Is Sedentary Behavior Associated With Executive Function in Children and Adolescents? A Systematic Review

Background Prolonged time on sedentary behavior, especially screen-based sitting time, is associated with unfavorable health indicators in children and adolescents. However, the effects of sedentary behavior on cognitive function remain to be elucidated. Objective The purpose of this systematic review was to synthesize the evidence on the associations of sedentary behavior with executive function in children and adolescents. Methods Four electronic databases (i.e., PubMed, Web of Science, PsycINFO, and SPORTDiscus) were searched for studies examining the associations between sedentary behavior and executive function in children and adolescents. Study quality was assessed by the NIH Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies. Results A total of 1,151 records were initially identified through database searches and other searches. Twelve cross-sectional and four longitudinal studies met the inclusion criteria. Of the 16 studies, seven studies found significant negative associations between sedentary behavior and executive function, and two studies presented positive associations. Eight studies measured sedentary time using accelerometers and showed varied associations between objectively measured sedentary time and executive function. Nine studies measured screen-based sedentary behavior, of which five studies found negative associations of sedentary time with executive function. Conclusion The available evidence on the associations between sedentary behavior and executive function is not conclusive in children and adolescents. However, screen-based sedentary behavior may be negatively associated with executive function.


INTRODUCTION
Sedentary behavior is a distinct construct from physical activity, referring to any waking behaviors with an energy expenditure of <1.5 metabolic equivalent units (METs) while in a sitting, reclining, or lying posture (1). Common sedentary behaviors include prolonged sitting, screenbased behaviors (e.g., TV viewing, computer/tablet using, video gaming), etc. Time spent on sedentary behaviors can be self/parent-reported or be objectively monitored by wearable devices such as accelerometers. Currently, sedentary time remains high in children and adolescents, and the trend continues to increase over the past few decades in some countries. According to the Global School-based Student Health Survey among 97 countries, about 25% of boys and girls aged 13-15 years old reported sitting longer than 3 h per day, in addition to sitting at school and for homework (2). From 2007 to 2016, the estimated total sitting time increased from 7 h per day to 8.2 h per day among adolescents in the United States (3). In China, an increasing trend of the prevalence of screen-based viewing time was also observed in school-age children (4).
Accumulating evidence showed that sedentary behavior, especially prolonged TV viewing, have been linked with increased risk of a variety of chronic diseases, such as obesity (5), type 2 diabetes (6, 7), cardiovascular diseases (8), and certain types of cancer (9). In children and adolescents, sedentary behaviors have also been linked with unfavorable health indicators, such as lower physical fitness (10), higher fatness (11), clustered cardiometabolic risk scores (12), and lower self-esteem (13). Even worse, emerging evidence has shown that excessive sedentary behaviors are associated with mental illness and poorer cognitive function (14,15). However, the findings on the relationship between sedentary behavior and cognitive function are mixed. A systematic review included eight studies examining the associations of sedentary behavior with cognitive function in adults older than 40 years (15). It concluded that greater amounts of sedentary behaviors were associated with poorer cognitive function over the lifespan. A more recent systematic review including 13 cross-sectional and longitudinal studies suggested inconsistent evidence on the direction of the association of sedentary behavior with cognitive function in older adults with a mean age of 65+ years (16). Another systematic review in young children (≤5 years) found that different types of sedentary behavior may exert different influences on cognitive development (17). Screen time, particularly TV viewing, was either not associated with or negatively associated with cognitive skills. However, no existing studies have critically reviewed the literature of the association between sedentary behavior and executive function in children and adolescents.
Previous studies have shown that physical activity and fitness have beneficial effects on cognitive function in children and adolescents (18,19). The effects are disproportionately larger for executive function (20). Executive function refers to a set of top-down mental processes needed for goal-directed behaviors, such as inhibitory control, working memory, cognitive flexibility, planning (21). Executive function is critical for school readiness, academic performance, and future career success (21,22). However, evidence on the effects of sedentary behavior on executive function remains conflicting in children and adolescents. Some studies showed that sedentary behavior is not associated with executive function in childhood (23)(24)(25), whereas other studies found negative (26,27) or positive associations (28). No existing systematic reviews have addressed this research gap. In addition, a preview review suggested a type-specific association between sedentary behavior and health indicators in children and adolescents (29). Another gap in the literature is the lack of the associations between type-specific sedentary behaviors and executive function. Therefore, the current systematic review is aimed to synthesize the literature on the association of sedentary behavior (both objectively measured sedentary time and self/parent-reported screen-based behaviors) with executive function in children and adolescents.

METHODS
This systematic review was performed following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (30).

