The effects of body mass index on foot shape among U.S. children.

Understanding the three-dimensional shape of children's feet is valuable for designing appropriate footwear and orthotic devices, tracking growth trajectories, and supporting foot health and development. The most important anthropometric change in the U.S. in recent years is the increase in body mass at every age, but the effects of these trends on child foot shapes have not previously been studied. To address this gap, we collected high-resolution, three-dimensional (3d) foot surface data from 490 children ages 3-17 years across body mass index (BMI) from 12.4 to 52.6 kg/m2 and developed a statistical foot shape model to analyze the variations in foot morphology related to BMI. After accounting for age effects, a higher BMI was associated with differences in foot dimensions, including broader foot width, greater circumference, and increased thickness, as well as a larger cross-sectional area and volume. Higher BMI was also associated with lower arch height. Specific foot regions, including the medial and lateral sides, arch, and instep, showed more substantial shape differences with varying BMI levels. The observed differences were most pronounced in older children, with 10- and 17-year-olds showing the greatest discrepancy in foot dimensions. Differences in foot shape between median and 95th percentile BMI were much higher than the differences between the 5th percentile and median BMI, particularly concentrated in the hindfoot and anterior ankle area. This study provides the first comprehensive three-dimensional analysis of children's foot shape variations in relation to BMI, compared to previous research that focused primarily on linear dimensions. The findings suggest that the increasing body mass among children may necessitate the redesign of footwear to ensure a good fit. The foot shape model can be used to estimate foot shapes from demographic data, accurately fit low-resolution foot scan data, and aid in the design of footwear and orthotic devices. Future research should aim to collect more data from young children with high BMI, improve the representation of population subgroups, and include consideration of dynamic effects. The children's foot model is publicly available online at https://HumanShape.org/ChildFoot.