## 3 Results  
The mean and standard deviation of the analyzed variables are reported in Table 1.  
[PERSON]'s correlation and R square values between the sprint performances and RSIs are reported in Table 2.  
As reported in Table 2, RSI RUN, NRSI RUN, RSI VDJ and VLEI only showed significant correlations (r from \(-0.97\) to \(0.87\)) with sprint performance. RSI RUN and NRSI RUN showed a nearly perfect correlation with all sprint variables (r from \(-0.96\) to \(0.96\)), while RSI VDJ showed very large correlations with sprint performance (r from \(-0.77\) to \(-0.76\); \(p<0.05\)). In addition, VLEI showed a very large correlation with sprint performance (r from \(-0.82\) to \(0.87\); \(p<0.05\)).  
[PERSON]'s correlation and R square values between the different RSIs are reported in Table 3.  
Among the data reported in Table 3, some correlations were also noted among the different indices. Specifically, RSI RUN showed a very large correlation with RSI VDJ (r = \(0.850\), \(p=0.002\)) and a large correlation with RSI SLR (r = \(0.632\), \(p=0.05\)). RSI VDJ showed a very large correlation with RSI HDJ (r = \(0.742\), \(p=0.014\)) and RSI SLR (r = \(0.761\)  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Variables** & **Mean \(\pm\) SD** & **Variables** & **Mean \(\pm\) SD** \\ \hline \hline \multicolumn{3}{l}{Sprint performance variables} & Horizontal jump test \\
30 m (s) & 4.16 \(\pm\) 0.48 & Contact time (s) & 0.30 \(\pm\) 0.07 \\
60 m (s) & 7.54 \(\pm\) 1.04 & Jump length (m) & 1.39 \(\pm\) 0.33 \\
30–60 m split time (s) & 3.35 \(\pm\) 0.51 & Jump length norm (m/BH) & 0.77 \(\pm\) 0.17 \\ SB 100 m (s) & 11.59 \(\pm\) 0.91 & RSI HDJ (a.u.) & 4.84 \(\pm\) 1.64 \\ Max speed (ms\({}^{-1}\)) & 9.10 \(\pm\) 1.29 & NRSI HDJ (a.u.) & 2.69 \(\pm\) 0.84 \\ Contact time sprint (s) & 0.12 \(\pm\) 0.01 & Straight-leg running drill test (SLR) \\ Step length sprint (m) & 2.18 \(\pm\) 0.20 & Contact time (s) & 0.148 \(\pm\) 0.02 \\ Step length norm (m/BH) & 1.23 \(\pm\) 0.10 & Step length (m) & 1.73 \(\pm\) 0.19 \\ RSI RUN (a.u.) & 19.27 \(\pm\) 3.50 & Step length norm (m/BH) & 0.98 \(\pm\) 0.12 \\ NRSI RUN (a.u.) & 10.79 \(\pm\) 1.76 & RSI SLR (a.u.) & 11.83 \(\pm\) 2.26 \\ Vertical jump test & & NRSI SLR (a.u.) & 6.68 \(\pm\) 1.33 \\ Contact time (s) & 0.22 \(\pm\) 0.03 & Ultrasound measurements \\ Rebound height (cm) & 47.29 \(\pm\) 10.30 & VLMT (mm) & 15.25 \(\pm\) 2.63 \\ RSI VDJ (a.u.) & 2.15 \(\pm\) 0.57 & VLEI & 42.23 \(\pm\) 5.88 \\ \hline \multicolumn{3}{l}{BH = body height; N = normalized by body height; NRSI HDJ = normalized reactive strength index (HD); NRSI RUN = normalized reactive strength index (RUN); NRSI SLR = normalized reactive strength index (SLR); RSI HDJ = reactive strength index of horizontal drop jump; RSI RUN = reactive strength index of sprint; RSI SLR = reactive strength index of straight-leg running drill test; RSI VDJ = reactive strength index of vertical drop jump; SB = seasonal best; VLEI = echo intensity of vastus lateralis muscle thickness.} \\ \end{tabular}
\end{table}
Table 1: Mean and SD of the performance variables and ultrasound measurements.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c} \hline \hline  & RSI RUN & **NRSI RUN** & **RSI VDJ** & **RSI HDJ** & **NRSI HDJ** & **RSI SLR** & **NRSI SLR** & **VIMT** & **VLEI** \\  & **r** & **R2** & **r** & **R2** & **r** & **R2** & **r** & **R2** & **r** & **R2** & **r** & **R2** & **r** & **R2** & **r** & **R2** \\ \hline
30 m (s) & -0.95\(\pm\) & 0.91\(\pm\) & -0.91\(\pm\) & 0.  
\(p\)= 0.011). Therefore, it is interesting to note that RSIDVJ is the only index correlated with all other indices analyzed in this study.  
Pearson's correlations and R square values between RSIs and ultrasound measurements are reported in Table 4.  
The results reported in Table 4 show only a significant very large correlation between VLEI and RSI RUN and NRSI RUN (r = \(-\)0.80 and \(-\)0.70, respectively).