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Original research

BMJ Open: first published as 10.1136/bmjopen-2020-040158 on 7 December 2020. Downloaded from

http://bmjopen.bmj.com/

on December 18, 2020 at National Institute of Occupational

Health - DNLA. Protected by copyright.

Is hard physical work in the early

working life associated with back pain

later in life? A cross-sectional study

among 5700 older workers

Rúni Bláfoss

1,2

Sebastian Venge Skovlund,

Rubén López‐Bueno

Joaquin Calatayud,

1,4

Emil Sundstrup,

Lars L Andersen

1,5

1,3

To cite:

Bláfoss R, Skovlund SV,

López‐Bueno R,

et al.

Is hard

physical work in the early

working life associated with

back pain later in life? A cross-

sectional study among 5700

older workers.

BMJ Open

2020;10:e040158. doi:10.1136/

bmjopen-2020-040158

►

Prepublication history for

this paper is available online.

To view these files, please visit

the journal online (http://dx.doi.

org/10.1136/bmjopen-2020-

040158).

ABSTRACT

Objectives

Physically demanding work increases the risk

of developing musculoskeletal disorders during working

life, with low back pain (LBP) as the most prevalent and

debilitating musculoskeletal disorder worldwide. However,

a lack of knowledge exists about the role of early working

years on musculoskeletal health later in life. This study

investigated whether an exposure–response association

exists between physical demands in early working life and

risk of LBP in later working life.

Design

Cross-sectional study.

Setting, participants and outcome measure

In the

SeniorWorkingLife study, 5909 wage earners aged

≥50 years with currently sedentary work replied to

a questionnaire survey in 2018 about physical work

demands during their first working years (exposure) and

current LBP (outcome). Associations between physical

work demands in the early working years and current LBP

were modelled using general linear models controlling

for various confounders, combined with model-assisted

weights based on national registers.

Results

Hard physical work during early working life

was associated with more intense LBP later in life among

senior workers with currently sedentary jobs. In the fully

adjusted model, workers with ‘standing/walking work with

lifting/carrying’ and ‘heavy or fast work that is physically

strenuous’ during the first years of working life reported

higher LBP intensity than those with sedentary work

during their first working years (0.2 (95% CI, 0.0 to 0.4)

and 0.6 (95% CI, 0.4 to 0.9), respectively).

Conclusion

Work involving lifting/carrying or work that

is physically strenuous in early life is associated with

higher intensity of LBP among older workers with currently

sedentary employment. These findings suggest that early

working life may have an impact on later working years

and underscore the necessity for careful introduction

and instruction to the working environment for retaining

musculoskeletal health and prolonging working life.

Trial registration number

NCT03634410.

Strengths and limitations of this study

►

This is a cross-sectional study among currently

►

Received 06 May 2020

Revised 27 October 2020

Accepted 18 November 2020

►

sedentary workers aged ≥50 years, which does not

allow causal interpretations.

The workers retrospectively assessed the physical

work demands of their current work and their first

work in the early working years, entailing a risk of

recall bias.

However, the first job in working life usually leaves

an impression and it is therefore likely that most can

remember what they worked with.

The large sample size with all analyses performed

with model-assisted weights based on high-quality

national registers strengthens the study.

Using statistical weights reduces the effect of non-

response and ensures that the data are representa-

tive of Danish workers aged ≥50 years.

employer(s)) 2020. Re-use

permitted under CC BY-NC. No

commercial re-use. See rights

and permissions. Published by

BMJ.

For numbered affiliations see

end of article.

Correspondence to

Rúni Bláfoss; [email protected]

BACKGROUND

Many Western societies experience demo-

graphic changes in terms of growing older

populations, which has led to gradually

increasing retirement age to maintain a

sufficient number of active workers. In

Denmark, the retirement age is regulated

according to the current life expectancy

age minus 14.5 years.

Hence, in the future,

today’s 50-year-old workers are expected

to retire at the age of 69 years, whereas

30-year-old workers will work until the age

of 73 years.

To prolong their working lives

and work until the increasing state pension

age, the working population must preserve

good health, which may be challenging in

light of the well-known age-related physical

decline.

