## 4 Discussion
### Variations in Training Load According to Maturation  
The correlational analysis conducted in this study revealed a relationship between EL and IL for both LH and SH trainings, suggesting that the players' perceptions of effort were aligned with the external loads proposed by the researchers. This finding is consistent with previous research exploring the relationship between internal and external load in football [41]. Conversely, the subjective task intensity, assessed via RPE, correlated with the actual proposed intensity (vIFT) only for the LH, but not for the SH. This discrepancy may be attributed to the possibility that, in short-duration efforts, the rating of perceived exertion (RPE) might lack sensitivity to detect subtle variations in intensity, as the time spent at high intensity appears to be an influencing factor in effort perception [42]. In addition to this, the paired samples _t_-test revealed differences in intensity and load between LH and SH, indicating the appropriate application of the methods.  
However, the analysis of training load between players with different maturity statuses revealed that, despite the programmed EL being uniform for all groups, the individual responses (RPE and IL) to this stimulus differed significantly based on the players' maturity statuses. Post-PHY players report an inferior IL than their less mature (pre-PHY) peers for the same EL (same work:rest times and same % of vIFT). Several studies have highlighted the importance of distinguishing between EL (e.g., distance covered, number of sprints, or accelerations) and IL (e.g., s-RPE, heart rate) to adequately interpret the body's response to training [20,21]. This distinction enables coaches and strength and conditioning professionals to adjust and monitor loads to optimise adaptation and reduce injury risk [19]. In the present study, the strong correlation between EL and IL, as well as between time (regarded as a volume criterion) and IL, confirms the complex relationship between both types of loads.  
During this intervention, less mature players perceived higher physiological stress under the same EL. These results align with the findings of [PERSON] et al. [24], which expose differences in IL amongst maturation bands in a young football player sample [24]. The pre-PHY group of [PERSON]'s intervention reported a higher average heart rate and percentage of maximum heart rate as well; RPE was higher in pre-PHY players. Previous research has also described that RPE seems to be more associated with training level, maturation, and stage of development than with training conditions or demands [43]. Biological maturation influences in IL have also been observed in accumulated training loads throughout the season; [PERSON] et al. [44] indicate that when maturity status and accumulated training load were included as covariates, the statistically significant differences in physical performance variables disappeared [45]. Attending to EL variations, the previous literature suggests superior high-intensity runs performance during training tasks and matches, covering greater distances at high speed [22,46].  
These maturity-related differences in internal load perception may stem from both physiological and perceptual mechanisms. Less mature players may present lower running economy, reduced neuromuscular efficiency, and greater relative physiological strain, making the same external load feel more demanding. Furthermore, limited training experience, lower tolerance to discomfort, and developing self-regulation abilities may heighten the perceived exertion in pre-PHY players. These factors could explain the consistently higher IL and RPE values observed in less mature individuals exposed to standardised high-intensity stimuli.