adolescence is an important period with regard to establishing a healthy life - style ( 1 ) .
for example , approximately 80% of obese children remain obese even as adults ( 2 ) .
hence , many studies have investigated adolescents life - styles and the establishment of healthy habits such as increasing physical activity ( pa ) and decreasing sedentary activities ( 37 ) .
physical inactivity ( sedentary activity ) has adverse health effects such as increased risk for cardiac diseases , musculoskeletal disorders , stroke , type 2 diabetes , weight gain , and some cancers ( 36 ) .
conversely , physical activity , a variable that is independent of inactivity , has health benefits such as improved weight control , bone and muscle strength , mental health and mood , ability to perform daily activities , and life span ; moreover , the risk for cardiovascular diseases , type 2 diabetes , metabolic syndromes , some cancers , and falls is reduced ( 7 ) . recently
, several studies have shown that pa improves brain activities , including memory and cognitive functions ( 810 ) .
subsequent studies suggest that adequate pa improves school performance and academic achievements of adolescent students ( 11,12 ) .
though pa and its positive effects on academic activities through increased brain activity has been studied extensively , whether sedentary activity and physical inactivity is associated with academic outcomes has not been a research focus . furthermore , there is no epidemiological research involving clinical practice investigating the relationship between physical inactivity and academic record .
therefore , the purpose of this study was to examine whether physical inactivity is associated with academic record in korean adolescent students .
in 2009 , the 5th korea youth risk behavior web - based survey ( kyrbws - v ) was conducted as an epidemiological study with a complex sample design including multistage sampling , stratification , and clustering through the korea centers for disease control and prevention ( kcdcp ) to determine the prevalence of health - risk behaviors in korean adolescent students from the 7th to the 12th grades ( 13 ) .
the kyrbws - v collected data from 24,000 classrooms ( secondary sampling units ) in 400 middle schools and 400 high schools ( primary sampling units ) and from 135 strata identified using a stratified multistage cluster sampling method .
a 16-city cluster sampling strategy was utilized and the survey - sampling frame included all of korea in order to evaluate the relationship between physical inactivity and academic records of adolescent students ; potential covariate variables were taken into account ( 13 ) .
all details pertaining to the data collection procedures have been reported by the kcdcp , and the reliability and validity of the kyrbws - v have been evaluated ( 1315 ) .
students participating in the survey were assigned unique identification ( i d ) numbers by their classroom teachers .
the students then accessed the survey s internet webpage , logged on using their unique i d numbers , and responded to a question regarding their willingness to participate .
willing participants self - administered the questionnaire anonymously at their school , and those who were unwilling did not progress further . in total , there were 76,937 willing participants .
however , students who had dyslexia , dysgraphia , or who were absent from school for an extended period were excluded .
after the exclusions , 75,066 adolescent students participated in this study ( the participation rate was 97.6% ) .
ethical approval was not required for this study because kyrbws - v did not collect any personal information such as the students names , their school , home address , residential area , telephone number , or social security number .
the present study obtained data from the kyrbws - v to evaluate the relationship between physical inactivity and academic record , taking into account covariate variables such as gender , age , body mass index ( bmi ) , family s socioeconomic status , parents education level , and frequency of vigorous and moderate pa as well as muscular strength exercises .
physical inactivity was evaluated for each participant on the basis of their responses to the following 2 questions : ( q1 )  in the last week , how much time , on average , did you spend per day on weekdays engaged in physical inactivity such as watching tv or movies , surfing the internet , and playing video games ?  and ( q2 )  in the last week , how much time , on average , did you spend per day on the weekend engaged in physical inactivity such as tv or movie watching , internet surfing , video gaming , etc .
 the response options were : ( i ) < 1 hour per day , ( ii ) 1 to < 2 hours , ( iii ) 2 to <3 hours , ( iv ) 3 to < 4 hours , and ( v ) 4 hours .
self - reported academic record was evaluated for each participant by asking the question ,  in the past 12 months , what was your average academic record ?  .
