So BMC Public Health 2012, 12:258 http://www.biomedcentral.com/1471-2458/12/258 assess the associations between academic performance and PA, after adjusting for covariate variables such as age, BMI, parents’ education level, and family income status, multivariate logistic regression analyses were performed. Statistical significance was set at p < 0.05. Results Multivariate logistic regression analysis of academic per- formance in relation to PA patterns, after adjusting for co- variate variables including age, BMI, parents’ educational level, and family income status, is shown in Table 2. Compared with boys who did not engage in vigorous PA, the following participants had greater odds of report- ing an average or above-average academic performance: those who participated in vigorous PA twice a week (OR = 1.198, 95% CI = 1.080–1.329, p = 0.001), thrice a week (OR = 1.287, 95% CI = 1.156–1.433, p < 0.001), 4 times a week (OR = 1.265, 95% CI = 1.117–1.433, p < 0.001), and 5 or more times a week (OR = 0.886, 95% CI = 0.801–0.981, p = 0.020). Compared with boys who did not engage in moderate PA, the following participants had greater odds of reporting an average or above-average academic per- formance: those who participated in moderate PA once a week (OR = 1.121, 95% CI = 1.021–1.232, p = 0.017), twice a week (OR = 1.337, 95% CI = 1.216–1.471, p < 0.001), thrice a week (OR = 1.460, 95% CI = 1.315–1.621, p < 0.001), 4 times a week (OR = 1.363, 95% CI = 1.183–1.570, p < 0.001), and 5 or more times a week (OR = 1.135, 95% CI = 1.013– 1.271, p = 0.029). Compared with boys who did not engage in strengthening exercises, those who participated in strengthening exercises 5 or more times a week had lesser odds of reporting an average or above-average academic performance (OR = 0.828, 95% CI = 0.754–0.910, p < 0.001). Compared with girls who did not engage in vigorous PA, those who participated in vigorous PA 5 or more times a week had greater odds of reporting an average or above-average academic performance (OR = 1.532, 95% CI = 1.336–1.757, p < 0.001). Compared with girls who did not engage in moderate PA, the following partici- pants had greater odds of reporting an average or above- average academic performance: those who participated in moderate PA twice a week (OR = 1.113, 95% CI = 1.008–1.229, p = 0.035) and thrice a week (OR = 1.306, 95% CI = 1.161–1.468, p < 0.001). Compared with girls who did not engage in strengthening exercises, those who participated in strengthening exercises 5 or more times a week had lesser odds of reporting an average or above-average academic performance (OR = 0.739, 95% CI = 0.618–0.883, p = 0.001). Discussion The aim of this study was to investigate the relation between PA and academic performance in adolescent Korean students. The results showed that vigorous PA Page 4 of 7 was positively correlated with academic performance in the case of boys, and moderate PA was positively corre- lated with academic performance in both boys and girls. However, strengthening exercises were not positively cor- related with academic performance in boys or girls. Fur- thermore, PA and strengthening exercises—when performed 4 times a week or more—were negatively corre- lated with academic performance in both boys and girls. Three hypotheses have been proposed to explain the effect of PA or exercise on the brain. First, PA and exer- cise increase oxygen saturation and angiogenesis in the regions of the brain essential for performing tasks [22,23]. Second, they also increase the activity of neurotransmitters such as serotonin and norepinephrine in the brain, facili- tating information processing [24-26]. Third, PA and exercise upregulate neurotrophins such as brain-derived neurotrophic factor, insulin-like growth factor-I, and basic fibroblast growth factor. These neurotrophins support neuronal survival and differentiation in the developing brain [27]. The results of our epidemiological study indicate that PA may increase brain and memory function and that moderate PA has a positive effect on the academic per- formance of both boys and girls. However, vigorous PA has a positive effect on academic performance only in the case of boys. This difference might be due to the fact that boys undergo rapid physical growth during adoles- cence [28] and generally prefer sports-related activities such as soccer and basketball, which are defined as vig- orous PA according to the KYRBWS-V. In contrast, girls prefer leisure activities that are considered moderate PA [29]. For this reason, we believe that vigorous PA has a positive effect on the academic performance of boys. Strengthening exercises were not positively correlated with academic performance in the case of boys or girls. PA usually affects the circulatory system [1]. Thus, PA stimulates the brain by increasing blood circulation to the entire body. However, strengthening exercises focus on the musculoskeletal system and not on the circulatory system [2]; therefore, because strengthening exercises stimulate the muscles, they may stimulate brain func- tions only slightly through increased blood circulation to the entire body. We also found that vigorous PA in boys and strengthening exercises in both boys and girls per- formed at a frequency of 5 or more times a week nega- tively affected academic performance. Performing vigorous PA and strengthening exercises 5 or more times a week is time consuming and would potentially deprive students of study time that could be invested in improving their academic performance. This study has some limitations. First, it was con- ducted online; therefore, the family income status, par- ents’ educational qualifications, and the students’ height and weight were not directly measured; they were