## 4 Discussion  
The first purpose of this study was to develop a new RSI (RSI RUN) calculated for a sprint by horizontal measures (step length) and foot contact times and to correlate this index with the maximum speed and the 100 m sprint performance of competitive track and field sprinters and compare this index with the traditional vertical-based RSI. The results of the present study show that the RSI RUN and the NRSI RUN were highly negatively correlated with the 100 m sprint time (r between \(-\)0.90 and \(-\)0.96), and positively correlated with the maximum speed (r = 0.97 and 0.90 for RSI RUN and NRSI RUN, respectively). RSI RUN is determined by contact times and stride lengths in a sprint; these are parameters that have been indicated as crucial for sprint performance [5; 29; 30], and in particular for the maximum running speed [6]. [PERSON] and colleagues [14] showed that the RSI obtained by an ankle rebound test was more correlated with the 60 m sprint performance than the traditional RSI (r= \(-\)0.49 and r = \(-\)0.07, respectively), especially when this index was calculated in the advanced phases of athlete acceleration (from 23.4 \(\pm\) 1.0 to 33.7 \(\pm\) 1.4 m). This parameter supports the key role of the reactive strength of the calf muscles in achieving high running speeds. The investigations conducted to date on the relationship between sprint performances and the RSI, however, mainly included short sprints (5 to 50 m) [16; 22; 31], and only a few authors considered the 100 m \begin{table}
\begin{tabular}{l c c c c c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**RSI RUN**} & \multicolumn{2}{c}{**NRSI RUN**} & \multicolumn{2}{c}{**RSI HDJ**} & \multicolumn{2}{c}{**NRSI HDJ**} & \multicolumn{2}{c}{**RSI VDJ**} & \multicolumn{2}{c}{**RSI SLR**} & \multicolumn{2}{c}{**NRSI SLR**} \\  & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** \\ \hline VLMT & 0.62 & 0.38 & 0.67 & 0.44 & 0.46 & 0.21 & 0.50 & 0.25 & 0.53 & 0.28 & 0.46 & 0.21 & 0.42 & 0.18 \\ VLEI & \(-\)0.80 * & 0.63 & \(-\)0.70 * & 0.49 & \(-\)0.34 & 0.11 & \(-\)0.36 & 0.13 & \(-\)0.59 & 0.35 & \(-\)0.50 & 0.25 & \(-\)0.34 & 0.11 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Pearson’s correlation and R square between RSIs and ultrasound measurements. \begin{table}
\begin{tabular}{l c c c c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**RSI RUN**} & \multicolumn{2}{c}{**NRSI RUN**} & \multicolumn{2}{c}{**RSI VDJ**} & \multicolumn{2}{c}{**RSI HDJ**} & \multicolumn{2}{c}{**NRSI HDJ**} & \multicolumn{2}{c}{**RSI SLR**} & \multicolumn{2}{c}{**NRSI SLR**} \\  & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** & **r** & **R\({}^{2}\)** \\ \hline RSI RUN & & & & & & & & & & & & & & \\ NRSI RUN & & 0.96 ** & 0.92 ** & & & & & & & & & & & \\ RSIDVJ & 0.85 * & 0.72 * & 0.81 ** & 0.65 * & & & & & & & & & \\ RSIDJH & 0.53 & 0.29 & 0.43 & 0.19 & 0.74 * & 0.55 * & & & & & & & \\ NRSIDJH & 0.56 & 0.31 & 0.56 & 0.31 & 0.72 * & 0.52 * & 0.74 * & 0.55 * & & & & \\ RSI SLR & 0.63 * & 0.40 * & 0.70 * & 0.49 * & 0.76 * & 0.58 * & 0.43 & 0.18 & 0.68 * & 0.47 * & & \\ NRSI SLR & 0.48 & 0.23 & 0.60 & 0.36 & 0.64 * & 0.41 * & 0.31 & 0.09 & 0.61 & 0.37 & 0.98 ** & 0.95 ** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: [PERSON]’s correlation and R square between the different RSIs. The orange color highlights near-perfect correlations based on r, while the yellow color highlights very large correlations; the large correlations are highlighted with the light yellow. performance [32]. In addition, these authors calculated the RSI in multiple horizontal jumps [33], a task that requires good technique and motor coordination. In addition, NRSI RUN also showed near-perfect correlations with all sprint performance measures, showing that the RSI RUN is independent of the athlete's height.