Gymnastik- och idrottshögskolan, GIH

Physical activity (PA) plays a key role in children's health, yet it remains unclear why girls are often less active and more sedentary than boys. This study examined genetic and environmental (shared and non-shared) influences on device-measured PA and sedentary time (SED) across girls and boys. This cross-sectional study contained data on zygosity, sex, PA, and SED for 2046 9-year-old twins in Sweden. The accelerometer-measured PA was expressed as the proportion of time in SED, light (LPA), moderate (MPA) or vigorous PA (VPA). Structural equation modelling within a twin framework estimated the heritability of PA and SED, decomposing variance into additive genetic (A), shared environment (C), and non-shared environment (E). Quantitative sex differences were examined by allowing the variance components (A, C, E) to differ across sexes. In the total cohort, genetic contribution was highest in VPA (Atotal: 0.64) but lower in SED, LPA and MPA (Atotal: 0.50-0.55). Boys and girls had differing patterns of genetic contributions, with the largest contributions among girls found for LPA (Agirl: 0.44; Aboy: 0.37) while the genetic contribution in VPA was larger in boys (Agirl: 0.15; Aboy: 0.57), and the difference was statistically significant for VPA (p = 0.001). While the non-shared environment was similar in all models, the shared environment showed the opposite pattern to the genetic contributions. This twin study shows that genetic influences on PA and SED differ by sex, highlighting the importance of considering how biological mechanisms that drive activity patterns might differ across sex, especially in vigorous physical activity.