According to the Domestic Internet Usage Survey conducted by the Ministry of Science and ICT of South Korea, the smartphone ownership rate among adolescents aged 6 to 19 was 95.5% in both 2018 and 2023. The usage rate of Social Network Services (SNS) among adolescents was 59.4% in 2018 but increased to 65% in 2023, showing a continued upward trend compared to the past. The previous studies have indicated that heightened SNS usage contributes to a growing interest in appearance. According to Jiotsa et al. [1] stated that using SNS such as Facebook and Instagram increase body dissatisfaction, and eating disorder symptoms may appear due to the increased desire for a thin body. In addition, Sampasa-Kanyinga et al. [2] reported that adolescent females who use SNS for more than two hours a day are more likely to experience dissatisfaction with their weight. Therefore, the increased use of SNS among adolescents in South Korea appears to have amplified their interest in subjectively perceived appearance and body shape.

Body shape perception is “an individual’s perceptions, thoughts, and feelings about their body” [3]. It is influenced by diverse factors, such as media exposure, cultural norms, and personal experiences. For instance, the repeated exposure of thin models in Western media perpetuates the idealization of thinness and reinforces internalized body standards [4]. Additionally, personal experiences, including feedback from peers and parents, often possibly contribute to reinforcing idealized body shape standards [4]. Previous research indicates that negative perceptions of body shape can have adverse effects on both mental and physical health. For instance, negative perceptions of body shape have been linked to elevated levels of stress and depressive symptoms [4]. Notably, research suggests that developmental instability during adolescence intensifies the effects of negative body shape perceptions, thereby heightening the risk of mental health issues [5]. Adolescents with negative body shape are also more likely to adopt unhealthy weight control practices, which may exacerbate both physical and psychological distress [6].

Adolescence is a critical period characterized by rapid physical growth, including the development of secondary sexual characteristics, increases in height and weight, and the growth of bones and muscles [7]. During this time, lifestyle habits such as physical activity and sedentary behavior are also established [8]. Physical activity during adolescence positively influences bone growth [9], mental health aspects such as self-esteem and stress [10], and academic performance [11]. In contrast, insufficient physical activity is associated with overweight [12], insulin resistance [13], and an increased risk of developing cardiovascular diseases [14], among other adverse outcomes. Moreover, sustained physical activity during adolescence has been shown to promote continued participation in physical activity during adulthood [15], highlighting the importance of maintaining consistent physical activity during this critical period. However, sedentary time among adolescent students in South Korea is on the rise due to the increased use of electronic devices, such as computers and smartphones, and extended study hours [16]. The increase in sedentary time is associated with various issues, including obesity [17], depression and anxiety [18], reduced academic performance [19], and insomnia and other sleep disorders [20]. Furthermore, previous studies have indicated that sedentary behavior during adolescence can establish habits of sedentary behavior that persist into adulthood [21]. Therefore, efforts to reduce sedentary behavior during adolescence are essential.

Previous studies have shown that research on sedentary behavior based on subjective body shape perception is limited, with most studies on physical activity focusing primarily on female high school or college students [22,23]. However, the use of SNS among middle and high school students continues to rise [24,25], and interest in appearance is growing, regardless of gender [26]. As a result, research that reflects these trends is currently lacking. This study, therefore, aims to confirm the relationship between physical activity, sedentary behavior, and subjective body shape perception among adolescents in South Korea.

Across the 2018–2023 KYRBS cycles, we delimited the analyses to youth between 12 and 18 years. Among those with complete data on the study variables such as age, sex, academic records, economic status, height, and weight, sedentary time, and physical activity, the total sample size was 307,926 (157,751 boys). Table 1 presents the study variable characteristics. The weighted gender proportions were consistent across the respective cycles, with boys ranging from 51.48% to 51.94% and girls ranging from 48.06% to 48.52%. The range of weighted economic status proportions varied across the respective cycles, with similar distributions observed for the Upper (10.59%–11.76%), Middle Upper (28.85%–31.83%), Middle (44.88%–49.07%), Middle Lower (8.82%–10.62%), and Lower (1.66%–2.09%) categories. The average weighted age ranged from 15.08 to 15.22 years. The average weighted height and weight were 170.25–170.71 cm and 60.02–66.40 kg for boys and 160.07–160.89 cm and 53.24–53.55 kg for girls, respectively.

