
Age, years 49.8 (5.4) 47.4 (5.1) <0.001
Included in the table are only factors significantly associated with HLDD risk in
Table 1. Values presented are mean (SD) or frequency in per cent. Significant
associations (p < 0.05) are highlighted (bold).
a
: p-values of unpaired t test or
Chi-square test (likelihood ratio) where appropriate.
Table 3 Predictors of hospitalisation due to HLDD including
all factors from Table 2 including physical fitness
Covariate HR (95% CI)
Strenuous work (work resulting in sweating)
Seldom/never (n = 2,328) 1 (reference)
Occasionally (n= 1,186) 2.37 (1.36-4.13)
Often (n= 247) 3.91 (1.82-8.38)
Height, cm
- 171 (n = 1,283) 1 (reference)
172-177 (n = 1,355) 2.18 (1.10-4.32)
178 + (n = 1,186) 1.88 (0.92-3.86)
Not in the final model, p > 0.10:
Physical fitness (VO
2
Max)
a
Low, - 32 (n= 2055) 1 (reference)
High, 33 + (n= 1778) 0.88 (0.51-1.50)
Weight, kg
a
- 72 (n = 1,282) 1 (reference)
73-80 (n = 1,278) 1.40 (0.69-2.86)
81 + (n = 1,271) 1.81 (0.89-3.66)
Cox proportional hazards regression using backward elimination. Variables are
ranked after statistical strength of association with the outcome following
adjustment. Significant associations (p < 0.05) are highlighted (bold).
a
: association with herniated lumbar disc disease at time of withdrawal from
the model.HR = hazard ratio; CI = confidence interval.
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to compare our results with those of others. Previous
studies on low back pain may indicate that a high level of
physical fitness may be associated with less LBP as demon-
strated by Heneweer et al. [5] in a cross-sectional case
control study. However, as mentioned, only a small fraction
of people who suffer from low back pain do so due to
HLDD [20]. Therefore, it is plausible that high physical
fitness may prevent low back pain, but not HLDD.
As previously shown and indicated in Table 3 in this
paper, strenuous occupational work is a significant
predictor for risk of hospitalization due to HLDD. It is
well known that strenuousness at work is a combined
factor of both the external physical work demand and the
physical fitness of the worker [21]. However, strenuous
work tasks in this study include lifting of heavy burdens
and awkward body positions, which may rather call for
high muscular capacity than aerobic capacity. Occu-
pational exposure to strenuous work tasks may not neces-
sarily induce a training effect in muscles and connective
tissues in the low back region. This is supported by a
former study among elite athletes [22]. In contrast, strenu-
ous exposures may be harmful because it may overload
tissues of the lower back [19]. Instead, physical training
during leisure has shown to improve capacity and re-
duce risk of low back disorders [23], but Table 1 also
shows that no effect is seen from leisure time phys-
ical activity on HLDD. This indicates that strenuous -
ness at work is the most important modifiable risk
factor for hospitaliz ation due to HLDD among men.
Men in sedentary jobs may have HLDD not leading
to hospitalization, but being treated as out-patients
more often than men in strenuous jobs, a possibility
which cannot be e valuated in this study.
Methodological considerations
This study has the advantage of data from objectively
measured physical fitness and an objectively evaluated
low back disease. Furthermore, the study has the advan-
tage of a prospective design among individuals without
back disorders at baseline. In this study, physic al fitness
is given as aerobic capacity relative to body weight and
therefore, naturally, fitness levels are highly associated
with weight levels. The univariate analysis presents a
high association between body weight and HLDD
suggesting that weight might modify a possible associ-
ation between high aerobic capacity and HLDD. Future
studies should therefore investigate the role of dire ctly
measured aerobic capacity and the risk of HLDD and
separately evaluate the possible modifying effect of body
weight. Fitness varies considerably across age groups. For
example, previously defined fitness categories suggest that
a physical fitness of ~30 ml O
2
*kg
-1
*min
-1
can be defined
as very poor among males below the age of 30 and very
good among males above the age of 75 [24]. In the
current study, fitness above 32 ml O
2
*kg
-1
*min
-1
has been
defined as high fitness both for younger as well as for
older males as the stratification of fitness in low and high
categories in the current study was also statistically
derived (i.e. based on the median value of the whole popu-
lation). This may introduce a skewed distribution of age in
the two fitness groups i.e. many young males will fall in
the category of high physical fitness and many older males
will fall in the category of low physical fitness. However,
since the age in this study ranged from 4058 years, the
median fitness of 32 ml O
2
*kg
-1
*min
-1
only ranged from
poor to fair according to previous categorisations. Thus,
due to the relatively narrow age span, the skewness can be
overcome by controlling for age in all analyses. The
sufficiency of this action is supported by t he fa ct that
the current stratification of fitness previously has proven
highly valid for the determination of other health outcomes
[7,25]. However, the lack of repeated measures of physical
fitness during the relatively long follow-up period may
contribute to misclassification of physical fitness. One
further limitation is the relatively small number of end-
points, narrowing the possibility of conducting interaction
analyses and increasing risk of a type-2 statistical error.
Moreover, the inclusion of only middle-aged Caucasian
men may make our results less relevant for other age
groups, ethnic groups, and women.
Conclusion
Among men without a history of low back pain or
injury to the back, no association wa s found between
the level of physical fitness and risk of hospita-
lisation due to HLDD during a 30-year follow-up.
Strenuousness at work is obser ved to be the domi-
nant modifiable risk factor for hospitalization due to
HLDD, and initiatives at the workplace fo r reducing
the strenuousness may be an important pre vention
method.
Competing interests
The authors declare that they have no competing interests.
Authors contributions
FG led the data collection. AH had the initial idea for the current study. PS
led and conducted the statistical analyses. MBJ led the writing process. All
authors contributed to the design of the study and the interpretation of
data. All authors discussed the results and read and approved the final
manuscript.
Author details
1
The National Research Centre for the Working Environment, Lersø Parkallé
105, DK-2100, Copenhagen, Denmark.
2
The Copenhagen Male Study,
Epidemiological Research Unit, Department of Occupational and
Environmental Medicine, Bispebjerg University Hospital, Copenhagen,
Denmark.
Received: 3 September 2012 Accepted: 20 February 2013
Published: 9 March 2013
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http://www.biomedcentral.com/1471-2474/14/86
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doi:10.1186/1471-2474-14-86
Cite this article as: Jørgensen et al.: Physical fitness as a predictor of
herniated lumbar disc disease a 33-year follow-up in the Copenhagen
male study. BMC Musculoskeletal Disorders 2013 14:86.
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    • "Physical capacity such as muscle strength and endurance, flexibility and aerobic fitness has been related to risk of musculoskeletal pain32333435363738 . Also low leisure time physical activity has been considered a risk factor for musculoskeletal pain [39], however studies present mixed results, possibly due to inadequate use of objective measurements [34,40]. Therefore, both of these potentially important confounders should be considered and measured objectively in studies of the association between work demands and musculoskeletal pain. "
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