## 3 Results
### Group \(\times\) Time Interaction Effects  
A two-way mixed ANOVA (group \(\times\) time) was conducted to examine whether the trajectory of physiological and cognitive outcomes differed between the HIIT and control groups across testing phases. Significant interaction effects were observed for several metrics, indicating differential adaptations over time between groups (Figure 2) (Table 1). A statistically significant interaction was found for OBLA (onset of blood lactate accumulation) velocity (F(2.22) = 6.35, \(p\) = 0.0040, \(\eta^{2}\) = 0.24). The HIIT group showed a \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variable** & **F-Value** & \(p\)**-Value** & **Domain** & **Description** \\ \hline
**OBLA speed (km-\(\mathrm{h^{-1}}\))** & 6.34 & 0.004 & Metabolic & OBLA speed—indicator of lactate threshold \\ \hline
**CRR at fatigue threshold** & 4.86 & 0.012 & Neuromuscular & Cardiac recovery ratio during fatigue threshold test \\ \hline
**Exercise-induced BDNF (14 km-\(\mathrm{h^{-1}}\))** & 6.28 & 0.005 & Cognitive & Exercise-induced BDNF concentration \\ \hline
**Resting blood pH** & 4.83 & 0.014 & Metabolic & Resting blood pH as a buffering indicator \\ \hline
**Post-exercise hemoglobin** & 5.70 & 0.007 & Metabolic & Post-exercise hemoglobin concentration \\ \hline
**AStandard bicarbonate** & 4.04 & 0.027 & Metabolic & Change in standard bicarbonate \\ \hline
**Acute cognitive load index** & 5.66 & 0.007 & Cognitive & Aggregate cognitive task demand \\ \hline
**Lactate accumulation index** & 5.57 & 0.008 & Metabolic & Rate of lactate accumulation per speed increment \\ \hline
**Lactate rise slope** & 6.70 & 0.003 & Metabolic & Slope of lactate increase with speed \\ \hline
**Reaction time at final stage** & 3.62 & 0.037 & Cognitive-Physical & Psychomotor speed at final test stage \\ \hline
**Cumulative cognitive load index** & 4.89 & 0.013 & Cognitive & Cumulative cognitive load over time \\ \hline
**Anaerobic fatigue index** & 3.77 & 0.035 & Metabolic & Drop in speed after lactate peak as fatigue index \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Variables Showing Significant Group \(\times\) Time Interaction Effects (\(p\) < 0.05). Figure 2: Longitudinal changes in key interaction-significant metrics. Line plots show mean values \(\pm\) 95% confidence intervals across R1–R3 phases for both HIIT and control groups. Each subplot (**A–F**) represents a metric with a statistically significant group \(\times\) time interaction. (**A**) Acute Cognitive Load Index, (**B**) Exercise-Induced BDNF (14 km-\(\mathrm{h^{-1}}\)), (**C**) Anaerobic Fatigue Index, (**D**) Lactate Accumulation Index, (**E**) OBLA Speed (km-\(\mathrm{h^{-1}}\)), and (**F**) Reaction Time at Final Stage. greater increase in OBLA speed (from \(\sim\)14.2 to 14.5 km h\({}^{-1}\)) compared to the control group (from \(\sim\)12.8 to 13.3 km\(\cdot\)h\({}^{-1}\)). Similarly, both the lactate accumulation index (F(2.22) = 5.58, \(p\) = 0.0084, \(\eta^{2}\)\(\approx\) 0.26) and the lactate rise slope (F(2.22) = 6.71, \(p\) = 0.0037, \(\eta^{2}\)\(\approx\) 0.30) demonstrated significant interactions. A large interaction effect was also found for exercise-induced BDNF levels (F(2.30) = 6.28, \(p\) = 0.0053, \(\eta^{2}\)\(\approx\) 0.30), with divergent trajectories between groups. The cognitive load index (F(2.34) = 5.67, \(p\) = 0.0075, \(\eta^{2}\) = 0.25) and cumulative cognitive load index (F(2.34) = 4.90, \(p\) = 0.0135, \(\eta^{2}\) = 0.22) also showed significant group \(\times\) time effects. The cerebrovascular index CRR at PFT (F(2.40) = 4.86, \(p\) = 0.0129, \(\eta^{2}\) = 0.20), the final-stage psychomotor reaction time (PSY\({}_{\rm last}\); F(2.34) = 3.62, \(p\) = 0.037, \(\eta^{2}\) = 0.18), post-exercise hemoglobin (F(2.32) = 5.70, \(p\) = 0.0076), the resting blood pH (F(2.32) = 4.83, \(p\) = 0.0147), standard bicarbonate change (F(2.32) = 4.04, \(p\) = 0.027), and the Anaerobic Fatigue Index (F(2.28) = 3.78, \(p\) = 0.035) all demonstrated statistically significant interactions.