## 4 Discussion  
This study was conducted to investigate the changes in hop-RSI across multiple consecutive unilateral horizontal hops. Only a limited number of studies to date on hop-RSI have been conducted, and all have used only triple hops. Thus, it was important to \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline  & & & & & & & & & & \\ \cline{2-10}  & **10 m** & **20 m** & **30 m** & **40 m** & **50 m** & **60 m** & **70 m** & **80 m** & **90 m** & **100 m** \\ \hline RSI (Hop 1) & \(-0.23\) & \(-0.20\) & \(-0.19\) & \(-0.19\) & \(-0.17\) & \(-0.16\) & \(-0.15\) & \(-0.14\) & \(-0.13\) & \(-0.12\) \\ RSI (Hop 2) & \(-0.10\) & \(-0.08\) & \(-0.06\) & \(-0.06\) & \(-0.03\) & \(-0.02\) & \(-0.02\) & \(-0.01\) & \(-0.01\) & \(0.00\) \\ RSI (Hop 3) & \(-0.21\) & \(-0.21\) & \(-0.22\) & \(-0.23\) & \(-0.22\) & \(-0.22\) & \(-0.21\) & \(-0.21\) & \(-0.20\) & \(-0.19\) \\ RSI (Hop 4) & \(-0.17\) & \(-0.18\) & \(-0.20\) & \(-0.21\) & \(-0.20\) & \(-0.21\) & \(-0.21\) & \(-0.21\) & \(-0.21\) & \(-0.20\) \\ RSI (Hop 5) & \(-0.32\) & \(-0.34\) & \(-0.36\) & \(-0.38\) & \(-0.38\) & \(-0.38\) & \(-0.38\) & \(-0.38\) & \(-0.37\) & \(-0.38\) \\ RSI (Hop 6) & \(-0.27\) & \(-0.29\) & \(-0.32\) & \(-0.34\) & \(-0.34\) & \(-0.34\) & \(-0.34\) & \(-0.34\) & \(-0.34\) & \(-0.34\) \\ RSI (Hop 7) & \(-0.24\) & \(-0.24\) & \(-0.26\) & \(-0.27\) & \(-0.27\) & \(-0.28\) & \(-0.28\) & \(-0.29\) & \(-0.29\) & \(-0.29\) \\ RSI (Hop 8) & \(-0.14\) & \(-0.12\) & \(-0.14\) & \(-0.15\) & \(-0.15\) & \(-0.15\) & \(-0.15\) & \(-0.15\) & \(-0.16\) & \(-0.16\) \\ \hline \end{tabular}
\end{table} 
Table 2: Association between the hop-RSI metrics (8 hops) and sprint split times (100 m with 10 m increments). Figure 2: Contact time, flight time, and reactive strength index throughout the first 8 steps in the sprint task. investigate how many successive hops should be performed for comprehensive evaluation. On a group level, the RSI (and the underlying contact and flight times), plateaued around hop 5, which was in accordance with our hypothesis. However, a significant proportion of individuals exhibited variable performance all the way to the last hop. Therefore, the hop-by-hop behavior of RSI seems to be stable on a group level after five hops, but still exhibits within-individual variability in later hops. As a secondary goal, we aimed to examine if hop-RSI is related to sprint-RSI and sprint performance. Our hypothesis was confirmed, however, the associations were mostly small to moderate. Horizontal hops for distance have been commonly applied in a rehabilitation context as a method of analyzing the progression of lower limb injury rehabilitation [22; 23], with some interest to apply hop tests to assessments of athletes as well [24]. [PERSON] et al. [25] reported that hop-RSI and hop flight times showed larger differences between the operated and non-operated leg (effect size = 0.65-0.85) than hop distance (effect size = 0.45), indicating the potential utility of calculating additional outcomes other than hop distance. Subsequently, [PERSON] et al. [17] and [PERSON] et al. [14] reported moderate-to-excellent reliability of the hop-RSI. However, all of the previous studies were limited to triple-hop tests, wherein only two RSI values can be calculated [14; 17; 25]. We aimed to extend the protocol from the previous studies by including multiple hops and examining how RSI behaves across consecutive hops. In previous studies, the mean RSI values from the first and second hops were \(\sim\)1.0 and \(\sim\)1.3-1.4, respectively [14; 17], which is very close to our results (Figure 1, bottom chart). The primary novelty of the present study is the finding that hop-RSI values continue to increase up until about the fifth hop. This implies that after the fifth hop, the performance of the hops is relatively stable, which means that a five-hop test could be sufficient for a comprehensive evaluation of single-leg horizontal reactive strength.