Relationships between physical activity and chronic pain: The role of endogenous pain sensitivity - A population-based perspective: The Tromsø Study

Background and aims: Chronic pain is a major global health concern, yet causal mechanisms are not well understood. Physical activity is popular as prevention and treatment, possibly acting through a positive effect on endogenous pain modulation. This has not been examined in the general population. Materials and methods: We used data from the sixth and seventh surveys of the populationbased Tromsø Study (Tromsø6 and Tromsø7, respectively) to perform cross-sectional and longitudinal analyses of the association between physical activity and pain tolerance. We also performed counter-factual mediation analyses to assess the direct effect of physical activity on chronic pain types, and any potential indirect effects on chronic pain mediated through the effect of physical activity on pain tolerance. Results and conclusions: This thesis found evidence supporting higher habitual PA in leisure time, and higher exercise intensity and duration, to be associated with higher coldpain tolerance. This association appeared to be dose-response shaped in leisure-time PA. The same was not seen in accelerometer-assessed PA. Leisure-time PA relationships appeared to be stable when measured in the same individuals at multiple time-points, and more PA over time was related to higher pain tolerance compared to being less active. There were indications that direction of PA change matters. However, PA did not appear to counteract an overall drop in pain tolerance over time. Effect estimates appeared in general to be slightly larger for men than women. Higher PA levels were associated with lower risk of moderateto- severe chronic pain types. For such chronic pain types, a small part of this effect was mediated through an effect on pain tolerance, suggesting pain tolerance might have a mechanistic role in the effect of PA on chronic pain. The clinical significance of this indirect effect is unclear.