Drop jump test and Reactive Strength Index (RSI), explained

A drop jump is a plyometric test where the athlete steps off a box, lands, and immediately jumps as high as possible while spending as little time as they can on the ground. It measures reactive strength — how efficiently someone absorbs and re-uses force through a fast stretch-shortening cycle — and it’s summarised in a single number: the Reactive Strength Index (RSI).

This guide explains what the drop jump is, how it differs from the countermovement jump, how RSI is calculated from a force plate, and how to run the test reliably.

What is a drop jump?

In a drop jump, the athlete stands on a box (commonly 20–40 cm, sometimes higher for advanced athletes), steps off — they drop, they don’t jump off — and on landing rebounds into a maximal vertical jump. The instruction that defines the test is: spend as little time on the ground as possible while still jumping high.

That constraint is the whole point. Landing from the box pre-loads the muscles and tendons of the lower limb, and a short, stiff ground contact lets that stored elastic energy snap back into the jump. The shorter the amortization phase — the brief transition from landing to take-off — the less of that energy leaks away. It’s a direct measure of the “fast” stretch-shortening cycle — the same quality a sprinter uses at foot-strike or a jumper uses off the floor.

How is a drop jump different from a countermovement jump?

Both jumps use the stretch-shortening cycle (SSC), but at opposite ends of it:

	Countermovement jump	Drop jump
Stretch-shortening cycle	Slow (>250 ms contact)	Fast (<250 ms contact)
Pre-load	Self-generated dip	Landing from a box
Primary quality	Explosive power	Reactive strength / stiffness
Headline metric	Jump height, peak power	Reactive Strength Index (RSI)

Put simply: the CMJ asks how much power can you produce?; the drop jump asks how quickly can you turn a landing back into a jump? An athlete can be strong on one and unremarkable on the other, which is exactly why testing both is informative.

A useful middle ground is the countermovement rebound jump (CMRJ) — a CMJ immediately followed by a maximal rebound — which pairs jump performance with reactive strength off the landing without needing a box. ForceMate supports the drop jump, CMRJ, and repeated hops, so you can profile reactive strength however suits your athlete.

What is the Reactive Strength Index (RSI)?

The Reactive Strength Index expresses reactive strength as one number: how much height (or air time) an athlete produces relative to how long they spent on the ground producing it. A high RSI means a springy, efficient athlete who rebounds fast; a low RSI means force is leaking away during a long, soft ground contact.

How is RSI calculated?

RSI comes from two things a force plate measures precisely on every rep — the time the feet are on the plate (ground contact time) and the time the athlete is airborne (flight time):

RSI = flight time ÷ ground contact time

Because a force plate reads zero force during flight and rising force during contact, it pinpoints both moments to the millisecond and returns RSI automatically. A jump with 0.50 s of flight time and 0.20 s of ground contact gives an RSI of 2.5.

You may also see RSI defined as jump height ÷ ground contact time (in m/s). Both are valid, but they produce different numbers, so the one rule that matters is: pick one method and use it consistently — RSI is only comparable within the same calculation.

Don’t confuse it with RSI-modified (RSI-mod), either — that comes from a countermovement jump, not a drop jump. A countermovement jump has no rebound ground contact to time, so RSI-mod puts time to take-off where drop-jump RSI uses ground contact time (jump height ÷ time to take-off). It describes a related quality, but the two are measured from different jumps and aren’t interchangeable, so never compare one against the other.

What’s a good RSI?

RSI is highly sensitive to method, drop height, and the population tested, so treat the ranges below (flight time ÷ contact time) as loose orientation only:

Level	Approximate RSI (flight ÷ contact)
Developing / recreational	~1.0–1.5
Trained athlete	~1.5–2.5
Elite in an explosive sport	~2.5+

As with any force plate metric, the honest and more useful comparison is an athlete against their own baseline. A falling RSI across a training block often signals accumulated fatigue or loss of tendon stiffness well before it shows up in a race or a session.

One drop-jump-specific point: higher boxes are not better. Each athlete has an optimal drop height — the one that produces their best RSI. Above it, ground contact lengthens and RSI falls because they can no longer control the landing. Testing a couple of box heights to find that peak is more informative than always drilling from the tallest box available.

How to perform a drop jump on a PlateMate

A repeatable protocol matters even more here than on the CMJ, because RSI swings with small changes in technique:

1. Set the box just behind the plates at a sensible starting height (30 cm is a good default for most athletes). Keep the same height across a comparison.
2. Start with hands on hips to remove arm swing as a variable, unless you’re deliberately testing with arms.
3. Step off, don’t jump off. The athlete leads with one foot and drops — jumping upward off the box inflates the landing and invalidates the rep.
4. Land on both plates and rebound immediately, cueing “minimise ground contact, jump as high as you can.” Both cues together — chasing only height or only a fast contact each distorts RSI.
5. Take three to five reps with full recovery between them, and report the rep with the best RSI (or a consistent average — just keep the choice the same every time).
6. Read RSI, contact time, and flight time in ForceMate, and — because the PlateMate is a dual plate — check left–right landing asymmetry, which often reveals a limb that’s offloading on impact.

One measurement caveat: flight-time-based RSI can be inflated by pointing the toes or tucking the legs in the air, which stretches out the airborne time without adding real height. Cue neutral ankles and extended legs, and have the athlete land the way they took off.

For the exact step-by-step protocol and a demo video, see the drop jump instructions in our Test Library.

Why measure the drop jump with a force plate?

You can estimate a drop jump with a contact mat, but a mat only knows “on” or “off.” A force plate measures the landing force itself, so alongside RSI you see how hard each limb hits the ground, whether one side absorbs more than the other, and how the force builds through the contact — the detail that turns a single RSI number into a picture of how the athlete is being reactive.

The PlateMate captures all of that in seconds: RSI, contact and flight time, landing forces, and per-limb asymmetry on every rep, tracked over time in ForceMate. Because each plate reads one foot, the reactive-strength imbalance that matters most for injury risk comes out of the same test you’re already running.

Getting started

The drop jump involves repeated high-impact landings, so treat it as an advanced test: the athlete needs the landing control, eccentric strength and impact tolerance to absorb the drops before you start. From there, if reactive strength is new to your testing, start conservative: a 30 cm box, hands on hips, and RSI as your one headline number. Establish each athlete’s baseline, re-test on a consistent schedule, and watch the trend rather than the absolute figure. Once you’re comfortable, experiment with drop height to find each athlete’s RSI peak, and add landing asymmetry to your return-to-play criteria.

The drop jump is quick, low in volume, and tells you something the countermovement jump can’t — making the two together a compact, powerful picture of lower-body performance.

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Want to see how the PlateMate handles reactive strength testing in your setting? Get in touch with the CC Athletics team.