2–5

Physical capacity naturally declines

with ageing due to several physiological

mechanisms,

for example, sarcopenia leads

to 1%–2% of muscle mass reduction per year

after the age of 50 and a paralleled decline

in muscle function.

2–5

This decline in physical

capacity may particularly affect workers with

physically demanding jobs, for example, by

lowering work ability and increasing the risk

of future low back pain (LBP).

Bláfoss R,

et al. BMJ Open

2020;10:e040158. doi:10.1136/bmjopen-2020-040158

BEU, Alm.del - 2020-21 - Bilag 180: Orientering om publicering af NFA-artikel om, at fysisk krævende arbejde tidligt i arbejdslivet øger graden af lændesmerter senere i livet, fra beskæftigelsesministeren

Open access

In Europe, about 60% of the working force are experi-

encing musculoskeletal disorders, with back pain being

the most prevalent.

Specifically, LBP is the most preva-

lent and debilitating musculoskeletal disorder worldwide,

affecting most individuals at some point during their life-

time and with increased prevalence rates with ageing.

7–12

In particular, workers with physically demanding jobs

are at elevated risk of developing LBP.

13–17

Among the

general working population in Denmark, the prevalence

of musculoskeletal pain also increases with age.

Approx-

imately 24% of workers aged between 18 and 34 years are

experiencing pain several times per week, whereas this is

the case for approximately 40% of workers aged between

55 and 64 years. Thus, it is reasonable to expect that the

ongoing ageing of the workforce will lead to a larger

proportion of the workforce suffer from musculoskeletal

pain in the future, thereby constituting a larger societal

challenge that needs to be addressed.

Recent Scandinavian studies using technical measure-

ment methods to measure physical work demands during

work found that younger and older workers were exposed

to more or less the same physical demands at work.

19 20

However, in the 2018 round of the Danish Work Envi-

ronment and Health study, young workers in their initial

working life (18–24 years) reported their work to be more

physically demanding than older workers.

This can be

due to, for example, hierarchy, younger workers having

not adapted to the working life, lack of education and

unadjusted postures, or younger workers taking a bigger

share of the physically demanding work.

Another

possible reason may be that a proportion of workers

cannot cope with the physical demands of their occu-

pations and have therefore changed to a less physically

demanding occupation or left the labour market (healthy

worker effect). Nevertheless, musculoskeletal pain seems

to develop over time, because older workers have more

pain without having higher physical work demands. A

longitudinal study from Finland found that young workers

(18–24 years at baseline) with physically demanding work

had an increased risk of both radiating and local LBP in

midlife (21 years later),

indicating that being exposed to

hard physical work at a young age may have long-lasting

effects on LBP. Furthermore, findings from our research

group observed that hard physical work throughout

the working life gradually increases the risk of sickness

absence and early retirement later in life.

22–24

This high-

lights the importance of early preventive initiatives for

young workers to postpone the occurrence of musculo-

skeletal disorders and thereby prolong their productive

working years and good health. However, there is a lack

of knowledge about the role of high physical demands at

work during the early working years on musculoskeletal

health later in the working life.

Therefore, we aimed to investigate whether an expo-

sure–response association exists between physical work

demands in early life and risk of LBP later in life among

currently sedentary senior workers (≥50 years). We

hypothesised that hard physical work during the first

working years would associate in an exposure–response

manner with more intense LBP in later working life

among currently sedentary workers aged

≥50

years.

METHODS

Study design

This study is a cross-sectional study among Danish workers

conducted between July and October 2018. The study

employs data from the SeniorWorkingLife study that is

registered as a cohort study in ClinicalTrials.gov.

Statis-

tics Denmark drew a random sample of 30 000 Danes

aged

≥50

years (18 000 employed, 7000 unemployed,

3000 on voluntary early retirement, 2000 on disability

pension), who received an invitation to participate with a

personal link to the web-based questionnaire via e-Boks,

an online digital mailbox linked to their Danish social

security number.

The response rate of the employed

workers replying to the entire questionnaire was 56%.

