## 4 Discussion
### Hrv  
For the analysis of stress and recovery, handball players' vagal HRV parameter rMSSD was used to reflect the autonomous regulation of the athletes' nervous system. The rMSSD values at all measurement time points showed high standard deviations. Immediately after the acute intervention, a decrease of the mean rMSSD values in the supine and in the standing position was found for both age groups. This reflects the stress on the autonomic nervous system caused by the repeated high-intensity sprint exercise. During and immediately after the exercise, a high activation of the sympathetic nervous system. Consequently, greatly reduced rMSSD values compared to the baseline values were measured probably because the HR was still increased and the HRV normally decreases with a higher HR. One day after the exercise, the rMSSD values had already rebounded, and even risen above the PRE exercise values and remained elevated on the days following the exercise. This overcompensation of the rMSSD following an intensive exercise has already been shown in various previous studies [28; 29; 30]. Parasympathetic reactivity leads to higher cardiac vagal control, which improves the self-regulation of the system for further training stimuli [31]. This positive adaptation has only occurred with sufficient recovery in previous studies [28; 29]. However, the significantly higher mean rMSSD values of the U20s compared to the older players on the first day after the intervention could be explained by a faster recovery of youth players' autonomic system. These findings conform to the previously mentioned slower decrease of older players' CK values.  
There are some limitations of the current study and considerations for future research that should be addressed. Previous research that has investigated the effect of aging on muscle damage caused by resistance exercise, for example, has found contradicting results between men and women (e.g., [32; 33]). As the current study analyzed only male handball players, future studies should investigate for any differences between the genders, too. It should also be noted that the observations in the current study were differentiated between a group of \"young\" and \"older\" players, even though, the mean age of both groups was under 30 years. Therefore, it would be advisable for future research to compare the athletes below 20 years and above 30 years with each other in order to exclude the possibility that age transition is fluid, thereby making it difficult to reveal any stable age effects. Furthermore, the age span was significantly greater in the seniors (20-42 years) than in the U20 players (17-19 years). Attention should be paid in selecting groups with similar age ranges (e.g., a gap of 5 years within each group) in future studies. Another limitation of the current study is that some subjects had to be excluded from the statistical analyses since they skipped single tests at distinct measurement time points leading to differing sample sizes and hindering a generalization of the findings. The focus on semi-professional handball players, who had to be tested at five different measurement time points, made the data collection unique but also more difficult. Future research, therefore, could consider studying non-expert and amateur handball players' performance and recovery processes after the intervention. Changing or adapting the exercise could also be an option for future investigations. While the handball players in the current study performed a repeated high-intensity sprint exercise consisting of four sets of 6 \(\times\) 40 m all-out change-of-direction sprints, future research could adapt the sprint track, the intervals, or also the performance test, in ways that correspond more to the multidirectional requirements of the game.