RSI-mod similar to it's original cousin the Reactive Strength Index (RSI) looks at the ratio between time spent of the ground and jump height performed.
Unlike a normal RSI through where we use Ground Contact Time (GCT) as the measure of time spent on the ground, we now use the Time To Take Off (TTO) as the measure of this.
RSI-modified = Jump Height / Time To Take Off.
This gives us insight into the input strategy and output of the jump performed, that is input represented by time spent on the ground and often the counter movement depth generating the force to jump, and the height of that jump that is achieved.
RSI-mod can be useful in monitoring fatigue, determining best jump strategies, and overall scoring jump performance with a single metric as the best jump is usually defined by what achieves the highest result with the least amount of countermovement.
Research by Sole et al 2018 assessing 151 NCAA Div 1 collegiate male and female athletes found the following RSI-mod results with L being the lowest performers and U being the highest performers.
Males
Females
References:
Sole CJ, Suchomel TJ, Stone MH. Preliminary Scale of Reference Values for Evaluating Reactive Strength Index-Modified in Male and Female NCAA Division I Athletes. Sports (Basel). 2018 Oct 29;6(4):133. doi: 10.3390/sports6040133. PMID: 30380639; PMCID: PMC6315647.