The main purpose of the study was to test the associations between cardiorespiratory fitness with motor performance and body size in 7-14-year-old children. The main findings are: 1) cardiorespiratory fitness predicts between 1–40% of the variance in performance in multiple other physical fitness components in boys, 2) in girls, cardiorespiratory fitness predicts between 7–22% of the variance in performance in multiple other physical fitness components and 3) when adjusting for age, the percentage of variance shared between cardiorespiratory fitness and multiple other physical fitness components slightly declines.
Our results of cardiorespiratory fitness being most strongly associated to 10 × 5 shuttle run, standing long jump and sit-ups in 30 sec are in line with previous findings obtained among a large sample of Lithuanian school aged children . Specifically, a study by Venckunas et al.  has shown that variance of performance in 10 × 5 shuttle run and standing long jump were each explained by cardiorespiratory fitness the strongest (> 10%), followed by the association between cardiorespiratory fitness with the abilities in bent arm hang and sit-ups (cardiorespiratory fitness explaining ∼6.5% of the variance of the performance in these tests), as well as in balance and sit-and-reach tasks (aerobic capacity significantly explaining ∼3% of the variance). It has been hypothesized, that for 10 × 5 shuttle run and standing long jump performance, movement patterns are similar and the same muscle groups (i.e. leg extensors) need to be involved for locomotion . Another potential mechanism may be the nature of these activities, which require different jumping, accelerating and decelerating performances deserving for intrinsic musculoskeletal characteristics, synchronizing upper and lower body and gaining appropriate momentum . Also, the aforementioned tasks fall under weight-bearing exercises, which share similar moving patterns. Indeed, studies have shown that cardiorespiratory fitness is associated with anaerobic functional capacities required for performing agility and power/strength tasks .
The strongest associations between cardiorespiratory and muscular fitness are not surprising, since higher levels of these components reduce the risk of all-cause mortality [21, 22] and are often interrelated . From the perspective in sport, evidence suggests that low cardiorespiratory fitness may be compensated for additional muscle training stimulus in aerobic endurance athletes , pointing out that all physical fitness components should be equally developed and enhanced across the lifespan. This supports the findings from previous studies, stating that being involved in endurance sport is not associated with an increased life expectancy . Therefore, physical fitness, as a multifactorial construct, is the best non-communicable factor remotely associated to health [3–7]. Nevertheless, the critical period when physical fitness (especially cardiorespiratory fitness) should be trained is during the childhood period, since it successfully predicts the development of cardiovascular diseases in later life .
This study has a few limitations. First, by using a cross-sectional design, we cannot determine the causality of the association, that is multiple other physical fitness components were associated to cardiorespiratory fitness. Second, we randomly selected schools and classes for the purpose of this study and achieved an acceptable response rate. Nevertheless, more physically active families are more prone to participating in the studies of such nature . Thus, potential selection bias cannot be excluded. Third, the proxy of cardiorespiratory fitness was assessed through the 20-m shuttle run test. Although this test has been widely used and the reliability and validity properties have been confirmed , treadmill or bicycle ergometers may have given somewhat different maximal oxygen uptake values and associations between cardiorespiratory fitness with multiple other physical fitness components. Therefore, future research on the same topic needs to be longitudinal with more objective methods to assess the level of cardiorespiratory fitness in school aged children.