the response options were : ( i ) very high , ( ii ) high , ( iii ) average , ( iv ) low , and ( v ) very low . based on these responses
, the participants were then classified into the following 2 groups for multivariate logistic regression analyses : ( i ) < average academic record and ( ii ) average academic record .
the adolescents were asked to self - record their height and weight ; bmi ( kg / m ) was calculated from the raw data recorded by each participant .
the 5 response options were : ( i ) very rich , ( ii ) rich , ( iii ) average , ( iv ) poor , and ( v ) very poor .
the 4 response options were : ( i ) unknown , ( ii ) middle school or lower , ( iii ) high school , and ( iv ) college or higher .
frequency of vigorous pa such as digging , aerobics , heavy lifting , and fast cycling during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( iv ) 4 times , and ( vi ) 5 times or more per week .
frequency of moderate pa such as cycling at a regular pace , carrying light loads , and playing doubles tennis during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( v ) 4 times , and ( vi ) 5 times or more per week .
frequency of muscular strength exercises such as sit - ups , push - ups , and weight lifting or weight training during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( v ) 4 times , and ( vi ) 5 times or more per week .
multivariate logistic regression analyses were conducted to determine whether the amount of physical inactivity was associated with academic record after adjusting for covariate variables .
statistical significance was set at p < 0.05 , and all analyses were performed using spss ver .
in 2009 , the 5th korea youth risk behavior web - based survey ( kyrbws - v ) was conducted as an epidemiological study with a complex sample design including multistage sampling , stratification , and clustering through the korea centers for disease control and prevention ( kcdcp ) to determine the prevalence of health - risk behaviors in korean adolescent students from the 7th to the 12th grades ( 13 ) .
the kyrbws - v collected data from 24,000 classrooms ( secondary sampling units ) in 400 middle schools and 400 high schools ( primary sampling units ) and from 135 strata identified using a stratified multistage cluster sampling method .
a 16-city cluster sampling strategy was utilized and the survey - sampling frame included all of korea in order to evaluate the relationship between physical inactivity and academic records of adolescent students ; potential covariate variables were taken into account ( 13 ) .
all details pertaining to the data collection procedures have been reported by the kcdcp , and the reliability and validity of the kyrbws - v have been evaluated ( 1315 ) .
students participating in the survey were assigned unique identification ( i d ) numbers by their classroom teachers .
the students then accessed the survey s internet webpage , logged on using their unique i d numbers , and responded to a question regarding their willingness to participate .
willing participants self - administered the questionnaire anonymously at their school , and those who were unwilling did not progress further . in total , there were 76,937 willing participants .
however , students who had dyslexia , dysgraphia , or who were absent from school for an extended period were excluded .
after the exclusions , 75,066 adolescent students participated in this study ( the participation rate was 97.6% ) .
ethical approval was not required for this study because kyrbws - v did not collect any personal information such as the students names , their school , home address , residential area , telephone number , or social security number .
the present study obtained data from the kyrbws - v to evaluate the relationship between physical inactivity and academic record , taking into account covariate variables such as gender , age , body mass index ( bmi ) , family s socioeconomic status , parents education level , and frequency of vigorous and moderate pa as well as muscular strength exercises .
physical inactivity was evaluated for each participant on the basis of their responses to the following 2 questions : ( q1 )  in the last week , how much time , on average , did you spend per day on weekdays engaged in physical inactivity such as watching tv or movies , surfing the internet , and playing video games ?  and ( q2 )  in the last week , how much time , on average , did you spend per day on the weekend engaged in physical inactivity such as tv or movie watching , internet surfing , video gaming , etc .
 the response options were : ( i ) < 1 hour per day , ( ii ) 1 to < 2 hours , ( iii ) 2 to <3 hours , ( iv ) 3 to < 4 hours , and ( v ) 4 hours .
self - reported academic record was evaluated for each participant by asking the question ,  in the past 12 months , what was your average academic record ?  .
the response options were : ( i ) very high , ( ii ) high , ( iii ) average , ( iv ) low , and ( v ) very low . based on these responses
, the participants were then classified into the following 2 groups for multivariate logistic regression analyses : ( i ) < average academic record and ( ii ) average academic record .
