A comparison of vertical and horizontal reactive strength index variants and association with change of direction performance

This study sought to investigate the inter-relationship between different vertical and horizontal variants of reactive strength index (RSI) and change of direction performance. Thirty-one male volleyball players (age: 22.4 ± 3.9 years), performed bilateral drop jumps (DJ), bilateral and unilateral countermovement jumps (CMJ), and triple hops for distance. The RSI was calculated as the ratio of jump height and contact time (DJ), jump height and time to take off (CMJ), and flight time or hop distance and contact time (triple hop), and 505 change of direction test. RSI obtained from DJ and CMJ tasks exhibited excellent trial-to-trial reliability (ICC = 0.91-0.94), while triple hop based RSI had only moderate reliability (ICC = 0.67-0.74). The relationships among different RSI variants were moderate to high (i.e. DJ to CMJ: r = 0.57-0.69; p ≤ 0.004; DJ to triple hop: r = 0.54-0.66; p ≤ 0.021 and CMJ to triple hop: r = 0.42-0.63; p ≤ 0.037). For the triple hop, the associations between RSI based on hop flight time and RSI based on hop distance were high for hop 1-2 (r = 0.77-0.83; p < 0.001) and very high for hop 2-3 (r = 0.91-0.92; p < 0.001). All RSI variants were in small to moderate negative correlation with 505 test performance (r = -0.38 to -0.45; p ≤ 0.042). The agreement in inter-limb asymmetry direction between in RSI from unilateral CMJ and triple hop RSI variables was slight to moderate (Kappa coefficient = 0.06-0.36). In conclusion, although inter-relationships between RSI variants were moderate to high, the direction of inter-limb asymmetry was inconsistent, highlighting the notion of movement variability in limb dominance