The aim of this cross-sectional study was to examine physical fitness of YTK and OTK children versus keyage children in a sample of German third graders. Our findings indicate that (i) OTK children show impaired performance compared to keyage children, especially in coordination, (ii) OTK girls outperform OTK boys, (iii) YTK children show better results than keyage children, especially in coordination, and (iv) four of the five tests represent a common construct (i.e., correlate strongly and positively with each other), except for the ball push test that tests upper limbs muscle power.
The child-related CPs between test scores were the same for YTK and OTK children. Consequently, the five tests represent a latent construct “physical fitness” for differences between children. However, the correlations with powerUP were distinctly smaller. Of note, the ball-push test is the only non-weight-bearing test within the test battery. In other words, the participants do not have to accelerate their own body mass during the test. It seems that foremost OTK children are able to compensate their delayed fitness or cognitive development through other factors which positively affect the performance in a non-weight-bearing test such as the ball-push test. It is possible that the onset of maturation associated with increased body mass, body height and muscle mass39 may further the performance in the ball push test especially for OTK children. Ceschia et al.40 examined associations between performance in physical fitness and the body mass index in 2,411 healthy Caucasian children aged 7 to 11 years. The analysis revealed that performance in weight-bearing tests (e.g., 500 m run test, 20-m linear sprint test, standing long jump test) was negatively affected by high body mass. In contrast, performance in non-weight-bearing tests (e.g., medicine ball forward throw test, handgrip test) was positively associated with high body mass, irrespective of sex. The results were confirmed by other studies41,42. Therefore, it can be concluded that performance for upper limbs muscle power was positively influenced by high body mass. Unfortunately, anthropometric factors such as body mass were not assessed in this study which is why we were unable to confirm this hypothesis based on our own data.
Several studies confirmed a linear increase in physical fitness performance with age9–11, 15. For instance, in a study with 424,328 Greek children and adolescents aged 6 to 18 years, Tambalis et al.15 reported a linear increase in physical fitness performance with age for cardiorespiratory endurance (i.e., 20-m shuttle run test), lower limbs muscle power (i.e., standing long jump test), flexibility (i.e., sit-and-reach test), muscular strength (i.e., sit-ups test), and agility (i.e., 10 × 5 m shuttle run test). The development of physical fitness of keyage children (see predicted gray lines in Figure 2) is in accordance with the above reported results. For keyage children, physical fitness performance increased linearly with age. However, the development of physical fitness for OTK children is different. Impaired performance was found in OTK children aged 9.00 to 9.99 years compared with keyage children (i.e., 8.00 to 8.99 years) for all components of physical fitness, especially for coordination. This could be due to the fact that third graders aged 9.00 to 9.99 years (i.e., OTK children) are not representative for the “average” third grader which is why we observed a deviation from the typically reported fitness development with age in this cohort9–11, 15. We do not know the exact circumstances which lead to the delayed enrollment into first grade or to the repetition of a school year. According to our results, we can only speculate that maybe a delay in cognitive development might be the reason why children are late enrolled into first grade or must repeat a school year. Of note, the coordination test requires high levels of motor coordination. OTK children achieved the lowest scores compared with the other two groups in the coordination tests. This is in line with a study by Urschitz et al.27. In a cohort of 1,144 German third graders, these authors observed that impaired academic performance significantly increased with age for all school subjects (i.e., mathematics, science, reading, spelling, and handwriting) and that especially children who had repeated a school year were more prone to impaired academic performance. These results were confirmed by other studies25,26. Interestingly, in our study OTK girls showed better performance compared to OTK boys which is in accordance with Urschitz et al.27. These authors reported that except for mathematics, boys showed a larger prevalence for poor academic performance compared with girls27. As girls mature approximately two years earlier than boys, the better performance of girls compared to boys might be influenced by biological maturation. Girls enter the adolescent growth spurt at approximately ten years of age and peak height velocity at 12 years, whereas boys enter the growth spurt on average at age 12 and peak height velocity at 1439.
In contrast, YTK children outperformed keyage children especially in tests requiring motor coordination. Again, we do not know the exact circumstances which resulted in early enrollment into first grade or reasons for skipping a school year. According to our results, we speculate that accelerated cognitive development could be a reason why early enrolled children skip a school year. This is supported by the fact that in this study, YTK children showed the best performance in the coordination test which has an inherent large cognitive demand. Moreover, findings from Martin26 point in a similar direction by showing that in a cohort of 3,684 Australian high school students, YTK children outperformed keyage children in academic performance.
Our study is not without limitations. First, anthropometric factors such as body mass, body height, and sitting height were not assessed in this study so that associations between anthropometric factors, biological maturation, and physical fitness could not be calculated. These factors would have provided additional insight as there is strong evidence that children’s physical fitness is associated with anthropometric characteristics40–42 and biological maturation39. One explanation of the deviation of YKT and OKT children might be a difference between chronological and biological age. It appears plausible to argue that YKT children may be more mature and that OKT children are biologically somewhat younger than indicated by their chronological age. Thus, in a hypothetical plot of performance over biological age, the linear trend may well hold for all children. Second, we predicted the performance of the YTK and OTK children based on a linear extrapolation recently reported by Fühner et al.24. However, we do not know if this linear extrapolation exactly fits to the data of keyage children aged 7.50 to 7.99 / 9.00 to 9.99 years as we do not have such cross-sectional data.
To sum up, this study is the first study that examined differences in physical fitness development of YTK and OTK children compared to keyage children. Our study findings complement results reported in the literature on the development of academic performance in youth25–27. Politicians and decision makers, schools, (physical education) teachers, and parents should be aware that despite their age advantage, OTK children often show impaired fitness compared to keyage children. Therefore, it is important to specifically target this “minority” within the population of third graders to specifically offer health and fitness programs to compensate the gap in fitness experienced during prior years.
The EMOTIKON test battery is easy-to-administer, cost effective, requires only minimal school specific equipment (e.g., stopwatch, measuring tape, medicine ball, pylons), and the tests represent a common construct of physical fitness. Therefore, physical education teachers, coaches, or researchers can use the EMOTIKON test battery to evaluate children’s physical fitness. However, they should be careful with the interpretation of the ball push test since this test does not measure physical fitness to the same degree than the other tests.