the adolescents were asked to self - record their height and weight ; bmi ( kg / m ) was calculated from the raw data recorded by each participant .
the 5 response options were : ( i ) very rich , ( ii ) rich , ( iii ) average , ( iv ) poor , and ( v ) very poor .
the 4 response options were : ( i ) unknown , ( ii ) middle school or lower , ( iii ) high school , and ( iv ) college or higher .
frequency of vigorous pa such as digging , aerobics , heavy lifting , and fast cycling during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( iv ) 4 times , and ( vi ) 5 times or more per week .
frequency of moderate pa such as cycling at a regular pace , carrying light loads , and playing doubles tennis during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( v ) 4 times , and ( vi ) 5 times or more per week .
frequency of muscular strength exercises such as sit - ups , push - ups , and weight lifting or weight training during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( v ) 4 times , and ( vi ) 5 times or more per week .
the adolescents were asked to self - record their height and weight ; bmi ( kg / m ) was calculated from the raw data recorded by each participant .
the 5 response options were : ( i ) very rich , ( ii ) rich , ( iii ) average , ( iv ) poor , and ( v ) very poor .
the 4 response options were : ( i ) unknown , ( ii ) middle school or lower , ( iii ) high school , and ( iv ) college or higher .
frequency of vigorous pa such as digging , aerobics , heavy lifting , and fast cycling during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( iv ) 4 times , and ( vi ) 5 times or more per week .
frequency of moderate pa such as cycling at a regular pace , carrying light loads , and playing doubles tennis during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( v ) 4 times , and ( vi ) 5 times or more per week .
frequency of muscular strength exercises such as sit - ups , push - ups , and weight lifting or weight training during the week the 6 response options were : ( i ) none , ( ii ) once , ( iii ) 2 times , ( iv ) 3 times , ( v ) 4 times , and ( vi ) 5 times or more per week .
multivariate logistic regression analyses were conducted to determine whether the amount of physical inactivity was associated with academic record after adjusting for covariate variables .
statistical significance was set at p < 0.05 , and all analyses were performed using spss ver .
the results of the multivariate logistic regression analyses of the duration of physical inactivity per day for the < average academic performance and average academic performance groups of korean adolescents are shown in table 2 .
this table compares the korean adolescents belonging to these 2 groups with respect to the information on duration of physical inactivity per day that was provided ( ranging from   < 1 hour to   4 hours ) . during weekdays , the odds ratios ( or ) ( 95% confidence interval [ ci ] ) for reporting a higher than average academic record , as compared with < 1 hour of physical inactivity per day , was 0.796 ( 0.7610.832 , for 1 to < 2 hours , 0.632 ( 0.6030.663 , for 2 to <3 hours , 0.567 ( 0.5350.601 , for 3 to 4 hours , and 0.494 ( 0.4680.522 ) for 4 hours of physical inactivity per day .
the results show that , during weekdays , among those reporting a higher than average academic record , the number of participants with 1 to < 2 hours , 2 to <3 hours , 3 to < 4 hours , and 4 hours of physical inactivity per day decreased by 20.4% , 36.8% , 43.3% , and 50.6% , respectively , as compared to the number of participants reporting < 1 hour of physical inactivity per day . during the weekend
, the ors ( 95% ci ) for reporting a higher than average academic record , as compared to < 1 hour of physical inactivity per day , was 0.901 ( 0.8480.957 , p = 0.001 ) for 3 to < 4 hours and 0.785 ( 0.7430.830 , p <
the results show that during the weekend , among those reporting a higher than average academic record , the number participants with 3 to < 4 hours and 4 hours of physical inactivity per day decreased by 9.9% and 21.5% , respectively , as compared to the number of participants reporting < 1 hour of physical inactivity per day .
the aim of this study was to examine the relationship between the duration of physical inactivity and a higher than average academic record in korean adolescents .
the results show that a higher than average academic record was negatively associated with increased duration of physical inactivity .
previous studies have reported that physical inactivity is strongly associated with obesity ( 16,17 ) .
