Objectives Physical activity (PA) is important for children's health but entails an inherent risk of injuries. The objective of this study was to assess activity-related correlates of injuries in children of the general population under the age of 10 while accounting for PA behaviour objectively assessed with accelerometers.

Design Cross-sectional.

Setting Primary schools in Switzerland.

Participants 41 (56.9%) of 72 contacted schools were eligible. 11 (26.9%) of them agreed to participate. 3 more schools were recruited with a snowball system. On the individual level, 83.7% of the parents gave consent. Finally, 249 children with complete data (82.2%) from 20 grade 1–3 classes from 14 schools were analysed (mean age 7.9 years, 49.4% girls).

Primary outcome measures Outcome measures were retrospectively assessed injury incidence rates expressed as the number of injuries per 1000 h of objectively measured moderate-to-vigorous physical activity (MVPA) and injury risk depending on levels of PA, aerobic fitness and motor coordination, derived from logistic regression models.

Results 0.43 injuries/1000 h of MVPA (95% CI 0.28 to 0.58) were reported. After controlling for sociodemographic characteristics and accounting for exposure to PA, children with medium and high levels compared with those with low levels of aerobic fitness assessed with the 20 m shuttle run test were at decreased injury risk (OR=0.37 (95% CI 0.16 to 0.85)/OR=0.29 (0.16 to 0.63)). Children with high motor coordination scores assessed with the “Körperkoordinationstest für Kinder” test were at increased injury risk compared with those in the normal range (2.59 (1.04 to 6.32)). Levels of objectively assessed PA were not associated with injury risk; they were neither expressed as rates nor as cumulative incidence.

Conclusions This study provides novel data showing that low levels of fitness and high coordinative skills, but not objectively assessed levels of PA, were related to injury risk in children under the age of 10.