So BMC Public Health 2012, 12:258 http://www.biomedcentral.com/1471-2458/12/258 Background Physical activity (PA) and exercise enhance the functioning of the musculoskeletal and cardiovascular systems [1,2]. Additionally, PA is known to confer benefits such as im- provement in weight control, bone and muscle strength, mental health and mood, ability to perform daily activities, and life span as well as reduction in the risk of cardiovas- cular disease, type 2 diabetes, metabolic syndrome, certain cancers, and falls [3]. Furthermore, physical inactivity leads to a sedentary lifestyle that in turn may lead to overweight or obesity [4-6], and is known to have adverse health effects such as increased risk of cardiac disease, musculo- skeletal disorders, stroke, type II diabetes, and certain can- cers [7]. Therefore, adequate PA is considered important for good health. Adolescence is an important period for establishing healthy habits [8]. For example, approximately 80% of obese adolescents grow into obese adults [9]. Many stud- ies have investigated the relation between PA and health effects. The results of these studies suggest that it is im- portant to increase PA and decrease sedentary activities to improve health [10,11]. Further, PA was recently found to improve cognitive and memory functions [12,13]. These reports showed that regular PA might improve the aca- demic performance of adolescent students. In addition, several studies have reported that PA enhances academic performance and outcomes [14-17]. However, while stud- ies conducted in various regions of the world suggest that PA has beneficial effects on academic performance, and some practical meta-analysis-based evidence associates PA with academic performance [18], there is no epidemio- logical evidence that indicates whether there is an asso- ciation between PA and the academic performance of students in Korea. Furthermore, while some regional stud- ies have analyzed this relation in Korean adolescents, no nationwide study has been conducted in this regard. Therefore, this study was conducted to investigate the re- lation between PA and academic performance in Korean adolescent students. Methods Subjects The 5th Korea Youth Risk Behavior Web-based Survey (KYRBWS-V), which was administered in 2009, was a retrospective epidemiological study with a complex sam- ple design that included multistage sampling, stratifica- tion, and clustering. This was a national school-based survey conducted by the Korea Centers for Disease Con- trol and Prevention (KCDCP) to estimate the prevalence of health-risk behaviors among adolescent Korean stu- dents from the 7th to 12th grade [19]. The KYRBWS-V used a 16-city cluster sample strategy, and the sampling frame of the survey included the entire country. A total of 400 middle and 400 high schools were sampled to Page 2 of 7 evaluate the association between PA and the academic performance of adolescent Korean students, taking into account potential covariate variables. All the details of the data collection procedure have been reported by the KCDCP [19], and this survey has been reported to be valid and reliable [20,21]. The survey was administered to a nationally representative group of Korean adolescent students. Because the survey did not collect identifier in- formation, ethical approval was not required. Further, students were given the option to not participate. The students who chose to participate in the study completed the questionnaire anonymously through the Internet, at their school. Teachers at participating schools were sup- plied with unique sets of ID numbers printed on tags, and each participating student drew one of these at ran- dom before accessing the survey web page and logging in using that unique ID number. A total of 75,066 ado- lescent students (39,612 boys and 35,454 girls) partici- pated in the study, and the response rate was 97.6% (out of 76,937 students, 75,066 accessed the questionnaire online). The characteristics of the subjects are shown in Table 1. Independent variables ① Frequency of vigorous PA: The examples included dig- ging, soccer, basketball, aerobics, heavy weight lifting, fast cycling, and swimming. The response options were classi- fied into 6 groups: (1) no vigorous PA, (2) once a week, (3) twice a week, (4) thrice a week, (5) 4 times a week, and (6) 5 or more times a week. ② Frequency of moderate PA: The examples included cycling at a regular pace, carrying light loads, and playing table tennis, badminton, and doubles tennis. The response options were classified into 6 groups: (1) no moderate PA, (2) once a week, (3) twice a week, (4) thrice a week, (5) 4 times a week, and (6) 5 or more times a week. ③ Frequency of strengthening exercises: The examples included sit-ups, push-ups, weight lifting, and weight training. The response options were classified into 6 groups: (1) no strengthening exercises, (2) once a week, (3) twice a week, (4) thrice a week, (5) 4 times a week, and (6) 5 or more times a week. Dependent variables Academic performance was evaluated using the follow- ing question: In the past 12 months, how has your aver- age academic performance been? The response options were classified into 5 groups: (1) very good performance, (2) good performance, (3) average performance, (4) bad performance, and (5) very bad performance. This vari- able included only the assessment of the academic per- formance of students according to examination scores, not taking into account their behavioral conduct, at- tendance, or other achievements. On the basis of their