RSI is a single number that tells you how much of your bodyweight bounces back per millisecond on the ground, a key marker of running economy, sprint capacity, and tendon health. Drop a step, hit the floor, spring up, type your two timings in.
From a drop jump (step off a 20–40 cm box). Ground contact time = how long your feet were on the floor between landing and take-off. Flight time = how long you were in the air on the rebound jump. Use a contact mat, force plate, optical system, or 240+ fps video.
This calculator computes drop-jump RSI (one drop, one ground contact, one rebound). Do not enter data from repeated-hop / pogo / ground-hop protocols, those are a different formulation and produce a meaningless number here. Pogos are great training for RSI (see below); they’re just not the right measurement for this tool.
Two physics formulas, no magic.
h = g · tflight² / 8 with g = 9.80665 m/s². Assumes take-off and landing happen at the same height. Standard projectile motion.
RSI = h / tcontact. Height per second on the ground. The lower the contact time and the higher the bounce, the better.
RSI tracks closely with running economy, sprint mechanics, and tendon stiffness. It’s the cheapest single measurement that previews how well an athlete will respond to plyometric training.
Bad protocol = noise. Five details that make or break the number.
Same box every test. Higher boxes don’t mean higher RSI, for many athletes, the optimum is 20–30 cm. Test at the box where you produce the best score, then standardize.
Step off the box with a single leg without losing height. Land stiff, spring up immediately, "as fast and as high as possible."
Or the same arm action every rep. Arm swing adds height and noise, lock it down to test the spring system, not the swing system.
Use the best valid rep (or mean of best 2). Discard reps where contact time blows out or technique collapses (knees caving, hips folding).
Contact mat or force plate is the gold standard. Optical systems work. Video is fine if you can shoot at 240+ fps, below that the timing error eats your signal.
Three angles of attack, in priority order for most endurance athletes:
Track RSI every 4–6 weeks, same protocol, same box, same shoes, same time of day. Trends matter; one number doesn’t.
The bands below come from sport-science literature (Flanagan & Comyns), valid for general athletic populations. Sprinters and jumpers will sit higher than endurance athletes at the same training level.
Reactive strength under-developed. Not yet ready for moderate-intensity plyos, build with strength, eccentric work, and very short fast-SSC drills first.
Ready for moderate-intensity plyometrics. Solid base for most age-groupers.
Ready for high-intensity plyometrics, bounds, depth jumps, weighted jumps. Most well-trained endurance athletes top out here.
Diminishing returns from more plyo volume. Time to specialize the stimulus toward the demand, sprint, jump, fast running, depending on the sport.
Elite jumper / sprinter territory. Marginal gains only. Maintenance + sport-specific transfer becomes the focus.
Three related-but-distinct concepts that get blurred in conversation:
Things that move RSI even at the same fitness: acute fatigue, soreness, warm-up quality, time of day, caffeine, surface and footwear stiffness, box height, athlete intent (min-GCT vs max-height instruction), and technique (hip/knee collapse blows out GCT and tanks the score).
Plyometric volume, sets, rest intervals by training phase, pairs naturally with RSI.
Open → Running time predictorRSI feeds running economy, better RSI, better predicted finish times for the same VO2.
Open → All calculatorsBack to the full catalog, 30+ endurance calculators across performance, fueling, swim, bike, run.
Browse →The lab uses a contact mat with sub-millisecond timing, bilateral and unilateral RSI plus jump-height symmetry. Pairs naturally with VO2max and gait analysis to give you a complete running profile.
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