This study only included currently employed wage

earners with sedentary work who replied to the questions

about physical demands (current and at their first work)

and current pain intensity in the low back (N=5909). The

questions are specified below. All participants did not fill

in all the survey questions, resulting in a varying number

of participants for each analysis. The reporting of the

study followed the ‘Strengthening of the Reporting of

Observational Studies in Epidemiology’ guidelines.

Patient and public involvement

Because this study is a large cross-sectional survey study,

participants were not involved in the design of this study

or the design of the research question and selection of

outcome measures.

Questionnaire

Physical activity at work (exposure)

Participants replied to questions about the physical

demands of their current work and their first work in

their early working career. The questions on their first

and current work were ‘How will you describe your phys-

ical activity in your profession during your first working

years?’ and ‘How will you describe your physical activity

in your current profession?’, respectively. The response

options to both questions were ‘(1) mostly sedentary

work that does not require physical exertion, (2) mostly

standing or walking work that otherwise is not physi-

cally strenuous, (3) standing or walking work with some

lifting or carrying and (4) heavy or fast work that is phys-

ically strenuous’.

28 29

Our research group has previously

demonstrated strong agreement between the grouping of

occupational physical activity from the questionnaire and

grouping based on the Danish International Standard

Classification of Occupations (DISCO).

In DISCO, the

workers are classified into occupational groups based on

high-quality national registers.

LBP (outcome)

Pain intensity in the low back was assessed by the following

question: ‘Rate your level of pain during the past 3 months

Bláfoss R,

et al. BMJ Open

2020;10:e040158. doi:10.1136/bmjopen-2020-040158

BMJ Open: first published as 10.1136/bmjopen-2020-040158 on 7 December 2020. Downloaded from

http://bmjopen.bmj.com/

on December 18, 2020 at National Institute of Occupational

Health - DNLA. Protected by copyright.

Open access

in each of the following body regions, where 0 is no pain

and 10 is the worst imaginable pain: low back (lower

part of the back)’. The 11-point Numerical Rating Scale

(NRS) was used with the following response options: ‘0

(no pain), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 (worst imaginable

pain)’. The NRS has previously demonstrated excellent

test–retest reliability.

Control variables

The analyses were controlled for possible confounders

that previously have been associated with LBP.

9 12 14 31–34

The confounders included age (categorical), gender

(categorical), education (categorical), physical exercise

during leisure (categorical), smoking (categorical), diag-

nosed depression (categorical), psychosocial work factors

(single items on influence and recognition at work

from the Copenhagen Psychosocial Questionnaire

)

(continuous, 0–100 (0 worst, 100 best)) and body mass

index (BMI) (continuous). Diagnosed depression was

determined by the question ‘Have you within the past

year been treated or medicated for one or more of the

following diseases?’, with the response option ‘Depres-

sion’ confirmed. Data on confounders were collected

from registers (age, gender and education) and from

the SeniorWorkingLife questionnaire (physical exercise

during leisure, smoking, diagnosed depression, psychoso-

cial work factors and BMI) (questions specified below).

The question about leisure-time physical activity was:

‘Within the past year, what describes best your physical

activity during leisure?’, with the following response

options: ‘(1) reading, watching television or other seden-

tary activity; (2) walking, cycling or other low-intensity

activity at least 4 hours per week (eg, Sunday walk, light

gardening and cycling/walking to work); (3) exercise

sports, heavy gardening or similar at least 4 hours per

week; and (4) vigorous exercise and competitive sports

several times per week’.

Smoking was evaluated by asking ‘Do you smoke? (apart

from electronic cigarettes)’, with the following response

options: ‘(1) yes, every day, (2) yes, but not every day, (3)

no, I have quit and (4) no, I have never smoked’.

Psychosocial work factors were evaluated by questions

about influence at work and recognition from colleagues.

Two questions were asked to evaluate influence at work:

‘How often … (1) do you have any influence on how you

carry out your work tasks? and (2) Do you have influence

on the order in which you carry out your work tasks?’,

with the following response options to both questions:

‘(1) always, (2) often, (3) sometimes, (4) rarely and (5)

never’. The question about recognition from colleagues

was the following: ‘How often … do you and your

colleagues recognise each other’s work?’, with the same

response options as to influence at work.

