Matthew Smuck, MD

Abstract
Background Context
Evidence 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.

Purpose
This study was designed to examine the inter-relationship 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/Setting
A cross-sectional U.S. population-based study.

Patient Sample
A cohort of 6,796 adults from the 2003-2004 National Health and Nutrition Examination Survey (NHANES).

Outcome Measures
Demographic information, an in-depth health questionnaire, physical examination details and 7-day free-living physical activity monitoring using accelerometry (ActiGraph AM-7164).

Methods
LBP status was determined by questionnaire response. BMI was calculated during physical examination and divided here into 4 groups (normal weight <25, overweight 25-30, obese 31-35, and ultra-obese 36+). Summary measures of physical activity were computed based on intensity cut-offs, 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 (AIC). All displayed estimates are significant at level

Results
In 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 ultra-obese (36+). Smoking is consistently the strongest predictor of LBP across the BMI spectrum (OR 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 (OR 0.98 and 0.996 per SD 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 ultra-obese groups.

Conclusions
Increased 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.

Journal Abstract:  http://www.sciencedirect.com/science/article/pii/S1529943013016495