Physical activity and smartphone use were linked to subjective body-shape ratings in different ways for Korean adolescent boys and girls, according to a five-wave analysis. Among boys, when the same adolescents reported more physical activity over time, their body-shape scores were lower. Among girls, higher weekday smartphone-use scores coincided with higher body-shape scores. Weekend smartphone use was not significantly associated with body-shape scores in either group, and physical activity was not significantly associated with girls' body-shape scores.
The relationship between activity and smartphone use also differed by group. Among girls, higher physical-activity scores coincided with lower weekday and weekend smartphone-use scores. Among boys, the corresponding associations did not meet the study's statistical significance threshold. The reported p-values were below 0.001 for the girls' weekday association and below 0.01 for the weekend association, compared with 0.08 and 0.87 for boys.
How the comparisons were made
The researchers used five waves of the Korean Youth Health Behavior Panel Survey, collected from 2019 through 2023. The panel followed sixth-grade students in Korea and was drawn using multistage stratified cluster sampling. The final complete-case analysis included 4,518 adolescents: 2,341 boys and 2,177 girls. They contributed 18,721 observations, and each participant had to be observed at least twice.
To focus on change within people, the analysis used individual and survey-wave fixed effects, a method that accounts for repeated observations and differences between waves. One model included control variables; a second added physical activity and weekday and weekend smartphone use. Parallel models examined whether physical-activity changes were associated with changes in smartphone use.
The measures were short rating scales. Physical activity ran from 1 to 8, smartphone use from 1 to 9, and subjective body-shape perception from 1 to 5. The outcome was a report of perceived body shape, not a measure of overall body image. Because the key variables were single-item indicators, the scores may not capture the full range of adolescents' activity, phone use or body-related perceptions.
Why the findings need care
Across the five survey waves, physical activity generally declined while weekday and weekend smartphone use increased. Boys reported more physical activity, and girls reported more smartphone use. Subjective body-shape scores tended to decrease among boys but increase among girls. These broad trends describe what was reported across waves; they do not establish that one behavior caused the other or caused changes in body-shape perception.
That distinction matters when interpreting the boys' result. A lower subjective body-shape score should not automatically be read as a healthier body image, particularly among boys, because the study measured body shape rather than broader body image. Single-item measures may not fully represent these behaviors and perceptions.
Overall, the findings describe associations, not proof that one behavior caused changes in the other or in body-shape perception. The evidence is limited to how reported scores changed together within Korean adolescents over time.
Paper data and sources
Original title: Move more, scroll less? Physical activity, smartphone use, and subjective body shape perception in adolescents: within-person associations in gender-stratified analyses
Authors: Kim J, Kang S
Journal/Repository: Frontiers in public health
Status: Peer-reviewed
First online: 2026-10-07
DOI: Not available
Original paper · Full text