RSI: The Most Underused Jump Test in Sport Science
Summary
Reactive Strength Index combines jump height and ground contact time into a single number that tells you more about athletic readiness and fast SSC function than almost any other field test. Most coaches aren't using it.
Jump height gets all the attention. It is intuitive, easy to communicate, and satisfying to watch improve. But jump height alone tells you nothing about how the athlete got there — how long they were on the ground, how efficiently they used elastic energy, or how close to their reactive capacity they are operating. Reactive Strength Index (RSI) does all of that.
What RSI Actually Measures
RSI = jump height ÷ ground contact time
That ratio seems almost too simple, but it captures something that neither variable captures alone. An athlete who jumps 40 cm with 180 ms of ground contact is operating differently from one who jumps 40 cm with 280 ms. The first is applying force faster; the second is relying more on the concentric phase. RSI distinguishes between them.
Young (1995) described RSI in the context of drop jump testing as a practical field measure of fast SSC function. Flanagan and Comyns (2008) expanded on this framework, arguing convincingly that RSI provides more actionable information than jump height alone precisely because it is a rate measure — it tells you how explosively the athlete is moving, not just how far.
Why It Is Underused
RSI requires measurement of ground contact time. Historically, this meant a contact mat or a force plate — equipment not always available to field coaches. This practical barrier led to RSI being relegated to laboratory settings.
That barrier no longer meaningfully exists. Validated smartphone apps, contact mats in the sub-$500 range, and portable dual-beam timing systems all measure contact time accurately enough for field use. The reason RSI remains underused is largely habit and unfamiliarity — not a genuine logistical constraint.
How to Test RSI
The standard RSI test is the drop jump (DJ): the athlete steps off a box of standardised height (typically 30–45 cm), lands, and immediately jumps as high as possible with minimal ground contact. Contact time and jump height are both measured. Multiple trials are averaged.
The box height matters. RSI peaks at a specific drop height for each athlete — too low and the stretch load is insufficient; too high and the athlete cannot manage the eccentric load and contact time extends. If you only test one box height, use 30 cm as a conservative starting point and flag that the optimal may be higher.
What RSI Tells You That Jump Height Doesn't
Fatigue monitoring: RSI drops with fatigue more sensitively than jump height because fatigued athletes extend ground contact time to compensate for reduced force production. A 10% drop in RSI with minimal change in jump height is a cleaner fatigue signal than a 5% drop in jump height alone.
Training response: Athletes adapting to plyometric work show RSI improvements that often precede jump height improvements. They become faster and more reactive before they become higher-jumping.
Return to sport: Following lower limb injury, RSI limb asymmetry (measured on single-leg drop jump) is a more sensitive indicator of deficits than single-leg jump height. An athlete may restore absolute output while remaining asymmetric in their reactivity.
Practical Benchmarks
Elite team-sport athletes typically achieve RSI values of 1.5–2.5 (using jump height in metres and contact time in seconds, or equivalently expressed as cm/s ratios). Values below 1.0 in athletes who should be reactive suggest meaningful fast SSC deficits. These benchmarks are sport- and position-specific — use within-athlete trends rather than absolute cut-offs where possible.
Measure RSI. It takes thirty seconds and tells you more than a countermovement jump alone ever will.
References
Mark Fisher
Founder, Swift Performance
Mark Fisher is the founder of Swift Performance and has spent 30 years designing and building athlete testing equipment used by elite sport programmes and universities worldwide. He has worked alongside researchers and PhD candidates across biomechanics, sprint mechanics, and strength science — developing the hardware and software they use to collect and analyse performance data. His writing comes from three decades at the intersection of applied sport science and precision measurement technology.
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