Search Strategy
Two authors (SL and JG) independently searched PubMed, Web of Science, PsycINFO, and SPORTDiscus from inception to April 2021. The combinations of the following three groups of retrieval items were used: (1) sedentary behavi * , screen time, sitting time, sedentary time, TV viewing, video gam * , computer use; (2) executive function, cognitive control, working memory, inhibitory control, cognitive flexibility, planning; (3) children, adolescents. The specific search strategy was slightly adjusted according to the search builder of each database.

Study Selection
Two authors (SL and JG) screened all the retrieved titles and abstracts to exclude duplicate or irrelevant studies. The two authors screened the full text of the remaining studies after removing duplicate and apparently irrelevant studies. Any disagreements about the study selection were discussed with a third author (TH) until a consensus was reached. The two review authors also searched the bibliographies of all included articles to ensure that all relevant studies were captured. Only the longitudinal result was extracted for the cohort studies that conducted both cross-sectional and longitudinal analyses in the same population.

Inclusion and Exclusion Criteria
The studies must meet the following inclusion criteria to be included: (1) studies with cross-sectional or longitudinal design examined the associations between sedentary behavior and executive function; (2) sedentary behavior was self/parentreported (e.g., prolonged sitting, TV viewing, computer use, video gaming) or objectively monitored by wearable devices (e.g., accelerometers); (3) executive function was objectively assessed (paradigms including Flanker task, Stroop color-word test, Nback task, Tower of London task, Trail making task, etc.); (4) the participants were apparently healthy children and adolescents aged 5-17 years; (5) studies must be published in peer-reviewed journals; (6) English full text must be available.
Studies were excluded if the sedentary behaviors were not clearly classified or measured. Studies focusing on specific screenbased or non-screen contents (e.g., violent films, educational programs), screen-based active behavior (e.g., active video gaming), or specific learning behavior (e.g., reading, puzzles) were excluded. Studies were also excluded if the executive function was parent-or teacher-reported.

Data Extraction
Data collection was conducted independently by two authors (SL and JG). Publication year, country, study design, sample size, covariates, measurement of sedentary behavior, assessment of executive function, and results were extracted from each included study and recorded.

Methodological Quality
The two authors assessed the quality of studies by the NIH Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies (31). This tool includes 14 items, and the reviewer could select "yes, " "no, " "cannot determine, " or "not reported" on each item. The score for longitudinal studies ranges from 0 (the lowest quality) to 14 (the highest quality). For crosssectional studies, three items are not applicable (items 10, 12, 13). The classifications of methodological quality are rated as "strong" (≥80%), "good" (70-79%), "fair" (60-69%), or "poor" (<60%) based on the percentage scores which are calculated as the number of "yes" responses divided by the total number of applicable items (32,33). All discrepancies between reviewers were resolved through discussion among the reviewers or with a third reviewer if needed. The items of assessment tool are listed in Supplementary Table 1.

Search Results and Study Characteristics
A total of 1,149 records were identified through database searches, and additional two records were identified through reference list searches (see Figure 1). After removing duplicate records, 962 records remained. Following the screening of titles and abstracts, 37 articles were obtained for further full text review. Sixteen articles met the inclusion criteria after detailed assessment of the full-text, including 12 cross-sectional studies and four longitudinal studies.
Across these 16 studies, a total of 21 cognitive tasks were used, measuring various aspects of executive function, including inhibitory control, working memory, cognitive flexibility, and planning. Study sample sizes ranged from 77 to 1,001. The participants were aged 5-17 years old. The countries of study locations were Norway, China, South Africa, Canada, Spain, United States, United Kingdom, Finland, and the Netherlands.
Of the nine studies, three studies assessed the total screen time (26,37,39). Two of these studies (66%) showed that total screen time was not associated with executive function (working memory, cognitive flexibility) (37,39). Only one study (33%) observed a negative association between total screen time and N-back performance in girls (26). Seven studies examined the associations between TV viewing and executive function (23,24,26,37,40,41,43). Of them, two studies (29%) found that more time on TV viewing was associated with poorer executive function (40,43), and five studies (71%) did not find any associations between TV viewing and executive function (23,24,26,37,41). Three studies examined the association between general computer usages with executive function (40)(41)(42). One study found a positive association (40) and one study found a negative association (41). Two studies examined the associations between computer/video gaming and executive function (26,37).
Both of them found that spending more time on computer/video gaming was related to worse working memory.

Methodological Quality of Included Studies
The average score of cross-sectional studies was 6.67 ( Table 1). The average score of longitudinal studies was 10.
Detailed scores of quality assessment are also available in Supplementary Table 1.

