ORIGINAL ARTICLE

Ambiguous relation between physical workload and low back pain: a twin control study

J Hartvigsen¹, K O Kyvik², C Leboeuf-Yde³, S Lings⁴, L Bakketeig²

¹ Nordic Institute of Chiropractic and Clinical Biomechanics, Klosterbakken 20, DK-5000 Odense C, Denmark
² University of Southern Denmark, Institute of Public Health, The Danish Twin Register, Sdr. Boulevard 23A, DK-5000 Odense C, Denmark
³ The Medical Research Unit in Ringkjøbing County, Torvet 1, DK-6950 Ringkøbing, Denmark
⁴ Odense University Hospital, Dept of Occupational and Environmental Medicine, Sdr. Boulevard 29, DK-5000 Odense C, Denmark

Correspondence to:
Correspondence to:
Dr J Hartvigsen, Nordic Institute of Chiropractic and Clinical Biomechanics, Klosterbakken 20, DK-5000 Odense C, Denmark;
j.hartvigsen{at}nikkb.dk

Aims: To examine the association between self reported physical workload and low back pain (LBP) in younger twins. To investigate whether genetic factors interact with physical workload in relation to LBP.

Methods: A twin control study was performed within a population based twin register using 1910 complete monozygotic (MZ) and same sexed dizygotic (DZ) twin pairs aged 25–42 and discordant for LBP. LBP in the affected twins was divided into two groups: "LBP for 30 days during the past year", and "LBP for >30 days during the past year". Physical workload was divided into four categories: "sitting", "sitting/walking", "light physical", and "heavy physical". Data were analysed in a matched design using conditional logistic regression. MZ and DZ twins were analysed separately and together in order to determine possible genetic influences in relation to physical workload and LBP.

Results: Statistically significant graded relations were found for increasing workload and LBP of longer duration but not for LBP of shorter duration (30 days during the past year). In both LBP groups the "sitting" and "sitting/walking" groups were not statistically different. MZ and DZ twins did not differ significantly with respect to LBP in the various workload groups.

Conclusions: There is evidence for a dose-response relation between physical workload and LBP of longer duration. Attention to clinically relevant subgroups based on duration, for example, is necessary in epidemiological studies dealing with LBP. Physical workload might be more important than genetic factors in LBP.

Keywords: risk factor; twin study; workload; subgroup

Abbreviations: DZ, dizygotic; LBP, low back pain; MZ, monozygotic

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