## 4 Discussion
### Relationship Between Maturation and Training Adaptations  
The training intervention yielded an overall positive effect on 30 m sprint and vIFT performances, yet significant improvements were confined to the pre-PHY group, with no significant changes observed in the post-PHY group. This finding suggests that players with a less advanced maturity status may adapt more effectively to the HIIT stimulus employed in this study. Such differential responses could be underpinned by variations in perceived intensity and IL between maturity groups, as evidenced by the higher IL reported by pre-PHY players despite identical EL prescriptions. A greater internal response to training load, potentially reflecting heightened physiological stress or effort perception, may facilitate more pronounced improvements in sprint and intermittent endurance capacities, a notion supported by prior research linking IL to training adaptations in youth athletes [41]. Alternatively, the lower baseline physical performance levels of the pre-PHV group could indicate a greater adaptive potential, allowing for more substantial relative gains following training. This aligns with the principle that individuals with lower initial fitness levels often exhibit larger improvements due to a wider scope for physiological adaptation [47]. The significant enhancements in 5 m and 30 m sprint times within the pre-PHV group are consistent with traditional perspectives in the literature, which suggest that players prior to PHV may benefit more from training stimuli targeting neural and glyometric adaptations [48]. Pre-pubertal athletes may exhibit pronounced gains in sprint performance due to neuromuscular adaptations, such as improved motor unit recruitment and intermuscular coordination, rather than structural changes more typical in post-PHV individuals [49]. These findings are corroborated by studies demonstrating that sprint-specific interventions [50; 51] enhance sprint velocity in youth footballers, with effects potentially accentuated in less mature players. Long-term athlete development (LTAD) models, such as those proposed by [PERSON] et al. [52], further posit windows of opportunity for speed development prior to PHV, attributing these gains to the natural maturation of neuromuscular systems, which may be amplified by targeted training programmes [52]. However, conflicting findings have also been reported in this regard. For instance, the systematic review by [PERSON] et al. [53] demonstrated similar improvements across maturity groups following pyometric training interventions, despite the traditional assumption that pre-PHV individuals exhibit greater adaptability to such training stimuli [53]. Conversely, LTAD frameworks typically position the optimal window for endurance development during or post-PHV, driven by maturation-related enhancements in aerobic capacity and metabolic efficiency [54]. This contrasts with our results, where pre-PHV players exhibited significant vIFT improvements, suggesting that the HIIT protocol may have elicited endurance gains independent of traditional endurance-focused windows. The literature presents conflicting evidence regarding such windows for endurance trainability. For example, [PERSON] et al. [55] observed greater responsiveness to endurance training around PHV, whereas [PERSON] [56] identified substantial gains in pre-PHV. However, [PERSON] et al. [18] found that pre-, circum-, and post-pubertal children achieved comparable peak VO\({}_{2}\) improvements following intermittent endurance training, indicating that endurance adaptations may not be strictly tied to a specific maturational stage [52]. This variability underscores the influence of training intensity, duration, and individual factors over maturational timing alone, challenging the existence of a definitive endurance trainability window. An additional consideration is that the observed improvements in the pre-PHV group may primarily reflect enhancements in sprint velocity rather than cardiorespiratory or metabolic capacity.