Data on BMI were collected by asking about the partici-

pants’ height and weight, where they responded in centi-

metres and kilograms, respectively. Subsequently, BMI

was calculated for the data analyses (BMI=kg/m

Bláfoss R,

et al. BMJ Open

2020;10:e040158. doi:10.1136/bmjopen-2020-040158

BMJ Open: first published as 10.1136/bmjopen-2020-040158 on 7 December 2020. Downloaded from

http://bmjopen.bmj.com/

on December 18, 2020 at National Institute of Occupational

Health - DNLA. Protected by copyright.

Statistical analyses

The analyses were performed using the SAS statistical

software for Windows (SAS V.9.4; SAS Institute). Asso-

ciations were modelled using the general linear model

(Proc GLM, SAS V.9.4) controlling for confounders

and weighted based on information from high-quality

national registers at Statistics Denmark, which included

gender, age, occupation, highest completed education,

family income, family type and origin.

The residuals of

the outcome variable (LBP) were normally distributed

through visual inspection. Because a strong correlation

exists between a 0–10 NRS and a 0–10 Visual Analogue

Scale,

the 0–10 (11 point) ordinal NRS in this study is

treated as a continuous scale. Estimates are reported as

least square means pain intensity (NRS) and 95% CIs

and between-group least square means differences and

95% CI. An alpha level of <0.05 was chosen as statistically

significant differences.

RESULTS

The proportion of men and women included in this study

was 54% and 46%, respectively, with a mean age of 56.6

years (table

1).

The majority of workers had sedentary

work during their first years at the labour market (62%);

14%, standing or walking work; 16%, standing or walking

work with lifting/carrying; and 8%, physically strenuous

work.

We found an association between physically demanding

occupations during the first working years and higher

intensity of LBP among currently sedentary workers

(table

2).

Specifically, in model 1, workers having

‘standing/walking work with lifting/carrying and ‘heavy

or fast work that is physically strenuous’ during their first

working years had higher pain intensity in the low back,

with differences in pain intensity of 0.3 (95% CI, 0.1 to

0.5) and 0.8 (95% CI, 0.5 to 1.1), respectively. In the fully

adjusted model 2, workers having ‘standing/walking work

with lifting/carrying’ and ‘heavy or fast work that is phys-

ically strenuous’ had higher pain in the low back, with

differences in pain intensity of 0.2 (95% CI, 0.0 to 0.4)

and 0.6 (95% CI, 0.4 to 0.9), respectively. Using physical

exposure as a continuous variable, the exposure–response

association was highly significant (trend test p<0.0001).

DISCUSSION

The main finding of this study is that an exposure–response

association existed between physically demanding work

in early life and LBP intensity in later working life among

currently employed sedentary workers aged

≥50

years.

Interpretation of findings

The findings of this study suggest that the level of phys-

ical activity in the early working life influences musculo-

skeletal health later in life. Compared with hard physical

work in early life, less physically demanding work showed

less intense LBP in later life in an exposure–response

Open access

Table 1

Demographics, lifestyle and physical activity at the

first work

Age, years

Gender

Men

Women

BMI

Smoking

Yes, every day

Yes, but not every day

Ex-smoker

Never smoked

Physical activity at the first work

Sedentary

Standing or walking

Standing or walking with lifting/

bearing

Heavy or fast work that is

physically demanding

Physical activity during leisure

Sedentary

Light exercise at least 4 hours

per week

Sports or heavy physical

activity at least 4 hours per

week

Vigorous exercise or

competitive sports several

times per week

Low back pain (0–10)

Psychosocial work factors (0–100)

Influence at work

Recognition from colleagues

BMI, body mass index.

BMJ Open: first published as 10.1136/bmjopen-2020-040158 on 7 December 2020. Downloaded from

http://bmjopen.bmj.com/

on December 18, 2020 at National Institute of Occupational

Health - DNLA. Protected by copyright.

