Reactive strength index (RSI) measures reactive jump capacity by reflecting a person's ability to turn deceleration into acceleration in a short period of time. This is often referred to as the stretch-shortening cycle. Reactive strength is an important physical competency measure for acceleration, agility, and change of direction speed. A drop in RSI is a good indicator of fatigue, and an increase in RSI is a good indicator of performance readiness. RSI asymmetry can be an indicator of Return-To-Play status.
For single leg jumps, RSI is assessed on each leg independently of the other.
RSI is calculated as the ratio of flight time to ground contact time, so a higher RSI means either a shorter ground contact time or a longer flight time (higher jump), or a combination of the two.
RSI as a predictor of contralateral ACL injury - Single and double leg drop jumps performed 9 months after ACLR are a better predictor of contralateral ACL injury than typical clinical evaluators. The contralateral side in the cohort that went on to experience subsequent ACL injury showed lower quadriceps strength, sagittal plane control, and plyometric ability (RSI) on the contralateral limb.
King, Enda, Richter, Chris, Daniels, Katherine A.J, Franklyn-Miller, Andy, Falvey, Eanna, Myer, Gregory D, Jackson, Mark, Moran, Ray, & Strike, Siobhan. (2021). Can Biomechanical Testing After Anterior Cruciate Ligament Reconstruction Identify Athletes at Risk for Subsequent ACL Injury to the Contralateral Uninjured Limb? The American Journal of Sports Medicine, 49(3), 609–619. https://doi.org/10.1177/0363546520985283