Table 2 displays the adjusted odds ratios (ORs) between subjective body shape perception and physical activity and sedentary behavior, respectively. After adjustments, the OR was statistically significant between subjective body shape perception and physical activity. Youth with thin and normal subjective body shape perception were more likely to be physically active compared to youth in obese, OR=1.46, 95% confidence interval (CI): 1.38–1.55 and OR=1.76, 95% CI: 1.68–1.84, respectively. However, there was no statistical association between subjective body shape and sedentary behavior [OR=1.04 (thin vs. obese) and OR=0.97 (normal vs. obese)].

Table 3 presents the adjusted ORs for the association between subjective body shape perception and both physical activity and sedentary behavior among boys. Following adjustment, a statistically significant association was observed between subjective body shape perception and physical activity. Boys who perceived their body shape as thin or normal were significantly more likely to engage in physical activity compared to those who perceived themselves as obese, with ORs of 1.58 (95% CI: 1.47–1.69) and 2.03 (95% CI: 1.92–2.14), respectively. Furthermore, boys with a normal subjective body shape perception were less likely to engage in sedentary behavior compared to those with an obese body shape perception, with an OR of 0.93 (95% CI: 0.89–0.97).

Table 4 demonstrates the adjusted ORs for the association between subjective body shape perception and both physical activity and sedentary behavior among girls. After adjustment, a statistically significant association was identified between subjective body shape perception and physical activity. Girls who perceived their body shape as thin or normal were significantly more likely to engage in physical activity compared to those who perceived themselves as obese, with ORs of 1.43 (95% CI: 1.28–1.61) and 1.32 (95% CI: 1.22–1.44), respectively. In contrast, no statistically significant association was observed between subjective body shape perception and sedentary behavior, with ORs of 1.06 (thin vs. obese) and 1.02 (normal vs. obese).

This study examined the relationship between physical activity and sedentary behavior based on subjective body type perception in adolescents in South Korea. The main findings are as follows: 1) Adolescents who perceived themselves as having a thin or normal body shape were more likely to engage in physical activity than those who perceived themselves as obese. However, no statistical relationship was found between subjective body shape perception and sedentary behavior; 2) The analysis of physical activity and sedentary behavior according to boys’ subjective body type perception revealed that boys who perceived themselves as having a thin or normal body type were significantly more likely to participate in physical activity compared to those who perceived themselves as obese. boys who perceived themselves as having a normal body shape were also less likely to engage in sedentary behavior than those who perceived themselves as obese; 3) The analysis of girls’ physical activity and sedentary behavior based on subjective body shape perception showed that girls who perceived themselves as having a thin or normal body shape were significantly more likely to engage in physical activity compared to those who perceived themselves as obese. However, no statistically significant relationship was found between sedentary behavior and subjective body shape perception.

There is a lack of previous research solely focused on physical activity based on subjective body shape perception, making direct comparisons with our study challenging. However, our findings can be substantiated by previous research examining the association between body satisfaction and physical activity. Neumark-Sztainer et al. [29] reported that adolescents with lower body satisfaction engaged in less physical activity compared to their peers with higher body satisfaction. Similarly, Fountoulakis and Grogan [30] observed that individuals who were more satisfied with their body shape were more likely to exercise for reasons related to fitness and health. Consistent with these findings, previous research has shown that individuals who are more satisfied or positively perceive their body shape are more likely to engage in physical activity. Furthermore, studies indicate that individuals with thin or normal body types tend to report higher levels of body satisfaction compared to those with obese body types [31], and body satisfaction positively influences subjective body shape perceptions [32]. These insights align with our findings, suggesting that adolescents who perceive themselves as thin or normal are more inclined to engage in physical activity than those who view themselves as obese.