Mean (SD) %

5909 56.6 (5.5)

5909

3194

2715

5721 26.3 (5.0)

5789

619

161

1987

3022

5907

3854

782

854

417

5779

755

3450

1442

132

5743

2.4 (2.9)

5875 76.6 (21.7)

5879 79.8 (21.5)

fashion. Specifically, workers with ‘standing/walking work

comprising lifting/carrying’ and ‘heavy or fast work that is

physically strenuous’ during early working years reported

on average 0.2 and 0.6 points higher on the 0–10 NRS in

the fully adjusted model, respectively. A mean difference

of 0.6 points (from 2.6 to 3.2) in a large study popula-

tion should not be neglected. The present difference of

25% is regarded as clinically significant.

37 38

Importantly,

such higher pain intensity provides knowledge that the

first working years may play an important role in muscu-

loskeletal health in later working years. Notably, this study

only included sedentary workers, whereas the inclusion

of workers with physically demanding work, who are at

a higher risk of developing musculoskeletal pain,

13–15 17

could have resulted in even higher pain intensities. In

fact, a recent systematic review found increased sitting

time at work to associate with less intense LBP among

physical workers.

Besides, hard physical work during

early life is bringing workers closer to a musculoskeletal

pain threshold that is associated with an increased risk

of sickness absence due to LBP. Andersen and coworkers

found female eldercare workers to be at elevated risk of

sickness absence due to LBP when their pain intensity was

5.0 on the 0–9 NRS.

In this study, the average NRS score

for those with heavy and fast work in early working years

was 3.2 in the fully adjusted model, but the CI ranged

from 2.3 to 4.2. Thus, the included sedentary workers with

hard physical work in early life, especially heavy and fast

work, experienced pain in the low back that may impair

work ability and eventually challenge work participation.

These data can be valuable knowledge for work environ-

ment professionals and authorities when introducing

young workers. As LBP is the most prevalent musculoskel-

etal disorder and a major cost for societies around the

world,

8 9 11 12 21

the present results should instigate promo-

tion of early preventive measures at the workplace and

assist work environment professionals and authorities in

their decision-making.

Previous studies have reported associations between

physical work exposures during early working years and

increased risk of LBP, sickness absence and early retire-

ment.

16 23 24

Sundstrup and coworkers assessed the physical

exposures during the entire working life retrospectively

and found hard physical work during working life to asso-

ciate with increased risk of long-term sickness absence,

disability pension and early retirement.

Furthermore, a

prospective cohort study reported physically demanding

work during early working life to associate with increased

risk of LBP in midlife.

In sum, this study elaborates on

these previous findings, showing that early exposure to

hard physical work increases the risk of poor musculoskel-

etal health later in life.

16 22–24

A reason for this might be

that young workers in physically demanding occupations,

who may take a bigger share of the heavy work and possess

unadjusted work postures,

may initiate an early build-up

of microdamage or fatigue in the tissue that may increase

the risk of LBP even further later in life. This is valuable

information for policy-makers, work environment profes-

sionals and authorities. As young workers in Denmark rate

their work to be more physically demanding than older

workers,

there is potential for more qualified introduc-

tion and instruction of young workers entering the labour

market to reduce the physical demands, especially in phys-

ically demanding occupations. Specifically, more focus

could be targeted on instructing young workers entering

the labour market in working postures, ergonomics, the

use of assistive devices and so on. If the young workers

learn good practice within their physical working envi-

ronment, presumably they will carry it into their working

life. This can improve the general health among workers

and thereby lower the risk of musculoskeletal disorders

in later life, sickness absence and early retirement. This

should be a high priority for societies taken into account

the growing proportion of older workers and the gradually

Bláfoss R,

et al. BMJ Open

2020;10:e040158. doi:10.1136/bmjopen-2020-040158

Open access

Table 2

Associations between physical work demands in the early working years and low back pain intensity among currently

sedentary workers

Work types in early working years

Model 1: controlled for age and gender

Sedentary work (reference)

Standing/walking work that otherwise is not physically strenuous

Standing/walking work with lifting/carrying

Heavy or fast work that is physically strenuous

N (Total=5907) %

3854

782

854

417

NRS (95% CI)

2.3 (2.2 to 2.4)

2.3 (2.1 to 2.5)

2.6 (2.4 to 2.8)

3.1 (2.9 to 3.4)

0.0 (0.2 to −0.2)

0.3 (0.1 to 0.5)

0.8 (0.5 to 1.1)

Difference

(95% CI)

BMJ Open: first published as 10.1136/bmjopen-2020-040158 on 7 December 2020. Downloaded from

http://bmjopen.bmj.com/

on December 18, 2020 at National Institute of Occupational

Health - DNLA. Protected by copyright.