DISCUSSION
The present study was aimed to critically review the evidence on the association between sedentary behavior and executive function in children and adolescents. Out of the 16 studies, seven studies (44%) found a negative association between sedentary behavior and executive function, while two studies (13%) presented positive associations. Eight studies measured sedentary time using an accelerometer, and showed varied associations of objectively measured sedentary time with executive function. Nine studies measured screen-based sedentary behavior, of which five studies (56%) found negative associations of sedentary time with executive function.
Eight of the included studies objectively measured sedentary time. The current review presented mixed results regarding the associations between objectively measured sedentary time and executive function in children and adolescents. It is impossible to conclude of the direction of the association between objectively measured sedentary time and executive function. Our findings are inconsistent with a systematic review in older adults, which indicated that shorter objectively assessed sedentary time was associated with better global cognitive function (44). Although the accelerometer-based measurements provided an objectively assessed sedentary time, they cannot distinguish the types of sedentary behavior. Children and adolescents may engage in cognitively active sedentary behavior, such as reading, and learning, benefiting cognitive development (45). A study further supports this idea. Brain connectivity was positively correlated with reading time and negatively correlated with screen-based media time (46). Therefore, when it comes to the associations between sedentary behavior and executive function in childhood, the types of sedentary behavior should be considered.
In this systematic review, nine included studies surveyed screen-based sedentary behavior, which provided some evidence on the association between type-specific sedentary behavior and executive function in children and adolescents. The majority of evidence suggests that screen-based sedentary time has either no effects or a detrimental effect on executive function in children and adolescents. Recent evidence found that the deleterious effects of sedentary behavior on cardio-metabolic health are most notable for screen-based behaviors (47). Regarding mental health, a study showed that only leisure screen-based sedentary behaviors are linked to worse perceived stress and anxiety (48). In the current study, the negative associations of sedentary behavior with executive function are mainly observed in the included studies that measured screen-based sedentary behaviors (i.e., TV viewing, computer use, video games, total screen time). These findings are in line with the systematic review in early childhood.  In female students, changes in total screen time and time on video games were negatively associated with changes in N-back accuracy.
In male students, changes in screen time was not associated with performance on Flanker task and N-back task.
López-Vicente et al. Specifically, the systematic review concluded that screen time was either not associated with or had detrimental associations with cognitive function in young children (17). The biological plausibility for the observed negative association of screen-based sedentary behavior with executive function is not clear. There might be several potential explanations. First, most digital screens are backlit and emit blue light wavelengths. It can suppress melatonin secretion to influence sleep quality (49,50), which may, in turns, affect brain health (51,52). Second, sedentary behavior may increase the risk of some aspects of mental problems, such as depression (14), which may negatively influence cognitive development (53). Third, recent neuroimaging studies have linked screen-based sedentary behavior with brain structure and integrity, which further supports a detrimental effect of screen-based sedentary behavior. A study indicated that prolonged time on TV viewing was associated with lower gray matter volume in six brain regions in children (54). Increased screen-based media use was also associated with lower microstructural integrity of brain white matter in preschool-aged children (55).
Most of the included studies were of low to moderate quality. Of the 16 studies, four studies employed longitudinal study design, and only one study was rated as strong quality. Therefore, more studies with stronger design are warranted to further ascertain the effects of sedentary behavior on cognitive function in childhood. In addition, previous studies have suggested that physical activity and exercise were positively associated with executive function in children and adolescents (18,56). Sedentary behavior may also correlate with physical activity considering the 24-h movement continuum. However, of the 16 studies, most studies did not consider physical activity as a potential covariate. The results may have been subjected to residual confounding. Future studies should consider physical activity as covariates or investigate the combined effects of sedentary behavior and physical activity.
Although the conflicting results exist, this systematic review provided preliminary evidence which supports a negative association between screen-based sedentary behavior and executive function in children and adolescents. Therefore, from the perspective of children's physical health and cognitive development, families, schools, and policymakers should consider interventions for reducing and limiting screen-based sedentary behavior in childhood.
To the best of our knowledge, this study is the first to systematically review the associations between sedentary behavior and executive function in children and adolescents. However, this study also has its limitations. First, a meta-analysis cannot be carried out due to the heterogeneities in study design and outcome measurements of the included studies. Second, all of the included studies were observational in design, and there was no intervention study. Therefore, the causal relationship between sedentary behavior and executive function cannot be inferred. Third, the searching language was limited to English, which increases the risk of omitting important studies published in other languages.

CONCLUSION
The study suggests that the associations between sedentary behavior and executive function are not conclusive in children and adolescents. However, time on screen-based sedentary behavior tends to be negatively associated with executive function.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.