Reactive strength index (RSI)

The reactive strength index, or RSI, is a jump-performance measure that relates rebound height to time spent on the ground. In its common drop-jump form, jump height is divided by ground contact time, the interval from landing to the following takeoff. It is used to assess performance in rapidly changing from absorbing a landing to producing an upward jump .¹

Because RSI is a ratio, it reflects both height and contact duration. A larger value can result from a higher jump at the same contact time, a shorter contact at the same height, or changes in both. Different combinations can also produce the same score. Reading the component measurements alongside the ratio therefore helps describe how a jump was performed. Jump height may be measured or estimated from flight time, so the measurement method is part of interpreting the result .¹

RSI should be distinguished from modified RSI, commonly calculated during a countermovement jump. Modified RSI divides jump height by the time from the start of the countermovement to takeoff, rather than a rebound's ground contact time. The two measures describe different tasks and are not interchangeable. Drop height and jump instructions also influence the context in which an RSI score is obtained .²