## 3 Results
### Training-Induced Changes over Time  
In the control group (standard training), several fitness parameters declined significantly across the 4-week period, consistent with a detraining or insufficient stimulus effect (notable given the elite status of players). Most concerning, the control group's relative VO\({}_{2}\)max dropped by -11% from baseline (from \(\sim\)56.9 to 50.6 mL\(\cdot\)kg\({}^{-1}\cdot\)min\({}^{-1}\)), a significant decline (\(F\)(2.16) = 5.47, \(p\) = 0.0176, \(n^{2}\approx\) 0.27). The absolute VO\({}_{2}\)max also showed a downward trend (\(-\)6%, \(p\) = 0.064). Markers of anaerobic endurance deteriorated: the control group exhibited a significant increase in lactate accumulation (i.e., worse lactate handling), with the lactate accumulation index rising (time effect \(F\approx\) 15.3, \(p\) = 0.0003, \(\eta^{2}\approx\) 0.45) and the lactate rise slope steepening (\(F\approx\) 8.96, \(p\) = 0.0031), indicating that at a given work rate they accumulated lactate faster by the end of the study. Consistently, the control's peak blood [La] tended to increase (mean +0.8 mmol/L, \(p\) > 0.2) and the velocity at the OBLA (onset of blood lactate \(\sim\)4 mmol) improved only modestly (\(p\) = 0.074, n.s. ), despite normal soccer training. Maximal ventilation in the control group dropped significantly (\(F\)(2.18) = 3.89, \(p\) = 0.045), as did VCO\({}_{2}\)max (\(-\)14% by Week 4), suggesting a reduction in peak exercise capacity. Unlike the HIIT group, the control showed a blunted BDNF response to exercise over time: the BDNF measured during submaximal exertion (e.g., running at 14 km\(\cdot\)h\({}^{-1}\)) fell markedly in the control group (\(F\)(2.22) = 13.33, \(p\) = 0.0009), whereas it was maintained in HIIT. There were also slight impairments in heart rate recovery in the control group: the post-exercise HR drop (1 min recovery) became smaller (from \(\sim\)27 to 25 bpm, \(F\)(2.18) = 3.47, \(p\) = 0.0508), although the resting HR and HR\({}_{\rm reserve}\) remained unchanged. On the positive side, the control group did exhibit a small improvement in lactate threshold speed (+\(\sim\)1.2 km\(\cdot\)h\({}^{-1}\) on average) and maintained critical speed gains similar to HIIT (CS \(p\) = 0.020). However, these improvements did not reach statistical significance in the control group (e.g., lactate threshold \(p\) = 0.074), and were accompanied by declines in other domains. No significant within-group changes were seen in the control's cognitive performance scores or cognitive load metrics, though there was a non-significant trend of worsening cognitive-load tolerance mid-study. The HIIT group demonstrated significant improvements in several performance metrics over time. Notably, the critical running speed (CS) increased significantly (e.g., CS in m\(\cdot\)s\({}^{-1}\): \(F\)(2.14) = 6.58, \(p\) = 0.0096, \(\eta^{2}\approx\) 0.41; CS in km\(\cdot\)h\({}^{-1}\): \(F\)(2.14) = 6.60, \(p\) = 0.0096) in the HIIT group, indicating enhanced high-intensity running capacity. By contrast, the associated anaerobic capacity d\({}^{\prime}\) (running distance above critical speed) showed no significant change (\(p\) > 0.5). Aerobic metabolic performance was largely maintained in the HIIT group: neither the absolute VO\({}_{2}\)max (\(F\)(2.18) = 1.23, \(p\) = 0.320) nor the relative VO\({}_{2}\)max (mL\(\cdot\)kg\({}^{-1}\cdot\)min\({}^{-1}\)) changed significantly over the 4 weeks (\(p\) = 0.999)--reflecting that the intervention group sustained their aerobic fitness level while the control declined (see below). The maximal VE (ventilation) and VCO\({}_{2}\) showed no significant time effect in HIIT (\(p\) > 0.9), and HR\({}_{\rm max}\) remained unchanged as expected (no training effect on maximal heart rate). In terms of lactate dynamics, the HIIT group did not show significant changes in lactate threshold speed or peak blood lactate (both \(p\) > 0.2).