## 4 Discussion
However, the growth spurt is preceded by a rapid increase in body fatness, also known as the 'pre-pubertal fat wave' [36], and a rapid increase in fat free mass can subsequently be observed, particularly in boys [37]. However, a high variability in the timing of biological maturation can be observed between individuals of the same sex, resulting in early and late maturers [36; 37]. Handball players should be able to perform repeated high-intensity actions, and well-developed intermittent aerobic fitness is considered an important determinant for handball players at the elite level [27]. In the current study, Yo-Yo IR1 performance was similar between groups, although large variations existed within groups, which may have resulted in differences not reaching the \(p<0.05\) level. The difference in Yo-Yo IR 1 performance between age groups differed from 3% (U11 to U12) to 38% (U10 to U11). Significantly better performance (11%) for an endurance shuttle run was found by [PERSON] et al. [38] in elite U16 handball players but not within our sample. The results of the current study confirm that intermittent aerobic fitness should be well-developed in youth handball players in order to maintain intermittent efforts during games [27]. Unfortunately, to the best of the authors' knowledge, no information is available in the international scientific literature regarding Yo-Yo IR1 test performance in youth, male team handball players. In addition to competitive level and gender, differences in Yo-Yo IR1 performance may also be dependent on the period of the competitive season when the test is performed. For throwing performance, the average difference between groups was 6% (U10 to U11) and 15% (U11 to U12). Team handball is characterized by explosive actions performed at high velocities; therefore, success in competition depends on the well-developed muscular power of the upper and lower limbs [39; 40]. The higher values of maximum power and strength of the upper limbs provide a clear advantage for maintaining repeated muscle contractions during an entire match. Becausestrength and power are thought to be important for throwing velocity, the medicine ball throw is an optimal test/movement to predict the power of upper limbs in handball players. These findings provide information for the assessment and evaluation of player performance, and they may help to develop and optimize positional training regimes. These findings may also be beneficial in the prevention, evaluation, and treatment of injuries commonly sustained by handball players. In the present study, body fat was negatively associated with all performance variables with the exception of sprint performance. We observed that a lower body fat was negatively associated with sprint performance. In contrast to body fat, the BMI was not found to be associated with performance parameters (15 vs. 2 relevant correlation coefficients). Interestingly, in contrast to the U10 and U11 groups, a large body height led to a relevant reduction of 15 m sprint performance in the U12 group. This could have been a result of the growth process in conjunction with a temporary reduction of the coordination of the lower and upper limbs. The main limitation of the present study was its cross-sectional design and that sexual maturation was not considered. A previous study indicated that maturation status explains a small part of the variance in aerobic fitness independent of %BF [41]. We believe this increased the variability within our sample, as demonstrated by the large SDs and 95% CIs in anthropometry measures. To rectify this limitation, an increased sample size or maturation status may add additional insight in future investigations. Thus, future studies should include an assessment of sexual maturation when examining the relationship between anthropometry and physical performance in youth. Since the design of this study was cross-sectional, it was hard to investigate causality in the studied relationships.