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Monitoring fatigue and performance is important for adjusting training loads in soccer. Therefore, knowing the status of the player when applying a training stimulus is key to optimizing the players’ development. This study aims to evaluate the interaction between internal and external load, during training and matches, in an elite youth soccer team. Methods: seventeen youth players of the highest Spanish category were monitored with GPS devices during training and matches, as well as recording their nocturnal heart rate variability (HRV). We employed a linear mixed model to assess the physical demands between training and matches, and to compare the HRV variables. Results: a higher total distance (+2993.35–5746.56 m; ES = 1.4), distance at high intensity (+641.24–1907 m; ES = 1.5), sprint distance (+350.46–795.05 m; ES = 2.1), number of sprints (+18.38–41.58; ES = 1.9), and number of repeated sprints (+5.91–15.30; ES = 1.7) (all p < 0.001), but not in the number of accelerations, were reported during the matches when compared to the training sessions during the 11 weeks. The analysis of the HRV variables showed no significant differences between the accumulated values during a training week, providing similar results pre-match or post-match (p > 0.05). The LF/HFRATIO showed a negative influence on the total distance ran, distance at high intensity, distance in sprint, number of sprints, and repeated sprint. RRMEAN was positively related to the sprint number. Conclusion: the results of the present study suggest that nocturnal HRV variables are not different between pre-match and post-match. Furthermore, it suggests that LF/HFRATIO and RRMEAN during pre-match can determine the external load that the player will be able to complete during the match

It is well established that different types of exercise can provide a powerful stimulus for mitochondrial biogenesis. However, there are conflicting findings in the literature, and a consensus has not been reached regarding the efficacy of high-intensity exercise to promote mitochondrial biogenesis in humans. The purpose of this review is to examine current controversies in the field and to highlight some important methodological issues that need to be addressed to resolve existing conflicts