Relationship between reactive strength index variants in rugby league players

Two reactive strength index (RSI) variants exist, the RSI and RSI modified (RSImod) which are typically calculated during the drop jump (DJ) and countermovement jump (CMJ), respectively. Both RSI variants have been used to monitor athletes' ability to complete stretch-shortening cycle actions quickly, but they have never been compared. The purpose of this study was to determine if they yield relatable information about reactive strength characteristics. Male professional rugby league players (n = 21, age = 20.8±2.3 years, height = 1.82±0.06 m and body mass = 94.3±8.4 kg) performed three DJs (30 cm) and CMJs on a force plate. RSI and RSImod were subsequently calculated by dividing jump height by ground contact time (GCT) and time to take-off (TTT), respectively. All variables were highly reliable (intraclass correlation coefficient ≥0.78) with acceptable levels of variability (coefficient of variation ≤8.2%), albeit larger variability was noted for DJ variables. Moreover, there was a large relationship between RSI and RSImod (r=0.524, P=0.007), whereas very large relationships were noted between jump heights (r=0.762, P<0.001) and between GCT and TTT (ρ=0.705, P<0.001). Additionally, RSI (0.90±0.22) was largely and significantly (d=2.57, P