Dickerson et al. BMC Public Health 2011, 11:926 http://www.biomedcentral.com/1471-2458/11/926 R E S E A R C H A R T I C L E Open Access The association of physical activity, sedentary behaviors, and body mass index classification in a cross-sectional analysis: are the effects homogenous? Justin B Dickerson1*, Matthew Lee Smith2, Mark E Benden3 and Marcia G Ory4 Abstract Background: While much is known about the benefits of physical activity (PA) and the consequence of sedentary behaviors relative to body mass index (BMI), little is known about the homogeneity of these effects across individuals. The goal of this study was to determine if PA and sedentary behaviors have the same effect on individuals of all BMI classifications. Methods: Data from a community health assessment were analyzed and a sample was selected to include respondents who self-reported a chronic disease associated with obesity (n = 2,840). Descriptive statistics were used to describe the association between selected independent variables and BMI. Simultaneous quantile regression was used to identify the degree of homogeneity in the effect of demographic independent variables, minutes per week of moderate PA, and hours per day spent watching television on BMI classification. In studies using simultaneous quantile regression, the word “effect” is used to describe association, not causation. Results: Minutes per week of moderate PA had a significant effect on lower BMI, but only when respondents were at least classified as obese-class I (b = -0.001, p = 0.006). The change in effect of moderate PA in lower BMI increased significantly when respondents were classified as obese-class II versus obese-class I (F = 4.54, p = 0.033). Hours per day spent watching television had a significant effect on higher BMI, but only when the respondent was at least classified as overweight (b = 0.87, p < 0.001). The change in effect of watching television on higher BMI increased significantly when respondents were classified as obese-class I versus overweight (F = 5.57, p = 0.018). Conclusion: PA and watching television were more related to BMI for obese individuals than those who were just overweight. Customized interventions for specific BMI classifications should be developed to maximize public health benefits. Background The last decade witnessed a noteworthy increase in body mass index (BMI) with average age-adjusted BMI for American adults now reported as 28.5 kg/m2 [1]. This is alarming considering the World Health Organization (WHO) recognizes a BMI of 30.0 kg/m2 as obese [2]. The effects of obesity on mortality are substantial. Not only is obesity a risk factor for cardiovascular disease * Correspondence: dickerson@srph.tamhsc.edu 1Department of Health Policy & Management, School of Rural Public Health, Texas A&M Health Science Center, College Station, Texas, USA Full list of author information is available at the end of the article [3], it has been shown that more time spent as an obese person is directly related with increased risk of all-cause mortality [4]. As a result, obesity places an economic strain on the healthcare system and society as a whole. It is estimated that total economic effects of obesity on the American economy are more than $200 billion annually [5]. The WHO uses BMI to classify individuals as either underweight (BMI < 18.5 kg/m2), normal weight (BMI of 18.5-24.99 kg/m2), overweight (BMI of 25.0-29.99 kg/ m2), obese class I (BMI of 30.0-34.99 kg/m2), obese class II (BMI of 35.0-39.99 kg/m2), or obese class III © 2011 Dickerson et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.