Physical fitness (PF) is closely related to body constitution, with either height-to-mass ratio (HMR) or body mass index (BMI) as indicators. We compared these indicators for 24,777 third-grade children from 2017-2022 cohorts in cardiorespiratory endurance (6-min run), coordination (star-run), speed (20-m sprint), lower-(standing long jump) and upper limb (ball-push test) muscle power, and static balance (one-legged-stance test; eyes closed). Quadratic HMR predicts children's physical fitness better than cubic BMI after adjustment for secular trends and the COVID-19 pandemic effects on PF. Except for powerUP, PF scores decreased with increasing body mass relative to height (low HMRs). Boys outperformed girls in five tests (exception balance), but their performance losses were larger with increasing overweight. Surprisingly, after adjustment for quadratic HMR trends, the main effects of sex favored girls (exception powerUP). We propose children's motor performance, usually normed with age- and sex-specific categories, should be assessed with body height-to-mass ratio.

The dissociation of effects of age, time of assessment and cohort is a well-known challenge in developmental science. We examined effects of time of assessment in the school year on children’s physical fitness using data from 75,362 German third-graders from seven cohorts. Children were tested once either in the first or second school term of third grade. Tests examined cardiorespiratory endurance (6-min run), coordination (star-run), speed (20-m sprint), lower (standing long jump) and upper (ball-push test) limbs muscle power, and flexibility (stand-and-reach test). We estimated the effect of time of assessment using a regression discontinuity design specified in a linear mixed model with random factors child and school and adjusted for age and cohort effects. Coordination, speed, and upper limbs muscle power were better in second compared to first school term, with boys exhibiting a larger increase of upper limbs muscle power than girls. There was no evidence for changes in cardiorespiratory endurance, lower limbs muscle power, and flexibility between assessments. Previously reported age and sex effects as well as secular fitness trends were replicated. Thus, there is evidence for improvement of some physical fitness components beyond age and cohort effects that presumably reflects the benefit of physical education. Effects of assessment time should be taken into consideration in performance-based grading or norm-based selection of children.

Background. In spring of 2020, the Sars-CoV-2 incidence rate increased rapidly in Germany and around the world. Throughout the next 2 years, schools were temporarily closed and social distancing measures were put in place to slow the spread of the Covid-19 virus. Did these social restrictions and temporary school lockdowns affect chil- dren’s physical fitness? The EMOTIKON project annually tests the physical fitness of all third-graders in the Federal State of Brandenburg, Germany. The tests assess cardiorespiratory endurance (6-min-run test), coordination (star-run test), speed (20-m sprint test), lower (powerLOW, standing long jump test), and upper (powerUP, ball-push test) limbs muscle power, and static balance (one-legged stance test with eyes closed). A total of 125,893 children were tested in the falls from 2016 to 2022. Primary analyses focused on 98,510 keyage third-graders (i.e., school enrollment accord- ing to the legal key date, aged 8 to 9 years) from 515 schools. Secondary analyses included 27,383 older-than-keyage third-graders (i.e., OTK, delayed school enrollment or repetition of a grade, aged 9 to 10 years), who have been shown to exhibit lower physical fitness than expected for their age. Linear mixed models fitted pre-pandemic quadratic secu- lar trends, and took into account differences between children and schools. Results. Third-graders exhibited lower cardiorespiratory endurance, coordination, speed and powerUP in the Covid pandemic cohorts (2020–2022) compared to the pre-pandemic cohorts (2016–2019). Children’s powerLOW and static balance were higher in the pandemic cohorts compared to the pre-pandemic cohorts. From 2020 to 2021, coor- dination, powerLOW and powerUP further declined. Evidence for some post-pandemic physical fitness catch-up was restricted to powerUP. Cohen’s |ds| for comparisons of the pandemic cohorts 2020–2022 with pre-pandemic cohorts 2016–2019 ranged from 0.02 for powerLOW to 0.15 for coordination. Within the pandemic cohorts, keyage children exhibited developmental losses ranging from approximately 1 month for speed to 5 months for cardiorespi- ratory endurance. For powerLOW and static balance, the positive pandemic effects translate to developmental gains of 1 and 7 months, respectively. Pre-pandemic secular trends may account for some of the observed differences between pandemic and pre-pandemic cohorts, especially in powerLOW, powerUP and static balance. The pandemic further increased developmental delays of OTK children in cardiorespiratory endurance, powerUP and balance. Conclusions. The Covid-19 pandemic was associated with declines in several physical fitness components in German third-graders. Pandemic effects are still visible in 2022. Health-related interventions should specifically target those physical fitness components that were negatively affected by the pandemic (cardiorespiratory endurance, coordina- tion, speed).

Previous research has shown that children who were enrolled to school according to the legal key date (i.e., keyage children, between eight and nine years in third grade) exhibited a linear physical fitness development in the ninth year of life. In contrast, children who were enrolled with a delay (i.e., older-than-keyage children [OTK], between nine and ten years in third grade) exhibited a lower physical fitness compared to what would be expected for their age. In these studies, cross-sectional age differences within third grade and timing of school enrollment were confounded. The present study investigated the longitudinal development of keyage and OTK children from third to fifth grade. This design also afforded a comparison of the two groups at the same average chronological age, that is a dissociation of the effects of timing of school enrollment and age. We tested six physical fitness components: cardiorespiratory endurance, coordination, speed, power of lower and upper limbs, and static balance. 1502 children (i.e., 1206 keyage and 296 OTK children) from 35 schools were tested in third, fourth, and fifth grade. Except for cardiorespiratory endurance, both groups developed from third to fourth and from fourth to fifth grade and keyage children outperformed OTK children at the average ages of 9.5 or 10.5 years. For cardiorespiratory endurance, there was no significant gain from fourth to fifth grade and keyage and OTK children did not differ significantly at 10.5 years of age. One reason for a delayed school enrollment could be that a child is (or is perceived as) biologically younger than their chronological age at the school entry examination, implying a negative correlation between chronological and biological age for OTK children. Indeed, a simple reflection of chronological age brought the developmental rate of the chronologically youngest OTK children in line with the developmental rate observed for keyage children, but did not eliminate all differences. The mapping of chronological and biological age of OTK children and other possible reasons for lower physical fitness of OTK children remain a task for future research.

Timing of initial school enrollment may vary considerably for various reasons such as early or delayed enrollment, skipped or repeated school classes. Accordingly, the age range within school grades includes older-(OTK) and younger-than-keyage (YTK) children. Hardly any information is available on the impact of timing of school enrollment on physical fitness. There is evidence from a related research topic showing large differences in academic performance between OTK and YTK children versus keyage children. Thus, the aim of this study was to compare physical fitness of OTK (N = 26,540) and YTK (N = 2586) children versus keyage children (N = 108,295) in a representative sample of German third graders. Physical fitness tests comprised cardiorespiratory endurance, coordination, speed, lower, and upper limbs muscle power. Predictions of physical fitness performance for YTK and OTK children were estimated using data from keyage children by taking age, sex, school, and assessment year into account. Data were annually recorded between 2011 and 2019. The difference between observed and predicted z-scores yielded a delta z-score that was used as a dependent variable in the linear mixed models. Findings indicate that OTK children showed poorer performance compared to keyage children, especially in coordination, and that YTK children outperformed keyage children, especially in coordination. Teachers should be aware that OTK children show poorer physical fitness performance compared to keyage children.