Association and Agreement between Reactive Strength Index and Reactive Strength Index-Modified Scores [Article Review]

The impetus for this study stemmed from the frequent and misleading practice in sport science to use the Reactive Strength Index (RSI) and its modified version (RSI-mod) interchangeably when assessing reactive strength and agility across athlete populations. While both terms share similar nomenclature, RSI is calculated from depth jump (DJ) performance, featuring high-impact and short ground contact times, whereas RSI-mod is derived from countermovement jump (CMJ), which involves longer durations and lower-impact transitions. The two movements place distinct demands on the stretch-shortening cycle (SSC) and the neuromuscular system, raising important questions on whether their respective indices actually measure the same qualities of reactive strength. To address this, the authors recruited both NCAA Division I basketball athletes and active young adults, aiming to include a diverse sample representative of research and practice, and systematically evaluated not just the association, but also the agreement and bias between RSI and RSI-mod scores.

Key Takeaway + Results

The study found that while RSI and RSI-mod scores are moderately associated—showing 20–47% shared variance in regression models—there is a large and significant measurement bias between the two approaches, with RSI values consistently and substantially higher than RSI-mod. The mean RSI scores (from DJ) exceeded RSI-mod scores (from CMJ) by over 130%, largely due to shorter ground contact times in DJs compared to longer time-to-take-off in CMJs. Bland–Altman analyses reinforced the poor agreement, demonstrating wide limits of agreement that often surpassed the mean scores of both metrics themselves, and a performance-dependent effect in which increases in one metric were not reliably mirrored in the other. These results establish that RSI and RSI-mod, despite modest correlations, respond to the distinct biomechanical and neuromuscular demands of their respective jumping tests—a finding echoed in other recent research on countermovement depth and jump characteristics.

Conclusion + Practical Application

In practice, this study underscores the importance of selecting reactive strength metrics appropriate to the athlete's capabilities and the SSC demands of training or testing environments. RSI from depth jumping should be prioritized for athletes with sufficient neuromuscular conditioning and tolerance for high-impact plyometric exercise, as it more accurately quantifies quick eccentric-concentric transitions crucial for explosive performance in fast SSC movements. RSI-mod, calculated from countermovement jumps, may serve as a safer and still informative alternative for athletes in earlier stages of training, rehabilitation, or those at risk of injury from maximal depth jumps. Coaches, practitioners, and researchers are therefore advised not to use RSI and RSI-mod interchangeably, but rather to tailor their use to specific athlete groups, monitoring purposes, and training goals, ensuring a more nuanced and valid assessment of lower-extremity reactive strength in practical settings.

Article PDF

sports-09-00097 RSI RSIm.pdf

Download PDF • 2.30MB