Sport Science
RSI (Reactive Strength Index)
Reactive Strength Index is a metric that quantifies an athlete’s ability to rapidly absorb force and immediately redirect it into explosive output. It is calculated by dividing jump height by ground contact time during a drop jump or rebound jump task. A high RSI means the athlete produces significant force in a very short contact time, the defining quality of reactive, elastic athleticism.
Why it matters for team sport
Most sport performance happens in fractions of a second. Sprinting, cutting, jumping off one leg after a landing, all of these demand the stretch-shortening cycle operating at high speed under load. RSI captures this quality directly. An athlete may have excellent jump height in a countermovement jump but poor RSI if their ground contact time is long. These represent different neuromuscular qualities and respond to different training emphases: heavy strength work improves force production; plyometric and reactive training improve RSI.
How it is measured
Drop jump RSI is the most common format: the athlete steps off a box, lands, and immediately jumps as high as possible with minimal contact time. Jump height and contact time are captured with a force plate, jump mat, or high-speed video. RSI = jump height (m) divided by contact time (s). Typical values range from around 1.0 in developing athletes to 3.0+ in elite sprinters and jumpers. Tracking RSI over a training block shows whether reactive qualities are improving, degrading, or holding steady.
Related terms
Force Plate · CMJ (Countermovement Jump) · Plyometrics · RFD (Rate of Force Development)