furthermore , other studies have reported that obesity is associated with decreased brain activities including cognitive performance and impaired brain structure ( 1821 ) .
moreover , studies suggest that decreasing sedentary activities improves academic achievements and outcomes of adolescent students ( 22,23 ) .
there is evidence indicating that physical activity or inactivity affects neuronal function and structure in those areas of the brain that are involved in memory , learning , and performing tasks via changes in blood pressure regulation , oxygen saturation , and angiogenesis ( 2428 ) .
furthermore , physical activity or inactivity changes the activity of neurotransmitters such as norepinephrine and serotonin , thereby facilitating information processing ( 29,30 ) .
moreover , physical activity or inactivity modifies neurotrophins such as insulin - like growth factor - i , brain - derived neurotrophic factor , and basic fibroblast growth factor .
these altered neurotrophins sustain neuronal differentiation and survival in the brain ( 31 ) . however , the specific mechanisms fundamental to this association are still unclear .
nevertheless , at the epidemiological level , we hypothesized that physical inactivity such as watching tv would be negatively associated with academic record because physical inactivity causes obesity , which in turn leads to decreased brain activity .
this study certainly showed that those spending more time engaged in physical inactivity are predisposed to a below - average academic record .
we recommend that parents and guardians restrict the time spent engaging in inactive physical activities such as watching tv to less than 1 hour per day at home to prevent decreased academic performance in adolescents .
for this reason , we could not provide definitive evidence of a cause - and - effect relationship ; instead , we assessed the relationship between the duration of physical inactivity and academic record .
second , the questionnaire was self - administered , using online methods ; therefore , information such as family s socioeconomic status and parents education level was obtained from the adolescents , not from the parents .
third ,  academic record  data as obtained through the kyrbws - v did not include actual academic outcomes such as test scores or behavior at school ; these factors might contribute to academic record .
nevertheless , because this kyrbws - v sampled 75,066 students from all parts of korea , it was highly representative of the korean student population ; thus , the relationship between physical inactivity and academic record can be generalized to all korean adolescents .
we conclude that korean adolescents who spend more time engaged in physical inactivities are predisposed to a below - average academic record , regardless of gender , age , bmi , family s socioeconomic status , parents education level , and frequency of vigorous or moderate pa and muscular strength exercises .
ethical issues ( including plagiarism , informed consent , misconduct , data fabrication and/or falsification , double publication and/or submission , redundancy , etc ) have been completely observed by the authors .
 background : the purpose of this study was to investigate the association between physical inactivity and academic record in korean adolescents.methods:adolescent students from the first grade of middle school to the third grade of high school ( n=75,066 ) participated in the 5th korea youth risk behavior web - based survey project in 2009 . 
 the association between physical inactivity and academic record was assessed using multivariate logistic regression analysis after adjusting for gender , age , body mass index , family s socioeconomic status , parents education level , and frequency of vigorous or moderate physical activity ( pa ) as well as muscular strength exercises.results:during weekdays , the odds ratios ( ors ) ( 95% confidence interval [ ci ] ) for reporting a higher than average academic record , as compared with < 1 hour of physical inactivity per day , was 0.796 ( 0.7610.832 , for 1 to < 2 hours , 0.632 ( 0.6030.663 , for 2 to <3 hours , 0.567 ( 0.5350.601 , for 3 to < 4 hours , and 0.494 ( 0.4680.522 , p < 0.001 for all cases ) for 4 hours of physical inactivity per day . during the weekends , 
 the ors ( 95% ci ) for reporting a higher than average academic record , as compared with < 1 hour of physical inactivity per day , were 0.901 ( 0.8480.957 , p = 0.001 ) for 3 to < 4 hours and 0.785 ( 0.7430.830 , p < 
 0.001 ) for 4 hours of physical inactivity per day.conclusion:korean adolescents who spend more time engaged in physical inactivity are predisposed to a below - average academic record . 
Introduction
Materials and Methods
Subjects
Independent variables
Dependent variables
Covariate variables
Gender
Age
BMI
Familys socioeconomic status
Parents education level
Statistical analysis
Results
Discussion
Conclusion
Ethical considerations