###### Abstract  
The reactive strength index (RSI) is used to assess reactive strength in the lower limbs. Since previous studies have mostly focused on vertical RSI (derived from drop jumps), we investigated the RSI across eight consecutive unilateral horizontal hops and its associations with sprint performance. A sample of 104 male kinesiology students (age: 19.2 \(\pm\) 1.1 years) performed unilateral hops and 100 m sprints with split times recorded. RSI was determined as the ratio between contact time and subsequent flight time for each hop. On a group level, the horizontal RSI was statistically significantly (\(p\) < 0.001; \(\eta_{\text{p}}\)\({}^{2}\) = 0.49) and increased from hop 1 (1.04 \(\pm\) 0.17) to hop 5 (1.41 \(\pm\) 0.22), but then plateaued (1.43-1.44) for hops 6-8. However, on an individual level, variations in RSI were present all the way to the last hop. All sprint split times were in small-to-moderate correlation with RSI variables (r = 0.25-0.40), implying that individuals with higher hopping RSI presented with shorter sprint times. Future studies should expand the research on the relationship between horizontal hopping RSI and sports performance, and examine if unilateral RSI tests can provide practitioners with valuable information when performed alongside more common vertical RSI tests. triple hop; horizontal hop; horizontal strength; sports performance; unilateral jump +
Footnote †: journal: Applied Sciences  
relatively closely related, with correlation coefficients between the two metrics reported to be \(\sim\)0.6-0.7 [14]. Both measures are likely useful to assess vertical reactive strength and have shown associations with a change in direction performance (to a horizontally directed task) [4]. Considering the principle of specificity, it seems reasonable to investigate if reactive strength outcomes derived from horizontal tasks offer additional information relevant for horizontal performance (e.g., sprinting, agility tasks). Previous studies have suggested that exercise/testing outcomes have larger associations with performance tasks when they are executed in the same direction (e.g., hip thrust and sprint) than when they are not (e.g., vertical jump and sprint) [15]. Notably, single- (r = 0.74-0.76) and triple-hop (r = 0.84-0.89) distances (but not outcomes from vertical jumps) were both shown to be highly correlated with 10 m sprint performance [16]. Thus, it seems reasonable to assume that horizontal RSI would better relate to horizontally directed tasks than vertical RSI. However, the literature on RSI derived from horizontally oriented tasks is scarce. [PERSON] et al. [17] has demonstrated the high reliability of RSI derived from a horizontal unilateral triple-hop test. They also reported consistent magnitude, and the fluctuating direction of inter-limb asymmetries derived from RSI values [17]. Recently, it was reported that RSI from triple-hop tasks is moderately related to both RSI from drop jump tasks (r = 0.54-0.66) and countermovement jump tasks (r = 0.42-0.63) [14]. This suggests that all of these variables partially reflect the same general ability (reactive strength), but also contribute unique information, likely because of their direction- and velocity-specific differences [15; 18]. RSI from drop jump and triple-hop tasks, but not countermovement jump tasks, was shown to be moderately related to a change in direction performance [14]. In one of the rare studies on horizontal RSI, it was proposed that further studies are needed to investigate how many hops are needed in a horizontal hop test before flight time and distance performance plateau [14]. Namely, information obtained from each consecutive hop could have unique meaning and transfer to sports performance. Therefore, the primary purpose of this study was to investigate the behavior of RSI across multiple consecutive unilateral hops. As no similar study has previously been conducted, we arbitrarily hypothesized that contact and flight times (and by extension, the RSI) will plateau after the fifth consecutive hop.