How to Test Reactive Strength.

What RSI actually measures

The Reactive Strength Index (RSI) is the single most useful field number for fast plyometric ability. It quantifies how well an athlete tolerates and reuses elastic energy in a fast stretch-shortening cycle: how much spring you get back out of the ground for the time you spend on it. The modern definition is a clean ratio:

A higher RSI means better reactive strength. The athlete is producing more height while spending less time on the floor, which is exactly the quality that shows up in sprinting, change of direction, and any repeated bounding action. (You will still see an older variant expressed as flight time divided by contact time. Both describe the same idea, but jump height divided by contact time is the version we use here, because it reports the result in a unit coaches can picture.)

The two numbers that feed RSI, jump height and ground contact time, are exactly what a contact mat or a force plate is built to capture. Get those two right and the index takes care of itself.

DRI: comparing across box heights

RSI has one blind spot. The moment you change the drop height of a box, you change the impact the athlete has to absorb, so an RSI from a 12-inch box and an RSI from a 24-inch box are not directly comparable. That is the problem the Dynamic Rebound Index (DRI) was built to solve. DRI is Plyomat's drop-height-aware metric, co-developed with Lance Brooks:

By dividing through by the square root of the drop height, DRI normalizes the result so that drop jumps performed from different box heights can be compared on a single scale. If you test an athlete across a range of boxes, DRI is what lets you line those efforts up side by side instead of treating each height as its own island.

Protocol 1: the drop jump (DJ) test

The drop jump test isolates the fast stretch-shortening cycle from a known height. The cue that matters most: the athlete steps off the box, never jumps off it. Jumping up first changes the drop height and ruins the standardization.

Run it like this:

1. Set the box. Place the contact mat directly in front of the box so the athlete lands flat on it.
2. Step off, do not jump. The athlete stands at the edge, steps out, and drops, leading with one foot but landing on both at once.
3. Land and explode. On contact the athlete immediately drives into a maximal vertical jump, minimizing time on the ground. The classic cue is "imagine the floor is hot."
4. Capture the numbers. The mat records ground contact time and jump height, and RSI is calculated from the two.
5. Repeat across a range of box heights. Test a few heights to find where each athlete expresses their best reactive output.

That last step is the honest part of drop-jump testing. There is no single "optimal" box height that applies to everyone, so be wary of any chart that hands you a universal centimetre number. Test a range and individualize: some athletes peak from a low box, others from a higher one, and that profile is itself useful information. Because each height produces a different impact, this is exactly where DRI earns its keep, letting you compare those efforts on one scale instead of guessing.

"Step off, do not jump off. The moment an athlete pre-jumps, the drop height is no longer the drop height, and the test stops being standardized.

Protocol 2: the 10-5 repeated hop test (RHT)

The 10-5 repeated hop test measures reactive strength without a box, which makes it fast to run and easy to standardize across a roster. The athlete performs roughly ten maximal repeated bilateral rebound hops in place, then you take the best five for the score.

Run it like this:

1. Stiff legs, minimal knee bend. The athlete bounces on the spot like a pogo stick, keeping the legs stiff and the bend at the knee small so the work happens at the ankle and through the tendon.
2. Hands on hips. Standardize the arms by keeping the hands on the hips, which removes an arm swing that would otherwise inflate the result.
3. Perform about ten contacts. The athlete hops continuously for around ten reps, going for height and short ground contact on every one.
4. Take the best five. Score from the five most consistent contacts, which filters out the first few settling hops and any ragged ones at the end.

Because it needs no box and stresses repeated rebound rather than a single drop, the 10-5 is a clean way to profile repeat-hop reactive strength and to monitor it often. Many programs lean on it as their default RSI test for exactly that reason.

How to administer it consistently

RSI is only as trustworthy as your protocol. Because the index is so sensitive to ground contact time, small inconsistencies in setup move the number more than real changes in the athlete. Lock these down:

• Warm up first. Use a standard prep so the athlete is ready for maximal reactive efforts before the first scored rep.
• Standardize the cues. Say the same things every time, "floor is hot," "minimal knee bend," "go for height."
• Standardize the arms. Hands on hips, or a defined arm action used identically across athletes and sessions.
• Same surface, same device. Test on the same surface with the same mat each time, since RSI is sensitive to how contact time is measured.
• Familiarize, then measure. Give a few practice reps so technique is not the limiter on the scored trials.
• Take multiple trials. Record several efforts and keep the best, or the best consistent set, rather than a single attempt.
• Track over time. One session is a snapshot. The value of RSI shows up in the trend across a block, not in a one-off number.

On equipment: a contact or jump mat is the most practical tool for this. The Plyomat Controller 3.0 records ground contact time and jump height and returns RSI and DRI automatically in seconds, so the whole test runs in real time with the athlete on the mat. A force plate works too and adds force-curve detail, but for field and weight-room testing the timing a mat provides is what RSI actually needs.

Reading the numbers

Interpret RSI against the right reference, which is almost always the athlete in front of you. The most reliable read is an individual's own trend over time: is this athlete's RSI climbing, holding, or dropping across the block? After that, compare within your own population, your own team, sport, and testing setup, where the conditions are consistent.

What to resist is leaning on absolute cross-population thresholds. A single "good RSI equals X" number lifted from a different population, sport, device, or protocol can mislead, because so much depends on how the test was run and what it was run on. Use RSI to track change and to separate athletes within a shared context, not as a universal pass-or-fail gate.

"Track the athlete's own trend first, compare within your population second, and treat cross-population thresholds with caution.

Honest caveats

Two limits are worth stating plainly so you trust your data for the right reasons:

• Long flight times. Contact mats can underestimate flight time when the flight time is very long, which mainly affects elite jumpers clearing well above 0.70 m. For most athletes this is a non-issue, but it is good to know where the edge is.
• Contact-time sensitivity. RSI rises and falls with the contact-time measurement, so a consistent device matters more than a fancy one. Test the same athlete on the same equipment and your comparisons stay honest.

None of this undermines the test. It just reinforces the same discipline that makes any field measure useful: standardize the protocol, use one consistent device, and read the trend. Do that and the drop jump and 10-5 hop test will give you a fast, repeatable window into reactive strength that you can act on.

Want the math behind the ratio, or a place to plug in your own contact times? See our guide to Reactive Strength Index and the RSI calculator. For the hardware that captures contact time and jump height, see the contact mat and vertical jump mat pages.