Reactive Strength Index
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RSI quantifies how efficiently an athlete converts ground contact time into jump height during a drop or depth jump, calculated as jump height divided by ground contact time. Higher values indicate better reactive, elastic power in the stretch-shortening cycle.

Two athletes can jump the same height, but one who does it with a much shorter ground contact time demonstrates superior reactive strength and tendon stiffness. RSI specifically rewards speed of force production, not just the magnitude of the jump.

Drop heights of 30-45 cm are commonly used for testing, since too low a drop doesn't sufficiently load the stretch-shortening cycle and too high a drop can shift the movement from reactive to a slower, more controlled landing strategy. The optimal height for each athlete is sometimes found by testing across several heights and identifying which produces the highest RSI.

Coaches use RSI trends over time to gauge neuromuscular readiness and plyometric qualities, often incorporating it into fatigue monitoring since a sudden RSI drop can indicate accumulated fatigue before other markers show it. It also helps identify whether an athlete needs more strength work or more speed-of-force training.

RSI is sensitive to fatigue, warm-up quality, surface, and even footwear, so day-to-day variation is expected and doesn't necessarily reflect a real change in reactive ability. Trends across multiple sessions over weeks are far more meaningful than a single test result.