Physical fitness of children – Associations with time of school enrollment, the Covid-19 pandemic, school social status, and time of assessment within the school year

Background and research objectives. Physical fitness is related to children’s physical health, cognitive performance and well-being. Children’s fitness development is affected by child-specific characteristics like age or gender, and by factors associated with their living environment. The present dissertation examines four factors potentially associated with children’s fitness, namely (1) time of school enrollment, (2) the Covid-19 pandemic, (3) school social status, and (4) time of assessment within the school year. Time of school enrollment is associated with children’s academic performance, with some research indicating lower performances for children with delayed school entry (older-than-keyage children, OTK) compared to children enrolled based on the legal key date (keyage children). Research on the association between time of school enrollment and physical fitness is limited. We compared the fitness of keyage and OTK children at the same average ages and examined their longitudinal development over two years. The next research goal of this thesis was to examine whether temporary Covid-19 pandemic lockdowns were associated with changes in third-graders’ fitness. As previous reports indicated that pandemic effects may have differed by socioeconomic background, we tested whether Covid-19 pandemic effects and post-pandemic rebounds in the German Federal State of Brandenburg differed by school social status. The final goal of this dissertation was to dissociate effects of time of assessment within a school year from age effects on third-graders’ physical fitness. Dissociation of these effects is a methodological challenge, as better fitness later in the school year may be related to greater amounts of accumulated physical activity throughout the year, but also to children’s age-related growth. We thus tested whether children exhibited better physical fitness in the second than in the first half of third grade while adjusting for children’s ages. Methods. We used data from the EMOTIKON project, an annual assessment of third-graders’ physical fitness conducted in all public primary schools in the German Federal State of Brandenburg. Seven physical fitness tests assessed cardiorespiratory endurance (6-min run), coordination under time pressure (star-run), sprint speed (20m sprint), lower (powerLOW, standing long jump), and upper limbs muscle power (powerUP, ball-push test), static balance (one-legged-stance), and flexibility (stand-and-reach test). In the first paper, we compared keyage and OTK children at the average ages of 9.5 and 10.5 years (across different school grades), and examined their development from third to fifth grade. In the other papers, we used data from third-graders tested in different cohorts. In all papers, we conducted linear mixed models with random factors child and school. Regression discontinuity designs tested Covid pandemic effects or effects of time of assessment while adjusting for secular trends and children’s ages. Results. At the same average chronological ages, keyage children exhibited better physical fitness than OTK children. However, both groups developed positively and at similar rates from third to fifth grade in all fitness components (exception: cardiorespiratory endurance stagnated after fourth grade). The Covid-19 pandemic was associated with small fitness changes. In pandemic cohorts (2020–22), keyage third-graders exhibited lower cardiorespiratory endurance, coordination, speed, and powerUP, but better powerLOW and balance, compared to pre-pandemic cohorts (2016–19). When adjusting for secular trends, pandemic effects at the first day of school in school year 2020/21 were small and negative in cardiorespiratory endurance, coordination, speed, and powerUP. Post-pandemic improvements were restricted to coordination and powerUP. For OTK children, impairments in cardiorespiratory endurance, powerUP and balance were larger in pandemic than in pre-pandemic cohorts. Higher school social status was associated with better fitness, more pronounced pandemic effects in coordination and speed, and larger coordination rebounds. Finally, we found that physical fitness differs by time of assessment within the school year. When adjusting for children’s ages, keyage third-graders exhibited better coordination, speed, and powerUP in the second compared to the first school term. Fitness improvements from first to second school term were more pronounced in schools with lower average fitness. Conclusion. The present dissertation showed associations of children’s physical fitness with time of school enrollment, the Covid-19 pandemic, school social status, and time of assessment within the school year. Regular physical fitness assessments can inform about temporal health trends and identify groups or regions in need of health-promoting interventions. Children with delayed school enrollment and in schools with low social status may be at particular risk of low fitness. Pandemic-related fitness losses and assessment time effects illustrate the importance of sports opportunities for children’s fitness development. Interventions to improve fitness may target all systems relevant for childhood development, including the family, school, and governmental policies. Effects of time of school enrollment and time of assessment may be considered when developing norm values or when grading children’s performances.