Reactive strength index-modified: reliability, between group comparison, and relationship between its associated variables

Abstract

To investigate and compare the reliability of reactive strength index-modified (RSImod) and its associated variables (jump height [JH] and [time to take-off]) 20 combat fighters and 18 physically active men participated in this study. They visited the laboratory three times; firstly, for jump familiarization and two sessions for test-retest (2–7 days apart). For both groups, the between-day changes in performance were trivial to small (≤ 1.1%). The coefficient of variation (CV) comparisons ( i.e . CV ratio) demonstrated that combat athletes had a lower test-retest variation for RSImod (0.87) and JH (0.80) than non-athletes. Combat athletes demonstrated a greater JH than physically active men (0.43 vs 0.37; p = 0.03, g = 0.73), but small and non-significant differences were observed for RSImod (0.60 vs 0.55; p = 0.24, g = 0.38) and TTT (0.70 vs 0.72; p = 0.32, g = 0.33). RSImod was more positively correlated with JH ( r = 0.75–0.87; p < 0.001) than negatively correlated with TTT ( r = 0.45–0.54; p < 0.001). This study suggests that RSImod is a reliable variable obtained during CMJ testing in combat athletes and physically active men, with scores being slightly better for combat athletes. In terms of performance, combat athletes jumped higher than physically active men, but no differences in RSImod or TTT were observed. Lastly, RSImod was more strongly related to JH than TTT, and this was more evident in athletes than nonathletes. This indicates that the combat athletes were able to better utilize their (equal) time spent jumping (higher), possibly via greater utilization of the stretch shortening cycle, faster or more optimal motor unit recruitment, or an array of other factors.

Associations of eccentric force variables during jumping and eccentric lower-limb strength with vertical jump performance: A systematic review

“…RSImod, an indicator of jump performance, has been recently studied [ 38 , 39 ]. RSImod is calculated by dividing the CMJ height by the CMJ’s time (the time taken from the start of the action to the takeoff) [ 38 ] and is a good indicator of the ability to jump within a limited time, particularly when jumping vertically and against other athletes, such as in basketball [ 8 ]. Our results demonstrated a significant relationship between the RSImod and braking RFD, amortization force, eccentric work, and unloading force in CMJs, but not unloading RFD.…”

“…RSImod, an indicator of jump performance, has been recently studied [38,39]. RSImod is calculated by dividing the CMJ height by the CMJ's time (the time taken from the start of the action to the takeoff) [38] and is a good indicator of the ability to jump within a limited time, particularly when jumping vertically and against other athletes, such as in basketball [8].…”