Background: An increasing number of physical activity (PA) interventions have been implemented to tackle child obesity epidemic, yet many have shown only moderate effectiveness. This is possibly due to a lack of in-depth understanding of the intrinsic motivators/demotivators to PA for children. Therefore, a main aim of this paper is to explore the intrinsic facilitators and barriers to PA participation through the lived experience of UK children (Study 1). The latter findings will facilitate the development of a psychometric instrument that assesses children’s tendencies to engage in repeated negative thoughts about PA (termed rumination ) which may hinder participation. Hence, our second aim is to develop and validate the PA-specific Rumination Scale for Children (PARSC) (Study 2). Methods: For Study 1, pedometry PA data were collected from 143 children (aged 6-10 years) over 3 weeks (Sample 1). Twenty-one focus groups were formed based on participants’ year group, sex and PA level. Focus group discussions were thematically analysed. For Study 2, the themes identified for the intrinsic barriers were used to develop PARSC. This was completed twice by 382 children (Sample 2), together with the PA subscale of the Physical Self-Description Questionnaire (short version). Sample 1 also completed the avoidant coping subscales of the Children Coping Strategies Checklist. Results: For Study 1, four overarching themes were identified for the intrinsic facilitators – sense of competence/accomplishments, cognitive motivator, sensations and socialisation/social facilitation. Four main themes for the intrinsic barriers were lack of competence, fear of negative experiences, external constraints and lacking a sense of purpose. For Study 2, results from Rasch analysis demonstrated that PARSC possessed sound internal validity and consistency, and test-retest reliability. Self-perceived PA and avoidant coping were predictive of PA-specific rumination tendencies (15% of variance explained), but objectively measured PA was not. Conclusions: The themes identified from the current study can potentially inform future PA interventions and PE curriculum for UK children. Also, PARSC can be a useful tool to assess children’s PA-specific rumination tendencies and to advance our understanding of the role of rumination in PA behaviour, but its applicability to other cultures warrants further investigations.