The Reactive Strength Index in Unilateral Hopping for Distance and Its Relationship to Sprinting Performance: How Many Hops Are Enough for a Comprehensive Evaluation?

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 ± 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; ηp2 = 0.49) and increased from hop 1 (1.04 ± 0.17) to hop 5 (1.41 ± 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.