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Effects of measurement strategy and statistical analysis on dose-response relations between physical workload and low back pain

Background: In epidemiological studies on physical workloads and back complaints, among the important features in modelling dose-response relations are the measurement strategy of the exposure and the nature of the dose-response relation that is assumed. Aim: To evaluate the effect of these two features on the strength of the dose-response relation between physical load and severe low back pain. Methods: The study population consisted of 769 workers in nursing homes and homes for the elderly. Observations at the workplace were made of 212 subjects. These observations were analysed to determine exposure to physical load according to two measurement strategies: the individual approach and the group approach. The nature of the dose-response relation was evaluated with nested logistic regression models. Results: The group approach resulted in higher odds ratios for the associations between physical load and low back pain than the individual approach. Spline logistic regression models appeared to describe the dose-response relation between physical load and low back pain best. The corresponding curve showed small changes in risk for small changes in exposure, whereas the categorical model only showed sudden large changes in risk at predefined exposure values. Conclusion: The choice for a particular measurement strategy of physical load influences the strength of the associations between physical load and severe low back pain. Spline models allow changes in risk over the whole exposure range and are therefore a promising approach to identify quantitative dose-response patterns between physical load and low back pain.