The limited research on the relationship between subjective body shape perception and sedentary behavior poses challenges in interpreting the findings. This study found no significant relationship between sedentary behavior and subjective body shape perception. This is because an accurate standard for sedentary behavior specific to Koreans has not yet been established. The current sedentary behavior guideline in South Korea is “Minimize the amount of sitting time during the day” in the Ministry of Health and Welfare’s 2023 Physical Activity Guidelines for Koreans. Therefore, this study categorized sitting time into two categories: less than 2 hours per day and more than 2 hours per day based on existing Korean and international studies on sedentary behavior [28]. However, the classification criteria did not account for Korea’s cultural characteristics. There is a pressing need for sedentary behavior guidelines tailored to the unique cultural context of Korean adolescents, which includes low levels of physical activity and a strong emphasis on education [33]. Consequently, future research should be conducted based on the sedentary behavior guidelines specifically developed for South Korea.

The relationship between subjective body shape perception and physical activity and sedentary behavior revealed that both boys and girls who viewed themselves as thin or of normal weight were more likely to participate in physical activity compared to those who perceived themselves as obese. Finne et al. [34] reported that girls who experienced body dissatisfaction, perceiving themselves as too fat, faced a higher likelihood of being inactive. Among boys, body dissatisfaction was more prevalent not only in those who considered themselves too fat but also in those who believed they were too thin (lacking sufficient muscle), aligning with our findings that perceiving oneself as too thin encourages physical activity. In contrast, the relationship between subjective body shape perception and sedentary behavior differed between boys and girls. Among girls, there was no significant association between subjective body shape perception and sedentary behavior. However, boys who perceived themselves as normal were less likely to engage in sedentary behavior compared to those who perceived themselves as obese. These findings, along with Lim’s [27] finding that girls are more likely than boys to sit for more than 2 hours per day and Min and Jung’s [35] finding that female adolescents have notably low adherence to sedentary behavior guidelines of less than 2 hours daily, suggests that girls generally exhibit higher levels of sedentary time compared to boys. This tendency, regardless of body shape perception, may account for the absence of a significant association observed in this study.

This study has several limitations. First, as the KYRBS relies on self-reported data collected online, there is a potential for overestimation or underestimation of physical activity and sedentary behavior. Second, Korea currently lacks clear guidelines for sedentary behavior, so classifications were based on previous studies and Canadian guidelines. This approach may not fully capture the current trends in sedentary behavior within Korea. Lastly, due to the cross-sectional design of this study, it is not possible to draw conclusions regarding causal relationships. Despite these limitations, this study has notable strengths. It utilized a representative sample of Korean adolescents and included data from a large-scale, nationwide study conducted over six years (2018–2023). Unlike previous research, which primarily examined the relationship between adolescents’ subjective body shape perceptions and variables such as mental health, nutritional intake, and weight control behaviors, this study investigated the association between subjective body shape perceptions and both physical activity and sedentary behaviors using long-term data. Moreover, while most body shape perception studies have predominantly focused on females, this study included both boys and girls, allowing for a gender-specific analysis and comparison.

Future research should consider that taking into account the current lack of clear guidelines for sedentary behavior, it may be necessary to classify sedentary time using additional thresholds, such as 4 and 6 hours beyond the 2-hour criterion used in this study, in order to enhance the robustness of the relationship between subjective body shape perceptions and sedentary behavior.

In conclusion, this study aimed to examine the relationship between physical activity and sedentary behavior based on subjective body shape perception, reflecting the growing emphasis on physical appearance among adolescents in Korea. The findings revealed that adolescents who perceived themselves as thin or normal were more likely to participate in physical activity compared to those who perceived themselves as obese. These results highlight the significant influence of body shape perception on physical activity. Consequently, it is recommended that the Ministry of Education acknowledge the impact of body shape perception on adolescents’ physical activity levels and utilize this information to design programs that encourage physical activity among students at higher risk of inactivity. Additionally, investigating adolescents’ subjective body shape perceptions could aid in developing educational resources to promote accurate body shape perceptions and emphasize their connection to physical activity. However, no statistically significant association was observed between subjective body shape perception and sedentary behavior. This underscores the need for establishing clear sedentary behavior guidelines in Korea. Future research should explore sedentary behavior in relation to subjective body shape perception by categorizing sedentary time based on newly developed guidelines. Furthermore, subsequent studies should compare the alignment between actual body shape and subjective body shape perception and analyze their associations with physical activity and sedentary behavior.