Physical Exercise, Body Mass Index, and Risk of Chronic Pain in the Low Back and Neck/Shoulders: Longitudinal Data From the Nord-Trøndelag Health Study

American Journal of Epidemiology ª The Author 2011. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: . Vol. 174, No. 3 DOI: 10.1093/aje/kwr087 Advance Access publication: June 1, 2011 Original Contribution Physical Exercise, Body Mass Index, and Risk of Chronic Pain in the Low Back and Neck/Shoulders: Longitudinal Data From the Nord-Trøndelag Health Study Tom Ivar Lund Nilsen, Andreas Holtermann, and Paul J. Mork* * Correspondence to Dr. Paul J. Mork, Department of Human Movement Science, Norwegian University of Science and Technology, N-7491 Trondheim, Norway (e-mail: ). Chronic musculoskeletal pain constitutes a large socioeconomic challenge, and preventive measures with documented effects are warranted. The authors’ aim in this study was to prospectively investigate the association between physical exercise, body mass index (BMI), and risk of chronic pain in the low back and neck/shoulders. The study comprised data on approximately 30,000 women and men in the Nord-Trøndelag Health Study (Norway) who reported no pain or physical impairment at baseline in 1984–1986. Occurrence of chronic musculoskeletal pain was assessed at follow-up in 1995–1997. A generalized linear model was used to calculate adjusted risk ratios. For both females and males, hours of physical exercise per week were linearly and inversely associated with risk of chronic pain in the low back (women: P-trend ¼ 0.02; men: P-trend < 0.001) and neck/shoulders (women: P-trend ¼ 0.002; men: P-trend < 0.001). Obese women and men had an approximately 20% increased risk of chronic pain in both the low back and the neck/shoulders. Exercising for 1 or more hours per week compensated, to some extent, for the adverse effect of high BMI on risk of chronic pain. The authors conclude that physical inactivity and high BMI are associated with an increased risk of chronic pain in the low back and neck/shoulders in the general adult population. body mass index; exercise; low back pain; neck pain; prospective studies; shoulder pain Abbreviations: BMI, body mass index; CI, confidence interval; HUNT, Nord-Trøndelag Health Study. Chronic musculoskeletal pain in the low back and neck/ shoulders is a common cause of reduced quality of life, sick leave, and disability in Western industrialized countries, and the problem is expected to grow with the aging population (1–4). The negative consequences, for both the individual and society, highlight the importance of identifying primary preventive measures that are easily accessible for the general population. Promotion of regular physical exercise and prevention of obesity are initiatives assumed to reduce the incidence of musculoskeletal pain (2). For example, obesity has been associated with increased prevalence of low back pain in several cross-sectional studies (5). A few prospective cohort studies have shown that exercise may reduce the risk of musculoskeletal pain (6–8), while other studies have found moderate or no associations (9). This inconsistency may be due to methodological limitations, such as small study samples and inclusion of persons with musculoskeletal pain at baseline. Few studies have investigated the combined effect of exercise and excess body mass on future risk of chronic musculoskeletal pain. A recent study showed that overweight and obesity increased the risk of widespread chronic musculoskeletal pain (i.e., fibromyalgia) during an 11-year follow-up period, whereas physical exercise could compensate for this adverse effect to some extent (10). Whether physical exercise and excess body mass have a similar effect on risk of localized chronic pain in the low back or neck/ shoulders is unknown. Our primary aim in the current study was to investigate the association between physical exercise, body mass index (BMI), and risk of chronic musculoskeletal pain in the low back and neck/shoulders in a large unselected population of women and men without musculoskeletal pain or any 267 Am J Epidemiol. 2011;174(3):267–273 Initially submitted December 21, 2010; accepted for publication February 28, 2011. 268 Nilsen et al. physical impairment at baseline. We hypothesized 1) that an inverse relation exists between physical exercise and risk of chronic pain in the low back and neck/shoulders and 2) that physical exercise can compensate for the adverse effect of excess body mass on risk of chronic pain in the low back and neck/shoulders. MATERIALS AND METHODS Study population Study variables Physical exercise. At baseline (HUNT 1), the participants were asked to complete a questionnaire that included questions on frequency, duration, and intensity of leisure- Statistical analyses A generalized linear model for the binomial family (log link) was used to estimate risk ratios for chronic musculoskeletal pain in the low back and neck/shoulders. Participants who reported different levels of physical exercise at baseline were compared with the reference group of physically inactive participants. The risk ratio for chronic musculoskeletal pain between categories of BMI was estimated in similar models. The precision of the estimated risk ratios was assessed using 95% confidence intervals, and tests for trends across categories of physical exercise and BMI were conducted by treating the categories as an ordinal variable in the regression model. All analyses were stratified by gender. Am J Epidemiol. 2011;174(3):267–273 In Nord-Trøndelag County, Norway, all inhabitants aged 20 years or older were invited to participate in 2 waves of a large health survey (the Nord-Trøndelag Health Study (HUNT)), the first in 1984–1986 (HUNT 1) and the second in 1995–1997 (HUNT 2). Among 87,285 eligible persons, 77,216 (89%) accepted the invitation to participate in HUNT 1, filled in a questionnaire, and underwent a clinical examination. At the examination, body mass and height were measured, and the participants were given a second questionnaire to complete at home and return in a prestamped envelope. During HUNT 2 in 1995–1997, 94,187 persons were invited to participate, and 66,215 (70%) accepted the invitation. The procedures were similar to those described for HUNT 1, although both the questionnaires and the clinical examination were more comprehensive. More detailed information about selection procedures, participation, and questionnaires used in the HUNT Study can be found at http://www.hunt.ntnu.no. For the purpose of the present study, we selected all 24,357 women and 21,568 men who had participated in both surveys. We excluded 4,085 women and 3,446 men without baseline information on hours of physical exercise per week, 167 women and 113 men without data on musculoskeletal pain, and 13 women without information on weight. Moreover, we excluded 1,527 women and 1,528 men who reported being physically impaired at baseline because of a movement disorder or who had no inf (...truncated)