• Matthew Smuck, Ming-Chih J Kao, Nikhraj Brar, Agnes Martinez-Ith, Jongwoo Choi, and Christy C Tomkins-Lane.
• Department of Orthopaedic Surgery, Stanford Medicine Outpatient Center, 450 Broadway St, Pavillion C, MC6342, Redwood City, CA 94063, USA. Electronic address: msmuck@stanford.edu.
• Spine J. 2014 Feb 1;14(2):209-16.

Background ContextEvidence supporting an association between obesity and low back pain (LBP) continues to grow; yet little is known about the cause and effect of this relationship. Even less is known about the mechanisms linking the two. Physical activity is a logical suspect, but no study has demonstrated its role.PurposeThis study was designed to examine the interrelationship between physical activity, obesity, and LBP. The specific aims were to determine if obesity is a risk factor for LBP in the U.S. population, measure the strength of any observed association, and evaluate the role of physical activity in modulating this association.Study Design/SettingA cross-sectional U.S. population-based study.Patient SampleA cohort of 6,796 adults from the 2003-2004 National Health and Nutrition Examination Survey.Outcome MeasuresDemographic information, an in-depth health questionnaire, physical examination details, and 7-day free-living physical activity monitoring using accelerometry (ActiGraph AM-7164; ActiGraph, Pensacola, FL, USA).MethodsLBP status was determined by questionnaire response. Body mass index (BMI) was calculated during physical examination and divided here into four groups (normal weight <25, overweight 25-30, obese 31-35, and ultraobese 36+). Summary measures of physical activity were computed based on intensity cutoffs, percentile intensities, and bout. Demographics, social history, and comorbid health conditions were used to build adjusted weighted logistic regression models constructed using Akaike Information Criterion. All displayed estimates are significant at level <.05. No external funding was received to support this study. None of the authors report conflicts of interest directly related to the specific subject matter of this manuscript.ResultsIn the U.S. population, the risk of low LBP increases in step with BMI from 2.9% for normal BMI (20-25) to 5.2% for overweight (26-30), 7.7% for obese (31-35), and 11.6% for ultraobese (36+). Smoking is consistently the strongest predictor of LBP across the BMI spectrum (odds ratio 1.6-2.9). Physical activity also modulates these risks. In the overall model, the best physical activity predictors of LBP are in the moderate and high intensity ranges with small effects (odds ratio 0.98 and 0.996 per standard deviation increase, respectively). When broken down by BMI, time spent in sedentary and moderate activity ranges demonstrate more robust influences on LBP status in the overweight, obese, and ultraobese groups.ConclusionsIncreased BMI is a risk factor for back pain in Americans. More important, the role of physical activity in mitigating back pain risk is shown to be of greater consequence in the overweight and obese populations.Copyright © 2014 Elsevier Inc. All rights reserved.

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