The impact of physical load on the course of low back pain

This thesis is divided in three parts. Part 1 focuses on the methodology used to characterize physical work load as risk factor for LBP. In Part 2 dose-response relations of occupational exposure to physical and psychosocial factors and LBP are evaluated. In Part 3 the information on dose-response relations is used to simulate the dynamic course of LBP over time by continuing exposure to physical load. In Chapter 2 of Part 1 we review the scientific literature on work-related back disorders in order to evaluate the strength of the associations between physical load and LBP among different studies, and, secondly, to analyze whether the strength of the associations can partly be explained by the study design and methods used to characterize physical work load. In Chapter 3 we propose a novel approach to evaluate simultaneously level, frequency, and duration of lumbar posture during work in order to determine the essential characteristics in postural load. In Chapter 4, the last Chapter of Part 1, two important features in modeling dose-response relations between physical work load and LBP are evaluated: the measurement strategy of the exposure and the nature of the dose-response relation that is assumed. In Chapter 5 of Part 2 dose-response relations between occupational exposures to physical and psychosocial factors at baseline and the risk of incident LBP at follow-up are investigated. In Chapter 6 the effect of occupational exposure to physical and psychosocial factors on the aggravation of the course of low back pain is assessed. Here, three measures of aggravation of LBP are used: increase in duration, increase in frequency, and increase in pain intensity. In Chapter 7 of Part 3 the transient nature of LBP and its consequences for the long-term course of LBP and associated morbidity are investigated. LBP is characterized by a dynamic pattern of episodes and recovery but little is known about the long-term course of back pain and its consequences due to lack of cohort studies with sufficiently long follow-up periods. Thus, a Markov model was developed to extrapolate the results of the 2 year observational study to a hypothetical cohort study with life-long follow-up.

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