Journal of Medical Internet Research - International Scientific Journal for Medical Research, Information and Communication on the Internet #Preprint #PeerReviewMe: Warning: This is a unreviewed preprint. Readers are warned that the document has not been peer-reviewed by expert/patient reviewers or an academic editor, may contain misleading claims, and is likely to undergo changes before final publication, if accepted, or may have been rejected/withdrawn. Readers with interest and expertise are encouraged to sign up as peer-reviewer, if the paper is within an open peer-review period. Please cite this preprint only for review purposes or for grant applications and CVs (if you are the author). Background: The UK government recommends that children engage in at least 60 minutes moderate-to-vigorous physical activity every day. Despite associated physiological and psychosocial benefits of physical activity (PA), many youth fail to meet these guidelines due partly to technology-based activities displacing active behaviours. However, technological advances such as 3D printing have enabled innovative methods of visualising and conceptualising PA as a tangible output. Objective: The purpose of the formative study was to elicit children’s, adolescent’s, parents’ and teachers’ perceptions and understanding of 3D PA objects to inform the design of future 3D models of PA. Methods: Twenty-eight primary school children (8.4 ± 0.3 years; 15 boys) and forty-two secondary school adolescents (14.4 ± 0.3 years; 22 boys) participated in semi-structured focus groups, with inidividual interviews conducted with 8 teachers (2 male) and 7 parents (2 male). Questions addressed understanding of the PA guidelines, 3D model design, both motivation for, and potential engagement with, a 3D PA model intervention. Pupils were asked to use Play-Doh™ to create and describe a model that could represent their PAL (PAL). Data were transcribed verbatim, thematically analyzed and key emergent themes represented using pen profiles. Results: Pupils understood the concept of visualising PA as a 3D object, although adolescents were more able to analyse and critique differences between low and high PAL. Both youths and adults preferred a 3D model representing a week of PA data when compared to other temporal representations. Furthermore, all participants highlighted that 3D models could act as a motivational tool to enhance youths PA. From the Play-Doh™ designs, two key themes were identified by pupils, with preferences indicated for PA models of abstract representations of PA or bar charts depicting PA, respectively. Conclusions: These novel findings highlight the potential utility of 3D objects of PA as a mechanism to enhance children’s and adolescents’ understanding of, and motivation to increase, their PAL. This study suggests that 3D printing may offer a unique strategy for promoting PA in these groups.