## 2 Materials and Methods
### Statistical Analysis  
Data were tested for normality using the Shapiro-Wilk test and for homogeneity of variances using [PERSON]'s test. No major violations of assumptions were detected. Initial differences in physical performance variables across maturity groups were evaluated using a one-way analysis of variance (ANOVA) with a post hoc (Bonferroni) test to identify pairwise differences between maturity groups. A repeated-measures (RM) ANOVA was conducted, with post hoc (Bonferroni) analyses, to examine (1) the effect of the training programme by assessing differences in physical performance between assessment sessions  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Weeks** & \multicolumn{3}{c}{**Session 1 (LH)**} & \multicolumn{3}{c}{**Session 2 (SH)**} \\ \hline  & **Series** & **Work:Rest** & **\% vIFT** & **Series** & **Work:Rest** & **\% vIFT** \\ \hline
1–2 & 5 & 1\({}^{\prime}\):1\({}^{\prime}\) & 80\% & 10 & 30\({}^{\prime\prime}\):30\({}^{\prime\prime}\) & 90\% \\
3–4 & 5 & 1\({}^{\prime}\):30\({}^{\prime\prime}\) & 80\% & 10 & 30\({}^{\prime\prime}\):15\({}^{\prime\prime}\) & 90\% \\
5–6 & 5 & 1\({}^{\prime}\):1\({}^{\prime}\) & 85\% & 10 & 30\({}^{\prime\prime}\):30\({}^{\prime\prime}\) & 95\% \\
7–8 & 5 & 1\({}^{\prime}\):30\({}^{\prime\prime}\) & 85\% & 10 & 30\({}^{\prime\prime}\):15\({}^{\prime\prime}\) & 95\% \\ \hline \end{tabular}
\end{table}
Table 1: HIIT programme progression for LHs and SHs.  
1 and 2 for each group (EG and CG) and (2) the interaction effect (\"time \(\times\) group\") to determine whether the changes resulting from the training programme differed between the EG and CG. The effect size (ES) for ANOVA comparisons was calculated using partial eta squared (\(\eta\)p\({}^{2}\)). \(\eta\)p\({}^{2}\) is interpreted as follows: values of \(\eta\)p\({}^{2}\) = 0.01-0.05 indicate a small effect, values of \(\eta\)p\({}^{2}\) = 0.06-0.13 suggest a moderate effect, and values of \(\eta\)p\({}^{2}\) > 0.13 denote a large effect. ES at a 95% confidence interval (CI) for pairwise comparisons was calculated in Hedges g units, according to the recommendations of [PERSON] (2013) for small sample sizes and were interpreted as trivial (<0.24), small (0.25-0.49), moderate (0.50-0.99), and large (>1.00) [32].  
Regarding the analyses with the EG, the relationship between the variables related to the EL and the IL was analysed using the Pearson's correlation coefficient (r), and it was interpreted as trivial (\(\leq\)0.09), small (0.10-0.29), moderate (0.30-0.49), high (0.50-0.69), very high (0.70-0.89) and almost perfect (\(\geq\)0.90) [33]. The differences in EL and IL variables according to the kind of HIIT (LH and SH) were analysed by paired samples \(t\)-test. Another ANOVA (with a Bonferroni pairwise comparison) was used to analyse the differences in EL and IL variables between the maturity groups for each kind of HIIT separately and for the overall training sessions. For these analyses, each training session for every player was treated as an individual 'observation'.  
A further RM ANOVA (with Bonferroni post hoc testing) was conducted for each physical performance variable to examine the effect of the intervention (AS1 vs. AS2) across players with different maturity statuses (limited to two groups [Pre-PHY and Post-PHY]), as well as to assess the potential interaction effect of \"time \(\times\) maturational group\".  
All calculations were performed using Microsoft Excel (Microsoft, Seattle, WA, USA) and JASP software (JASP Team, Version 0.17.3), and the level of significance was set at \(p\) < 0.05.