Model 2: model 1+education, physical exercise, smoking, BMI, psychosocial factors (influence and recognition) and diagnosed

depression

Sedentary work (reference)

Standing/walking work that otherwise is not physically strenuous

Standing/walking work with lifting/carrying

Heavy or fast work that is physically strenuous

Statistically significant differences (p<0.05) from reference are marked in bold.

NRS, LBP intensity rated on Numerical Rating Scale (0–10).

3854

782

854

417

2.6 (1.8 to 3.5)

2.7 (1.8 to 3.6)

2.8 (1.9 to 3.7)

3.2 (2.3 to 4.2)

0.0 (-0.2 to 0.1)

0.2 (0.0 to 0.4)

0.6 (0.4 to 0.9)

increasing retirement age. Although recent studies from

Norway and Denmark using more objective measurement

methods did not find younger workers to be exposed to

higher physical demands at work,

19 20

the physical work

environment should still be of high priority because pain

becomes more prevalent with ageing.

9 12 18

Furthermore,

Oakman and coworkers found some older workers to

stand for a longer time during the working day compared

with younger workers.

However, more standing time

does not necessarily equal a highly physically demanding

work. Moreover, Merkus and coworkers defined young

workers to be aged <45 years,

which makes the compar-

ison with the present study less applicable. In the Danish

Work Environment and Health study, workers aged

18–24 years workers in their initial working life perceived

their work to be more physically demanding than older

workers.

In sum, these studies indicate that younger

workers may not have higher physical work demands

than older workers, but younger workers may experience

the work to be more physically demanding. These data

suggest that young workers in their very initial working

life need careful introduction and instruction to adapt to

the working life and/or that younger workers may take a

bigger share of the heavy work.

Also, because physical

capacity and function decline with ageing,

2–6 41

it is of vital

importance for workers to diminish risk factors for poor

health and to preserve good health throughout an entire

working career. Knowledge about risk factors during early

working life is therefore imperative to provide optimal

work environment throughout a prolonged working life.

Limitations and strengths

A limitation of this study is that workers had to recall the

physical demands in their first working years, which always

bears a risk of recall bias. Nevertheless, as the first job in

working life usually leaves an impression, it is likely that

most can remember what they worked with. Another risk

Bláfoss R,

et al. BMJ Open

2020;10:e040158. doi:10.1136/bmjopen-2020-040158

of recall bias is that participants might refer to different

individual time intervals when reading the question about

their ‘first working years’. Moreover, there are no data

on what work the participants have held between their

first job and their current job, that is, we do not know

whether participants have changed occupation after the

first working years or recently. The inclusion of sedentary

workers may also imply the possibility of a selection bias

because some workers may have changed from a physi-

cally demanding occupation to a sedentary occupation

as a consequence of their health. In addition, the results

may be biased due to the healthy worker effect where

some workers unable to cope with their work may have

left the labour market before the age of 50 years. Thus,

the estimates may be rather conservative because workers

with the most intense pain may have left the labour

market or changed occupation to a more sedentary job

before the age of 50 years, that is, the associations could

have been even stronger. The narrow CIs and the clear

findings that hard physical work demands in early life are

associated with higher intensity of LBP in later life in both

models indicate trustworthy findings. A further limitation

is the cross-sectional design, which does not allow causal

interpretations. Furthermore, a bidirectional relation-

ship between pain and psychological factors exists. Thus,

simply adjusting for such factors as confounders may not

mean that the observed associations are causal. Although

the present associations—for obvious reasons—cannot

be tested in a randomised controlled trial, longitudinal

studies including young healthy people entering the

labour market, followed until retirement age, are neces-

sary to truly test the hypothesis of this study. Finally, self-

reporting can lead to common method variance where,

for example, the participant’s general health, mood and

socioeconomic status can affect the answers.

