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

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 less than or equal to30 days during the post 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 (less than or equal to30 days during the post 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.