RSI is the cleanest field marker of reactive, elastic athleticism — but it’s only as good as the contact time behind it. Plyomat times ground contact to 0.001 seconds and reports your RSI score live in the free Plyomat 3.0 app. No subscription, ever.
Reactive Strength Index (RSI) is a single number that captures how well an athlete uses the stretch-shortening cycle — the rapid load-and-release of muscle and tendon that powers every jump, cut, and stride. In plain terms, it answers one question: how much height does this athlete produce relative to how long they spend on the ground?
The formula is simple: RSI = jump height ÷ ground contact time. It is commonly also expressed as flight time ÷ contact time, since flight time is what a contact mat actually measures and jump height is derived from it. Either way, a higher RSI means a more reactive, elastic, spring-like athlete — one who converts ground contact into height fast rather than sinking and grinding through each rep.
That makes reactive strength different from raw vertical jump. A squat-jump or jump-and-reach tells you how high someone can go when they have time to wind up. RSI tells you how explosively they recycle force in a fraction of a second — the quality that separates a quick, springy athlete from a strong-but-slow one. Because it pairs a height with a time, RSI is also a sensitive readout of plyometric quality and a practical tool for monitoring fatigue and return-to-play readiness as an athlete recovers.
RSI testing usually relies on one of two field protocols. The drop jump is the classic: the athlete steps off a box (often 30–40 cm to start), lands, and immediately rebounds as high as possible while keeping ground contact as short as they can. RSI is taken from that single rebound — rebound height divided by contact time. Testing across a few drop heights also reveals the height at which an athlete is most reactive.
The repeated-hop test — often run as a 10-to-5 — asks the athlete to perform a series of maximal pogo hops with stiff ankles and locked knees, like a human spring. The system averages the best contacts so a single mistimed hop doesn’t skew the result. It is fast, repeatable, and ideal for testing a whole roster.
Here is the catch with both: ground contact in a good reactive jump lasts roughly a fifth of a second, and contact time sits in the denominator of the RSI formula. A timing error of even a few hundredths of a second swings the score noticeably. That is why you should not eyeball RSI or estimate it from video frame counts — you measure it on a jump mat, not by hand. A pressure-sensitive contact mat registers the exact instant of takeoff and landing, and Plyomat’s Controller 3.0 resolves that contact to 0.001 seconds — the precision that makes an RSI score trustworthy enough to program from. It is the same hardware behind the Plyomat vertical jump mat, and a modern, app-connected upgrade for anyone moving on from an older mat (see the Just Jump alternative).
RSI is one number, but two athletes can reach the same score by very different routes. Plotting jump height against contact time sorts athletes into the Reactive Strength Quadrant (RSQ) — and points each profile toward the training they actually need.
High height off short contact. The spring is loaded and released fast — the elite reactive profile. The goal is to maintain it and keep it healthy across a season.
Good height, but off long contact. The athlete can jump but sinks and dwells on the ground. Programming leans toward stiffness and faster stretch-shortening turnaround.
Short contact, but limited height. The athlete is quick off the floor yet under-expresses force. The priority is building strength and power to convert that speed into output.
Lower height and longer contact. A broad base — general strength, mechanics, and plyometric progression — moves this athlete toward the reactive corner over time.
Every contact returns RSI, ground contact time, flight time, and jump height — the raw inputs behind the quadrant, captured live rather than reconstructed after the fact.
Asymmetry, DRI, and a Power Profile across eight assessment modes turn one RSI number into a full picture of reactive readiness and left/right balance.
Put a Plyomat jump mat under your athletes and turn every reactive jump into a trustworthy RSI score — live, to the thousandth of a second, with no subscription.
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