At last, a

limitation of the study is that analyses were not adjusted

Open access

for other comorbidities than depression. Poor health

has previously been associated with work limitations,

and not accounting for this could have led to residual

confounding. On the contrary, the analyses were adjusted

for both lifestyle factors (ie, smoking and BMI) and educa-

tional attainment, which also associate with poor health.

Thus, adjusting for multimorbidity could potentially

have led to an overadjustment. A strength of the study in

terms of generalisability is that Statistics Denmark drew

a probability sample among all eligible Danish residents

aged

≥50

years, where all analyses were performed with

statistical weights based on high-quality national regis-

ters. Performing all analyses with these statistical weights

reduces the effect of non-response and ensures that the

data are representative of Danish workers aged

≥50

years.

Competing interests

None declared.

Patient and public involvement

Patients and/or the public were not involved in

the design, or conduct, or reporting, or dissemination plans of this research.

Patient consent for publication

Not required.

Provenance and peer review

Not commissioned; externally peer reviewed.

Data availability statement

Data are available upon reasonable request. The

authors encourage collaboration and use of the data by other researchers. Data are

stored on the server of Statistics Denmark, and researchers interested in using the

data for scientific purposes should contact the project leader Prof. Lars L. Andersen,

[email protected], who is responsible for the study design, questionnaire development,

definition of population and data collection.

Open access

This is an open access article distributed in accordance with the

Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which

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ORCID iDs

Rúni Bláfoss

http://orcid.org/0000-0003-1843-6884

Rubén López‐Bueno

http://orcid.org/0000-0002-7865-3429

BMJ Open: first published as 10.1136/bmjopen-2020-040158 on 7 December 2020. Downloaded from

http://bmjopen.bmj.com/

on December 18, 2020 at National Institute of Occupational

Health - DNLA. Protected by copyright.

CONCLUSIONS

Workers with high physical work demands during early

life experience higher intensity of LBP later in life in an

exposure–response manner. This finding was observed

among currently employed sedentary workers aged

≥50

years, which indicates that early work exposure may

matter for musculoskeletal health in later working years.

This underscores the importance of careful introduction

and instruction to the labour market for retaining muscu-

loskeletal health and prolonging working life. Future

longitudinal studies should investigate this association

with continuous follow-up rounds.

Ethical approval

The Danish law does not require approval by ethical and

scientific committees, nor informed consents, in ques-

tionnaire and register-based studies. Statistics Denmark

deidentified all data, and data are stored on the server

of Statistics Denmark where the analyses are performed

through remote access by the project leader.

Author affiliations

Musculoskeletal Disorders and Physical Workload, National Research Centre for

the Working Environment, Copenhagen, Denmark

Department of Sports Science and Clinical Biomechanics, Research Unit for Muscle

Physiology and Biomechanics, University of Southern Denmark, Odense, Denmark

Department of Physical Medicine and Nursing, University of Zaragoza, Zaragoza,

Spain

Exercise Intervention for Health Research Group (EXINH-RG), Department of

Physiotherapy, University of Valencia, Valencia, Spain

Sport Sciences, Department of Health Science and Technology, Aalborg University,

Aalborg, Denmark

Acknowledgements

The authors are grateful for important discussions and

input during the development of the study to many people from Aalborg University,

Team Arbejdsliv, National Research Centre for the Working Environment, Statistics

Denmark and the members of the Advisory Board. The study participants replied to

the questionnaire and were not involved in the development of the study.

Contributors

LLA received the funding for this study. LLA and RB designed the

present study aim and LLA performed the analyses. RB wrote the draft of the

manuscript before SS, RL-B, JC, ES and LLA critically read and reviewed the

manuscript. Thus, all authors have read and approved the manuscript.

Funding

This study was supported by a grant from the Danish Foundation

TrygFonden. Award/Grant number is not applicable.

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2020;10:e040158. doi:10.1136/bmjopen-2020-040158